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Starting & Starter Problems
New starters, substitutions, fitment problems.
Bosch; Valeo; Denso..... Starter motors.  Overhaul?
 Converting from a Bosch to a Valeo (and vice-versa).

Starter relays, starter solenoids, notes, advice, etc.

Copyright, 2012, R. Fleischer


article 16-A


For more information:

Starters, alternators, plus all sorts of electrical parts for them and lots more.
   Owned by Ted Porter, one of the "guru's".   Very knowledgeable.      Have one good recommendation, no
personal experience.
They also have discounted alternators for BMW particular, K bikes.

I would check Beemershop and Euromotoelectrics first.

How the starter circuits & associated parts function:

The starter motor is a powerful electrically operated motor.   The starter motor may have to provide
upwards of 1/2 to 1 horsepower in mild temperatures with modest compression engines,...& even
more when the engine is quite cold, has thick oil/ The starter motors in our Airheads are not very
efficient motors, compared to modern electrical motors.  The Valeo is more efficient than the Bosch. 
In any case quite a lot of amperes might be needed.  Starter motors of any kind for our motorcycles
are, at best, 60% efficient, and can be even less.  750 watts is equal to ONE horsepower. If the
system is a nominal 12 volts during cranking (typical, with good battery, as read at the starter
terminals), then 750 divided by 12 equals 62.5 amperes.  Due to the efficiency losses, & the need
for many more amperes to START the engine rotating, it is NOT uncommon to require 150 amperes
OR MORE from the battery.

There must be a means of switching on/off the large amount of current to the starter.  Aheavy-duty
solenoid-operated switch is physically located on the starter.  It obtains electrical power from the
battery via a large diameter copper-conductors-cored cable. The starter motor is grounded to the
engine aluminum case.  The transmission is bolted to that engine case. The battery's other
connection is to a hollow bolt at the rear right side of the transmission...that bolt secures the
speedometer cable in place, provides venting for the driveshaft & transmission & provides the
place for grounding the mentioned large diameter black-colored battery negative wire.

There is NO FUSE.  THAT is common to all vehicle starter motor primary circuits.

The starter solenoid itself is activated by a modest current delivered a small starter relay contacts.  
That small starter relay is located along the top frame backbone.  With the ignition key ON, pressing
the starter button will cause the starter relay to close its contacts, which sends +12 volt power from
the starter relay via a fairly thin BLACK wire that goes to the male SPADE terminal on the starter
solenoid unit. The starter relay is, therefore, a small relay that sends electricity to another much
larger relay, called the solenoid (solenoid relay). The electricity causes a substantial magnetic field
in the solenoid coil.  The center of the coil has a large round piece of steel that is moveable.  The
"coil" in the solenoid is actually a dual coil, with two ends connected, so it is a three terminal coil. 
The reason is that the current just described is INITIALLY applied to the section of the coil that
draws a fair amount of current, and is called the 'Pull-in Coil'.  This substantial magnetic field moves
the large steel slug.  One end of the steel slug has insulated contacts or a contacting strip which is
pushed towards large copper contacts & the pressure causes those contacts to be jumpered. 
Once jumpered/connected, the large battery cable at the solenoid can now pass a very large
flow of electricity into the starter motor. At the same time, the Pull-In wiring inside the solenoid switch
disconnects. There is a secondary STAY-IN coil that draws less current and will not overheat.  Its
purpose is to keep the solenoid piece pulled-in and the massive switch in contacting (shorted)
position; so long as your finger is still pressing the starter button.   In your Airhead, the starter
solenoid is a high power electro-mechanical switch.   

As the starter begins to suddenly and powerfully rotate, a mechanical device called, generically,
a 'Bendix' or Bendix drive, is on the shaft so it begins spinning.  It is located on the rear of the
starter motor on the starter motor shaft.   This device, via spinning forces & guide grooves,
moves suddenly and very quickly rearward... with quite considerable force.   As it moves towards
the rear it engages its teeth into the engine flywheel teeth.  The flywheel is called a Clutch Carrier
in models from 1981.  The teeth are actually on the outer edge of a part called the Ring Gear. 
It is shrunk onto the outer diameter of the flywheel.  Engagement of
the rotating Bendix drive teeth and the ring gear teeth, causes the engine to rotate. 
The teeth of the Bendix drive, & on the flywheel gear, are shape-designed to enable
quick meshing at high levels of force.   

