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Diode Boards & Grounding Wires
On BMW Airhead Motorcycles
((including information on diodes testing))

copyright, 2014, R. Fleischer

Basic and advance diode board testing:

Diode board testing and information about repairing the faulty Wehrle-manufactured ones (I've seen a few Bosch boards with the same problems) (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.  Airhead owners should read the 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 and 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 and is a huge PIA!). Yes, it can be done with long tweezers or forceps, but is tricky,...again... a real PIA!  I can NOT recommend that method, and I won't even do it myself.   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 "actual" because many meters have - voltage on the + red lead on the ohms functions).  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). A few meters have both very low applied ohms functions voltage AND a higher voltage application function.

Some meters have a DIODE TEST function, & you do not then have to use the OHMS function, unless you want to. I do BOTH tests routinely. The meter in diode test mode will indicate forward voltage drop of the diode. 

One has to know one's meter, & testing on a known good diode is a good way to know.   The most important readings are that the forward connection be a LOW ohms 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, and if outside this range, the diode is probably bad.

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 the 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, if the leads connection is in the proper direction.  Some, usually more expensive ohmmeters, do not turn on diodes on an especially low voltage ohms test mode.  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.  If something else (besides the meter) is connected to the diode, testing can provide usable information, or not at all usable readings.  Ohmmeters can be RUINED if the vehicle power is on.    To use an ohmmeter on a vehicle, it is mandatory to disconnect the battery.  This is usually done by removing ALL the wires at the Negative battery post!...or, if only the one large wire, disconnect it at either end.

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.   Disconnect the battery before doing testing!!

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 better test:
This is a dynamic test.  It tests the diode board with some substantial, but safe, current flowing through the diode.    I do the ohmmeter test for front to back ratio first.  This test is for all diodes on the diode board...but pay attention to the details, below!
ESPECIALLY GOOD (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 and indicating type of resistance (a lamp!).   If you stay under one ampere, this test is safe for even the small diodes of the diode board.  Thus, I recommend TWO types of lamps.

Obtain TWO 12 or 14 volt lamps...a taillight run lamp (NOT brake lamp or turn signal lamp!) is fine for the small diodes, but you will want a more powerful lamp for testing the 6 large diodes...I suggest an old part-burned-out headlight lamp, either an old 12 volt sealed beam type or any 55/60 watt (even to 100 watts) headlight lamp, and a 6 to 12 volt transformer.  DO NOT test the small diodes in the diode board with large lamps! A suitable transformer is Radio Shack #273-1352 (12.6 volts, 1.2 amperes) or, better, the # 273-1511 (12.6 volts, 3 amperes); or, appropriate other transformers. An old bell ringing transformer might be just fine. So also might be modifying leads for/on an old large bulb type desktop high intensity lamp unit...maybe with its own lamp!   Use of large lamps will draw the voltage down from the small transformers, and burn the transformer out if left lighted for any longish period of time, but you will be using the setup only for moments at a time. The 3 ampere transformer is safer in this respect.

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 neat as modifying an old desktop high intensity lamp unit.   You can 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....just do not pass over 1 ampere through the SMALL diode board diodes.

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.  A few amperes or even more, is OK for testing JUST the 6 large diodes.

I test the six LARGE diodes using an old sealed-beam headlamp bulb (one side of which is burned out and I kept the lamp for this purpose).   You can get one from a wrecking/salvage yard from very old cars.  I use a transformer of several amperes, it being ideal for the 6 large diodes and for the small diodes too. 

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 too much current. 

You can also make a two-in-one tester.  Get a new run and brake lamp, such as the type 2257 or 2157, or 7528.  These lamps have a TWO filaments, one is low powered (generally ~8 watts, maybe 3/4 of an ampere at most) for the RUN section; and 21 or 27 watts (roughly 2 amperes) for the high power BRAKE section.  The pins are OFFSET, that is, staggered, so the lamp will plug into the socket (autoparts stores have sockets for them WITH WIRES) in only one direction.  The high power section is OK to test the diode board big diodes, and the low power section is OK for testing the low power diodes in the diode board. YOU must identify the leads so you do not mix up the sections.  You will have two wires from the lamp, plus the lamp shell wire.  You use the lamp shell wire (make up a wire if not so equipped) and one or the other of the two lamp wires. 

