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Diode Boards & Grounding Wires
On BMW Airhead Motorcycles
((including information on diodes testing))
© copyright, 2014, R. Fleischer
Diode board testing and information about repairing the faulty Wehrle-manufactured ones
(with non-bent-over large diode leads) is covered in depth in the June 1999 issue of
AIRMAIL, an article by BMW Guru Oak Okleshen. Some Airhead owners will want to read
this article. The Oak-recommended (& frankly difficult to do) board modification, could be
done if the solder joints are failing, especially if you have a R100 engine, which develops
more heat (worse if an RS or RT). I've had good luck with removing the 'paint' with
gel type paint remover, somewhat enlarging the solder pad area first, by CAREFUL
scraping with a sharp Xacto thin blade hobbyist knife (don't remove copper), &
then re-soldering using a higher temperature solder (50-50 plumber's solder +
rosin flux). No re-painting needed. This is vastly easier to do because drilling the
PC board & adding wires & soldering per Oak's method is quite difficult &best done
by total disassembly (which Oak did not get into). Yes, it can be done with long
tweezers or forceps, but is tricky, a real PIA! I can NOT recommend that method.
I've also seen several BOSCH boards with this problem, perhaps Wehrle made
them? There have been some other Bosch and Wehrle labeled products
that made me think either could have made them.
No matter the board maker's label, I suggest inspecting the solder joints
for overheating. More later in this article.
Diode testing is most often done with a multimeter. Using the OHMS function, voltage is
applied by the meter's internal battery. When this voltage is applied to a diode (if the
polarity of the test leads are such that the ACTUAL - (minus) lead is connected to the
end of the diode marked with a LINE (that end is called the cathode), then current can
flow through the diode. Mentioning this because many meters have - voltage on the + red
lead) The amount of current is limited by multimeter design. If the meter leads are then
reversed, NO current should flow (or, a super teeny current, called leakage current or
reverse current). Thus, in the conducting direction, the resistance is very low, and in
the reverse direction the resistance should be VERY high. Some multimeters have
a very low voltage applied on the ohms function, and thus will NOT test diodes, because
the meter can not 'turn on' the diode in the forward (conducting) direction, which requires
as much as 0.75 volts on some small power diodes (most are .4 to .6 volt).
Some meters have an actual DIODE TEST function, & you do not then have to use
the OHMS function, unless you want to (I do, for the reverse high resistance direction.
The meter in diode test mode will indicate forward voltage drop.
One has to know one's meter, testing on a known good diode. The most important
readings are that the forward connection be a LOW reading, and the reverse be a
VERY HIGH ohms reading. For the Diode Test function, most such meters read the
forward voltage drop on the various power-rated diodes at ~0.5-0.6 volt.
Re-stating, differently, and somewhat more in depth:
Diodes can be tested for forward current direction resistance on your ohmmeter. The
indication should be fairly low, and just about equal for all power diodes on your meter.
The actual reading could be 8 to 120 ohms perhaps, depending on the meter...this is
not an absolute on X1 range (if it has adjustments for range). Most ohmmeters have
one or two small internal batteries that enable "turning on" the diode under test.
Some, usually more expensive ohmmeters, do not turn on diodes on their especially low
voltage ohms test modes in the so-called forward mode of the diode, when the leads are
properly connected. The reverse mode test of the diode should indicate a very high
resistance and can be tested for this by reversing the ohmmeter leads to the diode &
raising the full scale range setting on the ohmmeter (if it has manually operated ranges),
& the reading should be in the millions. All Airhead diode board diodes can be tested in this
manner. In fact, most diodes can be tested this way. Ohmmeters can be RUINED if
the vehicle power is turned on, during ohmmeter testing. This is true of the Airheads
if the battery is even fully connected! To use an ohmmeter on an Airhead, remove
ALL the wires at the Negative battery post!
Tests can be VERY confusing if anything is connected to that diode besides just the meter.
Luckily, the ohmmeter test and the meter Diode Test functions both generally work OK
on the still-connected Airhead diode board, with some minor reservations. Better is
with the board removed and disconnected from all wiring, but it is not mandatory.
Very special diodes such as "zener diodes"; Schottky diodes; ETC., will indicate differently,
and for our motorcycle purposes (except for some special places like inside the electronic
tachometer and inside the electronic voltage regulator for the alternator, are unimportant
except as a mention here).