Later model Airheads may have a Valeo brand of starter motor which has permanent magnets
instead of field coils like the Bosch has; the Valeo may have a planetary reduction gear inside its
nose, which allows the starter to develop higher rpm and hence higher power.  The Bosch
starters use electromagnet field coils.
Nerdy: Starters with electromagnet field coils have an interesting electrical property.  As the
starter faces an intense load (thick oil, high friction, high compression, large pistons, etc), the
starter will slow down....which causes the field coils to draw MORE electricity, which boosts
the power output.

There are at least THREE types of Bosch starters used on the Airheads as original equipment.
I say at least three, because there are some very rare instances of other models used on
foreign-shipped bikes.  However, to my best knowledge, the number of teeth on the flywheel &
starter are always either 8 teeth on the starter with 93 tooth flywheels; OR 9 teeth on the starter
used with 111 tooth flywheels. The 93/8 gives a ratio of 11.625; the 111/9 gives a ratio of 12.33.
This later higher ratio enables the '9 tooth' Bosch starter to develop a fractional amount more
horsepower, mainly because it can spin faster.   The Valeo starter also uses this ratio, but may
contain a planetary gear set that enables the Valeo to internally spin even faster, developing a
bit more horsepower than the last of the Bosch designs that BMW used.   This method is common
in later starters used on many vehicles.

Bosch starters:
Up through 1974 were 8 tooth, rated 0.5 Kw and 290 A. 
The /6 bikes for 1975 & 1976 used an 8 tooth rated 0.6 Kw & 320 A.  The 8 tooth
starters are used ONLY with the 93 tooth flywheels; and are for practical purposes, interchangeable. 

BMW phased-in the Bosch starter at different times depending on the motorcycle
model, so it is possible for some 8 tooth starters to be on somewhat later bikes...SO IT IS A GOOD
teeth must be on the flywheel or clutch carrier to match the starter.

For SOME 1976 and all 1977 and later, the starter has to be 9 tooth, for use with the 111
tooth flywheels  (more properly called the clutch carrier from 1981).
The starter is, rated 0.7 Kw and 320 A.  Not the increased power and SAME amperes.

Nerdy:  Earlier, I mentioned that starter motors are not very efficient.  The reasons are
magnetic field losses & friction inefficiencies, as well as other things.  The power rating
of the most powerful of the three above Bosch units was 0.7 KW.  I will do the math for you
here.  0.7Kw is 525 watts; divided by 12 is 43.75 amperes.  Not bad, eh?  BUT, notice the
320 ampere rating!!  That is equivalent to 3,840 watts!...that is the maximum allowed...and
is equivalent to a bit over FIVE horsepower.  The battery, cold day, thick oil, ETC., may need
to provide a LOT of power!

Valeo starters were installed on later motorcycles; all are 9 tooth; early ones were troublesome; the
glued-magnets ccame loose.   See for fixes or new ones.


Problems starting BMW Airhead motorcycles can often be traced to
a bad battery or its connections, but there are other possibilities:

1.  Poor contacts on the starter relay under the fuel tank.  This can be sometimes be repaired by opening
     the can and burnishing the contacts.  It is possible for you to hear this relay's faint click, and yet the
     contacts are not making electrical connection.  Very rarely, the relay has corrosion INSIDE the base.
     MORE commonly, the starter relay has faint corrosion on its EXternal male prongs, & in its socket
     (on models having a socket).  Cleaning external connections will cure the No-Start-NO-Click.  A
     permanent fix is to JOIN, very cleanly & neatly, & insulate afterwards, all the RED wires (usually 3) at
     the socket.  The reason joining the EXTERNAL red wires works is because there are two terminals on
     the relay for these wires, and the jumpering occurs INSIDE the relay, thus any problem with the
     EXTernal spade connections will act like the relay is not there.

2.  Poor solenoid (located on the starter motor) internal contacts.  A replacement is best; but I will have
     more to say later, herein, about how you might be able to repair one.

3.  The starter itself may have worn brushes, worn bush/bearing, poor drive gear assembly, or an
     armature that needs undercutting at the commutator and truing on a armature lathe.  It may also be
     heavily grunged-up with dirt/filth.   

4.  Early Valeo starters had a nasty habit of the glued-in-place field magnets coming loose & jamming
     the starter.  Some have rebuilt them using Saturn car starter parts.  Better to change to a
     Bosch...or, get an updated Valeo, or EnDuraLast, both of which are the easiest thing to do, & not

      SOURCES, INCLUDING BMW.  I suggest you discuss the various options, not all of which
      are shown here, with Ted Porter at his Beemershop.

5.  You CAN change the original Bosch Starter to a late model improved Valeo (see 4 above).  You
      may have to do a tiny bit of metal work, described later in this article.  Be SURE to get the
     appropriate 8 or 9 tooth starter!!!   Same, regarding metal work & 8 or 9 teeth, if converting to
     the Denso ...or any other....starter.  I check it even if replacing with the original brand of starter.

6.  A more expensive solution is to install the modified Denso starter that is available from numerous
     sources.   I am not convinced it is worth the large cost.  Distributed/Sold by Motorrad Elektrik,; and possibly available from such as Ted Porter's BEEMERSHOP, etc.

     These are modified-nose versions of the DENSO-made starter used on Toyota's, etc.  Possible
     interference problems with this starter, & need for some mild metal grinding, is shown with a photo,
     at the end of this article. A MINOR possible problem.

7.  On many BMW's the starter + lead connection is the ground for the headlight relay coil, but this
      is not a problem for starting; as all it does is turn off the headlight during starting, yet leave the
      instruments & tail light on.

8. at item 4.D., has some further information on
     problems with the starter circuits.   There is more information, such as for the /5 problems, also
     in that long article.

WARNING!.....>>Fitting a starter motor; breaking nose castings; ETC.

Failure to properly fit ANY starter, squarely and solidly/securely, to the cradle in the
motorcycle can lead to a broken starter nose. 

The Bosch starter MUST have the forward plate affixed and tight to the bracket and engine.  

Do NOT "assume" anything.  Never install a starter motor, any brand, without carefully
checking how well it fits into the engine case cradle.  This caution applies to the original
starter, and any replacement.   

The Bosch starters have a bent metal plate at the forward end, using one bolt to the timing chest wall. 
This supports the starter to the timing chest wall, needed with the heavy Bosch.

Electrical problems can cause very heavy multiple impact loads on the starter motor & then be
concentrated at that 'breakable' nose casting.  By electrical problems I mean anything that causes
chattering when using the starter motor; such as a poor battery; as well as off-center forces that
cause excessive starter power to be needed. 

If the starter motor bearings are quite bad, that can cause tilting of the armature, not good for the
starter nose, and not good for the starter Bendix drive, flywheel teeth, etc. 

Other problems include a bad starter ring or fitment on the flywheel/clutch carrier, & too tight a
clearance between the Bendix gear and the ring gear.  These are rather rare, IMO.

If your bike kicks back (or, backfires backwards), that can be VERY hard on the starter, and the
problem needs immediate attention. 

Some, especially early engine cases, may need some filing to ensure a proper fit.  Further down
this article are some of the interference problems found on starter fitment, but these are not
necessarily the same fitment problems found with starters back in the seventies.

Starter relays:

See #1 above.
Later model Airheads (AFTER 1984) use a starter relay with a diode inside.  Some folks have
substituted a 0-332-014-118 relay (03-32-014-118), and some HAVE used a DF005 'Blazer' relay
from AutoZone stores (which has two 87 terminals & no 87a terminal).   The Bosch starter relay
uses two #87 terminals, and may sub to Bosch 03 32 019 150 for 1977+ bikes.  That is a common
Bosch accessory use relay.  Connector, if you need one, is 0 334 485 007, while the spring loaded 
terminals are 1 901 355 917.  It is certainly possible to substitute almost any common 20 or 30 ampere
12 volt relay for the starter relay on Airheads, but there are some complications, such as with the /5;
& with late models with 2 diodes in the relay, so ask ON THE AIRHEADS LIST about it.  Complications,
if any, are easy to deal with, so don't be discouraged with my remarks here. 