A much better test,
ESPECIALLY (but not exclusively) FOR THE SIX LARGE DIODES IN THE DIODE BOARD:
This is a test that applies A.C. current through the diode via a suitable current-limiting
resistance. If you stay under one ampere, this test is safe for even the small diodes
of the diode board.
Obtain a 12 or 14 volt lamp...a taillight running lamp is fine, and a 6 to 12 volt transformer.
A suitable transformer is Radio Shack #273-1352 or 273-1511. An old bell ringing
transformer might be just fine. So also might be modifying leads for/on an old desktop
high intensity lamp unit...maybe with its own lamp!
If making from scratch you can purchase a socket with wires attached for tail lamps
if you want to. Makes a neater setup, but not as need as modifying an old desktop
high intensity lamp unit.
Or, just solder wires to the low-power lamp section of a run/brake lamp if the lamp is the
two-filament type. Nothing critical here.
The lamp MUST be the type that pulls a bit of current, and very broadly maybe half an
amp to an ampere is OK for the small diodes....but even more, a couple or so amperes
or even more, is OK for testing JUST the 6 large diodes.
I personally test the six LARGE diodes using an old headlamp bulb (one side of which
is burned out and I kept the lamp for this purpose). I use a transformer of several
amperes, it being ideal for the 6 large diodes. DO NOT test the small diodes with the
higher powered lamp. The BRAKE lamp or section of the tail lamp that has the braking
section, is NOT safe for the SMALL diodes. This also applies to turn signal lamps,
which also draw a bit too much current. Headlight lamps will allow about 4 or 5 amperes
using a 12 volt output transformer, for the common 55/60 watt lamp. NOTE that you
do not need a transformer rated at 4 or 5 amperes, one rated at just an ampere or two
is fine, after all you are only using the transformer for a FEW seconds at a time.
With whatever lamp you are using, connect the lamp in SERIES with the transformer
low voltage secondary winding, that leaves two wires for testing the diodes, one diode
at a time.
When the wires from your transformer & lamp are touched together, the lamp will light up
brightly. When the wires are touched momentarily to a diode, the lamp will NOT BE LIT
if the diode is faultily open. It will be brightly lit with a shorted bad diode, and considerably
dimmed on a good diode. Getting a nice set of test prods from Radio Shack, and
making a nice little tester out of all this is a NICE idea. This type of test is called
a dynamic test, and is THE easiest to do best test for your diode board diodes.
If you follow up with an ohmmeter front to back ratio test, then it is highly unlikely
you will have any sort of diode faults that you did not find.....unless the diodes
fail at elevated temperature, which is quite rare.
BMW used rubber diode board mounts on MANY models, from ~1979 to ~1993,
with some exceptions. Airheads that ALREADY SHOULD HAVE solid casting
metal diode board mounts are: /5 models; /6 models; 1978-1987 R65 & R80 models.
I believe BMW thought that installing rubber mounts would cure the problems
they were having with the boards, which was actually NOT vibration which is
what they must have thought, and why they used rubber mounts on some bikes.
The problem, as earlier mentioned, was improperly formed & soldered leads on
some diode boards, with the solder joints failing especially badly on fairing
equipped models, and more especially on those if R100 models...all of which from
their extra heat (an additional factor was the lower flow of air through the early
front alloy covers).
Using rubber mounts meant that BMW needed extra grounding wires from the
top (grounded side) row of diodes. These rubber mounts were LOUSY at
transmitting diode board heat to the timing chest casting. BMW's grounding
wire method did NOT work as well as solid mounts types did, electrically. This
caused wrong voltage regulator operation, lowered output, ETC. The
rubber mounts deteriorated, & if the diode board mechanically disconnected
from the deteriorated mounts, sparks could fly, with resultant burned or
otherwise damaged boards. The answer to ALL the rubber mounts problems
are to replace them with solid metal aftermarket mounts, which have additional
grounding benefits as well as lasting forever & they remove more heat from the
diode board heat sink areas, which might add to life. Metal mounts are available
from Thunderchild, Motorrad Elektrik and Euromotoelectrics.
If you are too cheap to purchase four metal diode board mounts...they are NOT
expensive... from these companies, then you may want to make them on a lathe.