BMW has an SI on retrofitting the Valeo starter on 1985-1988 bikes that came with Bosch starters. 
BMW says that the original starter relay contacts are not up to the larger current draw of the Valeo
SOLENOID COIL.  It is my belief that the original relay will usually be OK for most, but if you want to
install one with larger current capacity, see this document, which I have here in pdf format for you: Valeo Starter-2383.pdf
...OR, install a common relay as a paragraph above.

This is applicable to 1985-1988 models officially:
If you install a Valeo in earlier models, ALSO pay attention!

That SI does not have ALL the information...but I do, below:

If the original starter relay is 61-31-1-244-019 (that's a BMW number, not a Bosch number!), it
contains a diode, and has other properties that BMW thinks do not match the Valeo starter, & the
relay contacts might stick.......and the starter continue to run!

BMW says that the proper relay is 61-36-1-391-397, which has a higher current carrying capacity,
and a stronger spring to help the contacts open.  If you have to replace the -019 relay, BMW will
supply the -397 relay.    It also has the diode inside. Some have substituted generic 30 ampere relays. 
I have heard of sticking relay problems, but maybe only rarely, since I began working on Airheads! 
So, it is likely a rare event.  On the other hand, it is my belief that if there IS a problem with putting a
Valeo into an Airhead that came with a Bosch, that the problem is much more likely to be only with
1985-1988 models; due to a change in the relay internals construction that is NOT mentioned by
BMW.   It is possible that lengthy starting can tend to overheat the relay contacts.

To avoid confusion, if you have a pre-1985 Airhead, the stock relays seem to hold up just fine with
the Valeo.  I have MEASURED the current draw of the Bosch & the Valeo solenoid coils under
actual starter use; they are NOT excessive & are just about equal!  The inductive kick-back voltage
is also comparable.  This solenoid coil current is the current that passes through the starter relay
contacts.  I would not be inclined to replace the early stock relay, although I might open it & burnish
the contacts.  It is my belief that the problem, if there really is one (per BMW), that it is confined to
SOME of the 1985-1988 models.

MORE on Bosch Starters used on BMW Airhead motorcycles:

Be sure
to use the Bosch (or other) starter with the proper number of teeth to match your

If you decide to use some starter from a car, salvage yard, SURE the starter has
the correct direction of rotation
, and will fit. If a Valeo, be sure the field magnets are secured.

The Airhead Bosch starter uses ELECTROmagnets, not permanent magnets, for the field coils.
Bosch does make a permanent magnet starter, but I have never seen one in the Western Hemisphere.

The Bosch electromagnet starter is very old-fashioned technology, reasonably rugged, reliable,
but less powerful than the "permanent magnet high rpm with planetary gear type" like the Valeo. 
A property of the Bosch field coil type is that as it 'sees' a heavier & heavier load (cold engine,
thick oil, high compression engine, etc), it draws more & more current, producing more & more
power.   Still, the ultimate power under most conditions is less than the Valeo, which spins faster,
& has the planetary gear reduction drive inside.   I consider it a tossup as to which is better,
considering all factors.
   The Valeo parts are less rugged, but that appears not to be any big factor,
even over many miles,
at least with the updated type with the secured field magnets.

It has been theorized that the Valeo permanent magnet type MIGHT loose some magnetism over
TIME and USAGE.   However, they have held up surprisingly well....and I have had NO reports of
any loss of magnetism. I suggest ignoring this 'supposed' problem.

The Bosch starters have a bent metal plate at the forward end, using one bolt to the timing chest
wall.  This supports the starter to the timing chest wall, needed with the heavy Bosch.

When installing a Valeo in place of a Bosch, you do not use the the plate nor bolt.  The Valeo
installation is ~ 6 pounds lighter than the Bosch.  Do NOT throw away that mounting plate, someone,
or you, may want it.   When installing ANY starter be especially careful that the starter is properly &
squarely mounted, you want to avoid breaking or otherwise cracking the nose (the casting that is the
rear part over the flywheel) during operation.  Be sure the mounting cradle, nose area, etc., is
CLEAN of filth and there is NO proud metal, etc. 
See later on that subject in this page!! 
This caution on fitting INCLUDES the original starter!

The Valeo Starter & some Bosch information:

The magnet failures on the early Valeo's are well-known.  Using epoxy to glue magnets may have
its place in this world, but there were lots of failures, especially in Airheads where the starter is
subject to so many repetitive heat-cool cycles from engine heat.  I do NOT like the ideas of epoxies
being subjected to constant wide temperature changes, their expansion-contraction rate is different
than the surrounding metal. The starter in an Airhead does, remember, sit right on top of the engine,
inside a cover!  The constant heating/cooling cycl can crack or otherwise ruin the bond of the glued
magnet.   Valeo addressed this problem (in 2001) by changing the type of glue & reshaping the side
of the magnet that fits against the starter shell. Late manufacture Valeo starters are adequate &
seem reliable.  BUT: 
There is an aftermarket type of Valeo housing available with a modification to
help keep the magnets from moving, spring-type separating pieces (clips).
   Valeo permanent magnet well as permanent magnet starters from many other manufacturer's, even Bosch (rare,
never seen one), work fine in cars.  
The proper aftermarket Valeo's with the updated magnets,
bonding, & clips, are available from
and from such as Ted Porter's

Watch out for cheaper made & cheaper construction "Chinese" starters, being sold, UNbranded,
in place of a real Valeo.


In the distant past I was outspoken in that I personally preferred the Bosch electromagnet type for our
However, I am open-minded on the latest Valeo starters....and think them now, if
you are making a decision between overhauling your Bosch, or replacing it with a Valeo.....or;
replacing a Valeo with a Bosch....I no longer have any especially strong preferences for the Bosch
over the Valeo (or the aftermarket Denso, for that matter).  Typically/usually, overhauling an existing
Bosch starter, assuming the armature is not going to have to be replaced, is going to be less costly
than replacing it with a Valeo, ETC. The stock Bosch starters are adequate....and almost any
starter/alternator shop can overhaul one, if you do not want-to, yourself. 
AVOID cheap construction "Chinese" starters, being sold, UNbranded, in place of a
real Valeo.

For parts for rebuilding for both Bosch and Valeo, see, John Rayski. 
John knows the in's and out's of all of them, and has a stock of all the pertinent Bosch & Valeo parts;
and has Valeo starters of the correct type in stock (inquire as to if they are GENUINE Valeo). 
Remember that there are both 8 & 9 tooth starters. BE SURE you have the correct number of teeth
on the starter...and if you are breaking nose cones, count the flywheel teeth too....just in case
something got switched.  Check the FITMENT!!!!

The Bosch is a common starter for many starter repair shops, easily worked on...but some might
not have the parts for the starter used in our Airheads.     The same sort of comment could be
made about the Valeo and Denso.

You may run into information leading you to think about using a Saturn or some other automotive
Valeo housing when rebuilding an original troublesome Valeo.  Unless you are trying to save the
absolute maximum amount of money, I recommend you get a new upgraded version Valeo.  

Repeating here: 
When installing a Valeo in place of a Bosch, you do not use the the plate nor bolt;
the Valeo installation is maybe 6 pounds lighter than the Bosch. When installing ANY starter be
especially careful
that the starter is properly & squarely mounted, you want to avoid breaking or
otherwise cracking the nose (the casting that is the rear part over the flywheel) during operation. 
Be sure the mounting cradle, nose area, etc., is CLEAN of filth and proud metal, etc. 
See later
on that subject in this page.  This caution on fitting INCLUDES the original starter!

NOTE! is possible to mis-fit a starter, and the nose cone does NOT break immediately.
That is rather common for the broken nose situations.

Here is a link to an article on working on the Valeo starters:
Use only the images and text for them.  The article does not get into some deeper technical
matters, but will be useful to you if you have not taken a Valeo apart before, & want to
do maintenance to fix loose magnets, or replace worn planetary gears, clean/lubricate, etc.

Going from a Valeo to a Bosch and vice-versa:
BE SURE you use the proper number of teeth on your starter motor, to match your flywheel!

If you decide to change your failed Valeo to a Bosch:  Brand New Bosch's are very pricey &
probably not available. If you want a Bosch, and yours is not, you can try to find a cheap good
used one, & rebuild it if needed. Generally you need only a set of brushes & bushings (bearings)
& a solenoid assembly. If the Bendix drive is questionable, replace it.  The solenoids can often
be rebuilt too but I do NOT recommend it, although I have the information in this article.
  You may
have to undercut the armature, farm this out or do it yourself, it is not difficult.  YOU MUST obtain
& use the forward bent metal plate when using the Bosch.