Please do NOT use stacked washers! Make the four mounts from 0.5" round
stock. You can use aluminum or brass. The "middle part" is to be 0.5" diameter,
0.58" (approximately) wide; and, each end MIGHT (read on...) have a threaded
portion sticking out from that 0.5" round piece. You could use a threaded piece
of rod at each end, & thread the inside of the round piece, or, just make the
entire piece on the lathe from one piece brass; thread the ends. If you use
threaded rod, or a cut-off screw thread, use ONE drop of Loctite RED on the
ends going INto the round piece. Another way of making these would be to
use an Allen screw (from the starter compartment side), and not have that
particular end of the mount have a stud...only the other end...or, actually, you
could have no studs (threaded material) at either end....just make the
0.5 x 0.58 piece, drill & tap it through; use Allen screws at both ends. DO
use Loctite RED or BLUE at the rear, the starter cavity screw. The threaded
stud, if you make or buy "all-thread" for its use, is 5 mm. You can certainly
cut the head off an old screw for this. The best thing is to purchase four
metal mounts from the sources I have listed, they are NOT expensive.
The following sources have solid metal mounts for your diode board:
All mounts, rubber or the above all-metal aftermarket ones, are a bit of a
pain to install. Some tricks to it for the difficult starter motor side of the
mounts, but the modification is VERY worthwhile. Once you install metal
mounts, you will NOT have to remove them ever again, never have to
replace rubber ones, have longer diode board life; & you gain electrical
benefits. Installing the mounts is very easy if you are also doing a timing
chest/chain/sprockets/guides type of job. In any event, it IS worthwhile,
& highly recommended!!! There are some photos, etc., showing one way
of accessing the mounts on the above Thunderchild's website:
I highly recommend that the new metal mounts NOT be tightened down
in the front NOR rearward areas until the diode board is pushed over the
forward threaded area of the new mounts. Failure to follow this CAN result
in a broken diode board mount area if you install the mounts first, and force
the board on later.
The two grounding wires assemblies are another story...
hence the rest of this article:
In September of 1988, BMW Service-Information bulletin 12-012-88 (2323)
came out, and covered complaints of:
a) diode board connections having solder melting
b) low battery charging rate
c) GEN lamp would glow around half intensity
That bulletin covered all boxer models FROM 1981, that had the electrostatic dip
black coating on the timing chain case cover where that fits against the engine.
BMW said that increased resistance at the diode board grounding points, due
to corrosion, ground wire connection deterioration; and, the exacerbating
effect of the insulation from the paint at the diode board mounting points on
the timing chain case, ETC. were the causes. Their fix was to remove the diode
board & scrape away the paint from all 4 diode board mounts on the timing cover.
Rubber mounted boards were to have the existing ground wires replaced if they
looked corroded or overheated & those ground wires were to be SOLDERED at
their lugs. They also recommended cleaning the paint from the cover around
bolt heads for the ground terminals, etc. I have seen many models after 1981
without that coating! STILL, the bulletin does apply...as far as it went.
In October 1993, another BMWNA SI came out, this time it was 12-019-93 (2611).
This bulletin supposedly covered all boxer models from 1980, EXCEPT R1100
models (obviously, there were no R1100 models to say no to, for the prior bulletin).
THIS DATE OF MODELS 'FROM 1980' IS WRONG!....BMW made no mention of the
1979 model, which had the same setup, but the BOSCH board was used (the Bosch
boards MOSTLY did NOT have the bent lead problems...see earlier). NOTICE
however, that it makes NO difference about the boards HERE. The bulletin
SHOULD have said 1979 onwards...and, the bulletin also forgot that BMW
changed back the type of mounts in 1992 (I think it was 1992, some may have
Well...that may be confusing, but OK, as another bulletin supersedes that one!
But, you will still need THIS bulletin information... so...here is the information
for this October 1993 bulletin:
COMPLAINTS: diode board grounding wires getting hot, stiff, solder melting,
melted insulation; alternator output falls below 13.2 volts.
CAUSE: increased resistance at the diode ground points due to corrosion of
the ground wires and bolts.
REMEDY: install ADDITIONAL ground "wire" 12-31-1-468-013.
That 'wire' is really a three wire arrangement, one end of each connecting to
a common lug. That means this harness that has 4 lugs total. This entire
assembly was to be installed such as to tie the diode board, timing case cover,
and the engine housing, all together, electrically.