The 'Bendix' drive should be clean & lightly lubricated. I like to use a silicone grease due
to its general non-hardening & wide temperature range. But, I have some mixed feelings
about the use of silicone's on fast moving parts, & maybe a lightly laced moly grease is
better...I am NOT sure about this.  Starter rebuilders may have ideas.
   I personally have
used a faint coating of common wheel bearing greases, and they work fine.

Fitment Problems:

When retrofitting a Valeo to a BMW Airhead that had come originally with a Bosch starter, there
can be an metal interference problem, which can be very slight & hardly noticeable, more
The problem comes about because the starter nose machining is NOT EXACTLY
the same between the Bosch & the Valeo starters, & BMW was, perhaps, a bit more careful
about machining the engine case when the Valeo was installed as the stock starter.   It is my
belief that when BMW shipped Valeo starters on the later Airheads, that the machining on their
Valeo nose might be very slightly different.  I have not made enough measurements to prove this. 

I strongly suggest that ANY time you replace or even re-install, ANY starter, that you
CAREFULLY check its fitment!!  There have been instances of a starter poorly fitted, & the
nose broke!  Remember, the forces are HIGH.

I THINK that most metal interference problems will be seen only when installing a Valeo in place
of a Bosch, but I am not 100% sure about that>>>>>there were instances of the Bosch
having problems with broken nose cones in the seventies models.  Best to check, find
the interference ....and using a common file, fix ANY potential problem.

Thus:  my advice here is generic, & assumes you will check fitment of any & all
starters, that includes the Bosch, the Valeo, and the Denso....and no matter what HAD
been fitted.  This includes refitting the same starter!
I think that anytime you remove
& replace even the same starter that was in the bike....that you should check for the
proper fitment!!!   This is particularly so if the nose cones have been breaking. 
They should NEVER break.

I suggest your CHECK the fitment before you remove the old starter....and then:
After you remove the old starter, LOOK carefully at the area the aluminum nose of the old
starter had recessed into, just forward of where the gear mates to the flywheel teeth.   On
some Airheads, BMW may not have cleaned up excessive metal over the last 1/2" or so of
the round cavity, at the top inside edge. When you try to install the starter (probably this
happens only with the Valeo, but I am NOT sure about that), you may find some combination
of the following:

The starter might not fit perfectly, may not want to go fully downwards at the nose
area & the Valeo threaded bolt holes (or even Bosch unthreaded ones??) may not
line-up perfectly.   Inspect carefully, & use a half-moon fine file, or a small electric
rotary tool of some sort, to make this inside area smooth at the top, with NO metal
that is 'proud' (or of lesser diameter, causing a reduced diameter).  You can feel this
with a fingertip, and usually see it with your #1 eyeball.   I have personally seen this
and had to remove about .020" (+-), on both sides, at that top area.   This is easy to
do, & then the Valeo (Bosch?) SHOULD fit/mount correctly.  

The Valeo, as opposed to the Bosch, has threaded holes, so you use the bolts you had from
the Bosch, sans nuts, from the rear. 

Note the excess material at the left, pointed at by the red lines
in this photo. Note the probably OK line at the right.  Don't
worry if you remove a bit too much material! 
DO NOT remove material at the bottom.   Check how the nose
fits, very carefully, at ALL POINTS.  Use a dentists mirror &
strong light.  I also use a feeler gauge.


The Denso fitment problems:
(check fitment everyplace!...what is below may not be the
only area for problems)

It was reported to me that the Denso aftermarket starter, such as sold by
Motorrad Elektrik, etc, does not, or may not, fit perfectly.   I obtained a photo. 
The installer noticed TWO problems with metal interference.

1.  Light interference with the solenoid to a rib on the starter cavity cover
     (sorry, no photo on that).

2.  Interference at the mounting ear on the back cover.  In the photo, below,
     you can see that the ear has been ground down so as to give about 1-2 mm
     of clearance between the crankcase and the starter ear.

This may not be the only interference problem; so check CAREFULLY!

ONE OTHER interference problem has been reported:  the ribs on the
underside of the BMW starter cover.

Overhauling the Bosch Starter Motor & Solenoid Switch:

Bosch starters are used in a variety of makes/models of vehicles. Sixties & Seventies
VW's, for one example, use similar starters to the ones in Airheads.  They vary some,
but are mostly the same.    So, it is not difficult, even at some auto-parts stores, to get
parts.  I do, however,  suggest you get them from
or Ted Porter,

Not shown anyplace that I know of, is how to overhaul a malfunctioning solenoid

That is because most people will replace the solenoid.   A malfunctioning solenoid
that has an open winding (there are two windings) should be considered as not
repairable.  However, most of the time the problem is poor contacts inside; although
poor contacts at the brushes of the starter can make the solenoid act like it was bad. 

Below are photos of a Bosch solenoid unit as used on our Airheads.  IF THE SOLENOID

Most of you will just replace a solenoid switch as a normal matter when overhauling a
Bosch Starter.  But, for those of you that wish to try to fix or improve such a switch, I
include the details here.  See the photo, below.

I have removed the terminal end cap.  To do so, you need a large soldering iron with a
big tip.  Even a 35 watt iron with a big tip mass will work.  It is the MASS of the tip that
is important.  A very large soldering gun (perhaps the 275 watt size) might work OK. 

You undo the 2 screws as shown; then put a slight to modest amount of pressure on
the end cap (as if trying to remove it), whilst you heat the solder joints until they release. 
Go back & forth to the soldering places; until the cap comes off.  Clean up the solder
It is VERY important not to mix up which wire goes to what terminal hole!!!
watch them as you remove the cap.   DONT GOOF HERE!!!  The easiest way to avoid this
is to mark the end cap and body before removal!

Inside you will see the flat switch contacts recessed in the cap you removed....& the
mating contacts held to the solenoid with a funny 'nut'.  I usually don't bother
disassembling further on the plunger side. 

Clean the contacts in the cap & at the end of the plunger.  You can use a very small
rotary stainless steel or brass cone-shaped brush, that you can put in your drill press
or electric drill.  They remove very little material if used BRIEFLY and with not too much
pressure; this takes only a few seconds at modest rpm.  Other methods will also do,
such as a flat end of a piece of doweling or other tool, covered in 220 grit sandpaper. 
Use what you want to, but do not make deep want a smooth clean
surface of maximum contacting area. 
You MUST keep the contacts FLAT & square to
the assembly, so maximum contact surface will be obtained.

There are 4 thin solid copper wires that have a thin enamel coating for insulation.  
The paired wires (push the insulation back a bit if you need to, to see it is a pair) go
to the SPADE terminal that has a black wire to the starter relay.  You've already marked
the case and cap so you won't mix them up, right?  There is one wire that is spot-welded
to the metal end case; that's the ground wire for the HOLD-IN coil.   The single wire
next to it
is the one that goes to the starter & one of the large solenoid switch contacts.   
You did note where the wires went when you removed the cap?  If you goofed & did not
mark the cap & body, the information here will guide you to the proper fitment of the
wires before you re-solder.

When re-assembling, you need to be sure the solenoid contacting plate fits into its
mating cap depressed area portion, & the wires go to the correct terminals! 
 You can
assemble the cambric ring & the wires to the cap, & lift the cap a wee bit, & rotate the
tang at the far end of the solenoid, to line up the contacting plate with the cap cavity.  
Push against the tang end, holding the cap end, on your bench top, to be sure the
contacting plate actually moves into the cavity for it.  Assemble cap (align wires &
pegs) to the plunger side of the solenoid unit carefully, using #1 eyeball; test that
the solenoid plunger mechanically works OK with your fingers, then screw the cap
on with the two screws (locate the pegs!)...and THEN solder the terminals. Use ONLY
a very hot large tip soldering iron.  Use standard rosin core electronics solder.    Be
careful to assemble correctly!

The solenoid should be tested with it mounted & connected-up to the starter.   Do
the testing on the repair bench, before you install the starter into the engine block. 
You will need heavy gauge wires.  Connect the + terminal of the battery to the battery
large threaded terminal (#30) on the solenoid.  Connect the - terminal of the battery
to the starter body...such as a nose mounting ear.   If you get a small spark from this,
you miss-wired the solenoid!