The second page of this bulletin listed the above wire, a hex bolt 07-11-9-913-015
(an error, it really should be 07-11-9-914-148); an allen bolt 07-11-9-901-023
(should be 07-11-9-919-792); two washers 07-11-9-931-029; and finally,
a 'cap' 46-62-1-453-668.
There was a sketch. I will describe where the connections are made, per BMW:
Facing the timing chest, the upper left & upper right diode board mounts were
each a connection point for this 'wire assembly', & these wire connections were
made onto the mounts. NOTE!! This is in ADDITION to the already existing
grounding wires at the top mounts!..in the same position. Since there are now
TWO lugs at each mount....this spaces the upper part of the diode board outward
a tiny amount, the two washers in the kit were to be installed on the lower diode
mounts to help equalize the spacing. SEE later note herein about those washers!!!
The hex bolt in the kit is used at the threaded area located directly below the
large hole behind the diode board. The final connection is directly to the right of
THAT...and the allen bolt in the kit is used there. This point is through the
sort-of large elliptical hole in the timing chest...and this point where this last lug
attaches is actually the inside engine case, the TRUE ground. Thus, everything
is tied together & grounded to everything else. It is important to note that it is
this last connection through that elliptical hole to the threads already available,
that makes the REAL grounding of the diode board to the engine itself.
NOTE: It is a GOOD idea to use some grounding wires even if you have solid
mounts, as the outer timing chest does not, because of painted joint surfaces,
provide a perfect electrical ground to the engine. This is not something a
simple ohmmeter will show up.
In November of 1993, BMW came out with a REVISED S-I bulletin 12-019-93 (2611).
NOTE that it has the same bulletin number(s). BMW carried forward the same
1979/1980 error. NOW, BMW is adding some additional 'causes'. Hot weather
driving, city riding, low battery voltage, full fairings...yeah, riiight!! BMW now
corrected the parts numbering errors...and made some additional errors! and,
no, they never fully corrected the 'corrected revised version'.
The information on the wire harness & connections is correct as I outlined them
above. BUT, the -792 allen bolt now comes with a captive waverly washer, and,
frankly, that M6 'cap' is not really needed...never really was anyway. You will
want to GET a waverly washer for the -148 hex bolt, however.
The MAIN reason I am having all this background discussion here is that if you
see something that looks strange in your timing chest outer area, you will know
why...and if you see no wires and you have rubber mounts, you will know of
the modification. Frankly, I HIGHLY advise folks to install METAL mounts if
you do not have them. I also advise extra grounding of the two top diode mount
screws, to the INNER WALL, that being the ENGINE wall.
As described, in the elliptical hole.
NOTE 1: There MAY be an existing M6 x 15 hex bolt with a waverly washer at
the position of the TRUE ground mentioned previously, so why not use it!
NOTE 2: I have seen some very minor differences between some stock bikes....
maybe an extra washer found at a mount, that type of thing.
NOTE 3: If you have the solid aftermarket mounts, just about all in this mess
of information is moot...except for the grounding wires, which MAY help.
Moot?....well......if the diode board fitting & mounts are CLEAN of paint,
corrosion, etc., ....& the timing chest & engine are making good electrical
contact (can be assured by adding a couple of grounding wires),...then this
automatically ties the diode board grounding top area to the timing chest
& engine case. I like to have ONE wire from the diode board grounding metal
to a lug fastened to the bolt, threaded into the hole in the case, as noted.
SO, you do not have to purchase the BMW special grounding wires, you can
make your own. The BMW ones ARE convenient though.
***It is possible that the grounding wire
12-31-1-468-013 is no longer available.
NOTE 4: See http://bmwmotorcycletech.info/electricalhints.htm; item #1, for
a lot of information on the diode boards.
Problems removing & replacing the diode boards & MOUNTS:
I have previously mentioned this:
You may, or may not, find that useful.
I have my own methods. I have done a bunch of these.
The nuts used on both ends of the rubber mounts, or the forward end of
cast-in-place mounts, or both ends of aftermarket metal mounts, require an
8 mm wrench. In order to do this job properly, you WILL want the following
tools, UNLESS you are totally removing the inner timing chest casting:
A small combination wrench ...open wrench at one end, box wrench,
preferably 12 point, at the other end of 8 mm size. You take an oxy-acetylene
torch (or?) & bend this wrench. SOME folks have used a small 1/4" drive
8 mm socket, & drive handle, etc., some modify those, some use a 1/4" universal
U-joint adapter in using that socket & drive handle. Whatever YOU want.