Hold the starter will tend to jump around as it starts!..... using a jumper
wire or screwdriver, etc, momentarily connect the SPADE terminal to the + battery
connection at the solenoid (#30).  The solenoid should clicking strongly & loudly
& the starter should run.

Here is another test, but just for the solenoid assembly, not mounted to the starter. 
This is a sophisticated type of test.
1.  Make a clamp, of wood, for your bench vise.  You can use wood as jaw pieces, or
     drill a hole in a piece of wood to ~match the case diameter of the solenoid assembly,
     then split it down the middle.  You want to be able to clamp the solenoid into your
     vise, without distorting the case.
2.  Connect the CASE to the battery - terminal.  I usually just put a bare wire around
     the case, during the clamping.
3.  Touch-connect terminal #50, the start terminal (male spade) to the battery + terminal.
     This will cause the assembly to be a fairly strong electromagnet, easily seen/tested. 
     This tests the HOLD-IN coil.
4.  New connections:  Connect the battery + to terminal 50 which is the male spade terminal.
5.  Momentarily connect the battery - to the terminal that normally goes to the starter motor. 
     This connection will draw more current, than in #3, and a much stronger magnetic field.
6.  Push/pull on the solenoid to see how the magnetism feels to you.

There is a test that can be done to determine that the Pull-In coil is not energized DURING
the starter motor's actual turning.  It is done by actual current testing, & isn't described here.

Another test is that you can use an ohmmeter instead of #1 to #6. Disconnect the battery. 
With unit NOT attached nor connected to the starter motor:
Moderately low ohms:  terminal 50, the male spade terminal, to the case.
LOW ohms:  Terminal 50, the male spade terminal, to the solenoid large terminal that goes
to the starter motor.


Overhauling the Valeo:

I was going to write up how to do this, but a fairly decent article with photos is on Anton
Largiader's website, so here is the direct URL:

Valeo starters can be purchased from  They have parts
for Valeo, and Bosch, & many other things.  I highly recommend them over Ace
Houston Warehouse, which is mentioned in Anton's article as well as the one
you are reading, well above this area.  
Another quite good source is:


02/06/2004:  add link to /5 article; minor other clarifications
05/30/2004:  add NOTE on the starter relay changes
10/27/2005:  updated in numerous places
04/22-2006:  add red note on Valeo installation needing special relay
11/28/2006:  correct URL spelling for euromotoelectrics
11/29/2006:  Change to 16-A, and do minor editing (16-B is being written)
04/28/2007:  Fix actual hyperlink for euromotoelectrics, display had been OK, link NOT.
09/28-10/1/2009:  Update hyperlinks, clarify some details that were unclear; re-arrange article a bit for
                           additional clarity, add all the information on the Bosch Starter solenoid overhaul.
10/17/2009:   photo of the metal interference problem.  Clarifications & editing throughout article.
11/23/2009:  reference to Anton's site for the Valeo overhaul, euromotoelectrics in that same area, etc.
04/25/2010:  Add photo and information on the Denso problem
05/24/2010:  Fix typo regarding forward plate that IS used with Bosch, NOT Valeo.   Clarify minor details.
05/12/2011:  Fix typo where word alternator should be starter. Add dividing line at one place to clarify a
                    new discussion section that was confusing. Fix a couple of mis-spellings.  Remove hyperlink
                     to 'thisoldtractor site (that has removed the starter motor overhaul information).
08/04/2011:  Fix note, DF005 relay now confirmed as substitute.
03/18/2012:  Clarifications
11/14/2012:  Clean up article, separate sections needing such, add some minor details, fix whole article
                    so will display a bit better on Chrome and Firefox, and on somewhat smaller screens.
08/11/2014:  MINOR changes
09/12/2014:  cleanup
02/16/2015:  Extend, somewhat, the warnings
04/04/2015:  Add information/section "How the starter circuits and associated parts really function:".  This
                    is the same section I expanded-upon, located in article startingprobs.htm.  Expand testing.
                    Fix wrong description on wire connections inside the solenoid assembly.
07/21/2015:  Re-work section on starter/flywheel teeth vs model #, for more clarity. Cautions added 11/15/2015.
01/14/2015:  Clarity. Metacodes. Left justify.  Narrowing.


  copyright, 2012, R. Fleischer

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