The bent wrench is to be used on the nuts on the starter motor side.
DO keep a wrench of some sort in your on-bike tool tray, that would enable
you to R/R the diode board, if it ever failed. You don't really need any special
tool to be in that tray for replacing the mounts...after all, you ARE using metal
An idea for a removal-of-the-diode-board tool, besides the 1/4" 8 mm
socket (perhaps ground a bit thinner) is as follows:
Obtain an 8 mm tubular socket wrench....really just a tube incorporating a
very small diameter socket at one end & a wooden or plastic handle, often
generically called a SpinTite wrench. Grind the end so its diameter is fairly
small, yet still strong enough. It is nice to grind the tip end so no INternal
taper is in the working 8 mm area. I also do NOT like like to have this hex
end too deep....shallower makes for easier starting a outer diode board nut
onto the threads when replacing the nut. There are versions of these
commercial wrenches that even have internal magnets, although difficult
to find in mm sizes.
Medium length hemostat: you may well want this for the installation of the
washer & nut for the solid mounts (or, replacing the rubber ones with new
rubber ones... if you are that stupid). Some folks glue the washer, then the
nut, to their fingers (Crazy Glue). I tend to use a steel screwdriver I have
temporarily magnetized, to touch-hold the nut, while I try to get it started
on the threads on the starter motor side of the mount. Removing the starter
motor area cover (gas tank first, please) is obviously needed. Removing the
starter itself is not necessary. NEVER remove the starter cover without
disconnecting the battery first....otherwise it is sometimes possible to
short the starter solenoid terminal to ground, & cause serious sparks.
NEVER remove the front cover of the engine without disconnecting the
battery first...that can also cause sparks...& destroy the diode board. Check
the starter motor and solenoid nuts...they need to be tight & the big
cable wire lug properly positioned so it won't touch the cover when it is
When trying to install the nuts & washers for the solid mounts in the
starter motor cavity area, they tend to fall down into cavities. They do
NOT fall into the real inside of the engine, but may be out of sight in the
cavity you will see. Thus, having a mechanics magnetic wand is a nice
tool to have. This is one of the reasons for folks gluing washer & nuts
to fingertips, tools, using of small magnetized screwdrivers....ETC!
For the METAL mounts, I recommend that you not only use the waverly
washers on the INside starter motor side, but have a wee drop of Loctite
on the threads.
Regarding the 5.5 mm flat washers, number 07-11-9-931-029: I have seen
ONLY black colored washers. These do NOT conduct electricity well.
That was fine, as they were in the KIT for use at the LOWER, electrically
hot from ground two mounts, & were to compensate for the thickness of
the wire lugs of the grounding wires at the top mounts. The diode board
fit OVER these lugs/washers/mounts. Under no circumstances use these
black washers at the top studs, on the underside of the board, nor by
mistake as a washer at the nuts. You MAY not even want these washers
on the lower studs as specified in the kit instructions. The reason is a
bit subtle & not easy to notice. The diode boards are NOT made the same,
although they look similar, & I mean Wehrle versus Bosch. LOOK at your
diode board UNDERside, & if the distance from the aluminum heat sink to
the rivet end is the same for the two heat sinks, then you need the washers
at the lower studs, they would go on first, before the board. If the lower,
insulated rivets measure slightly more height overall than the grounded
rivets, then just don't use washers. You will need at least a vernier or
something to measure the distance, as we are talking only a wee bit of
difference. If using washers, get shiny metal ones. The standard BMW
waverly washers are like that.
Waverly locking washers ARE used at the nuts...with Loctite Blue used
on ONLY the threads on the starter motor side of the mounts.
02/03/2003: review, upload final version
02/04/2003: add note and hyperlink regarding HINTS article re: diode boards.
04/04/2003: considerably edited for clarity
09/16/2003: additional clarity, many places
01/30/2009: more clarity...esp. item 2.
07/06/2012: Edit article to remove some messiness, and add clarity.
Add information on Diode Test Function of multimeters, etc.
09/25/2012: Add QR code, language button, update Google code
04/09/2013: Slight updating for clarity.
09/14/2014: Review, clean up, no tech changes.
12/28/2015: Meta codes, fonts, cleanup, justify leftwards, etc.
©copyright, 2014, R. Fleischer
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