Headlight lamps will allow about 4 or 5 amperes using a 12 volt output transformer, for the common 55/60 watt lamp. 

With whatever lamp you are using, connect the single lamp filament you wish to use in SERIES with the transformer low voltage secondary winding, that leaves two wires for testing the diodes, one diode at a time.

1.  When the wires from your transformer & lamp are touched together, the lamp will light up brightly.
2.   It will be dimmed to roughly half brilliance when connected to a good diode.
3.  The lamp will NOT BE LIT if the diode is faultily open. 
4.  The lamp will be brightly lit with a shorted bad 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.  

If you also do an ohmmeter front to back ratio test (disconnect battery in the bike!) 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.

Diode board mounts:

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 from the factory 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 comes from from their extra heat.  Additional factors were the lower flow of air through the early front alloy covers and the non-louvered early fairing center-lowers.

Using rubber mounts meant that BMW needed extra grounding wires from the top (grounded side) row of diodes.   These rubber mounts were poor at transmitting diode board heat to the timing chest casting (yes, I know about the minimal heat transfer at that point even by solid metal mounts).  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 diode board life. 

If you are too cheap to purchase four metal diode board mounts...they are NOT expensive ($15 or so per set of four)... from these companies... then you may want to make your own on a lathe.  Please do NOT use stacked washers!   Make the four mounts from 0.5" round stock.  You can use aluminum or brass. DO NOT USE STAINLESS STEEL.   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 (I suggest you do NOT use SS) 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 below, they are NOT expensive....under $20 delivered to you.

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.  Tighten the rear screws or nuts, as case may be, tightly.


The grounding wires assemblies are a story in themselves:


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. BMW's 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 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-labeled boards MOSTLY did NOT have the bent lead problems...but, 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 been later).

Well...that may be confusing, but OK, as another bulletin supersedes that one!   But, you will still need this second bulletin information, ... 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 'spider' 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 (see next paragraph); and finally, a 'cap'  46-62-1-453-668.

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 to use 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.

There was a sketch. I will describe where the connections are made, per BMW:
Facing the timing chest from the front of the motorcycle, 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; and this is an important connection.

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 have 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, 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 such wires and you have rubber mounts, you will know of the modification.  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 described, in a bolt in the threads located 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.

NOTE 4:  It is LIKELY that the grounding wire 12-31-1-468-013 is no longer available.

NOTE 5:  See; item #1, for a lot of information on the diode boards.

Problems removing & replacing the diode boards & MOUNTS, tools, etc:

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 the cover is replaced.

I have previously mentioned may, or may not, find that useful. 

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 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 now using metal ones! 

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 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.  Harbor Freight has these in sets, cheaply, and calls them Nut Drivers.  Be sure to get the Metric set.  Grind the end of the 8 mm tubular wrench so its diameter is smaller, yet still strong enough.  It is also nice to grind the tip end a small amount so no INternal taper is in the working 8 mm area.    

Medium length hemostat:  you may well want one, I like the curved type, for the installation of the washer & nut for the solid mounts (on the starter motor side of the inner timing chest) (or, replacing the rubber ones with new rubber ones... if you are that stupid).  Harbor freight has 12" long hemostats ("locking clamp pliers) for $7 or so, but they are not curved....but they do have a 10-1/2" curved type too....and it will work fine.  I think you will prefer one of these, rather than offset long nosed pliers.  This type of tool will enable you to start a screw, or nut, or washer, on the starter motor side of the timing chest. You may be using a screwdriver to hold the nut against the threaded post, with help of the locking clamp pliers...or, whatever wrench/hardware you will have.  I usually use such a tool to install the washer too, but some folks glue the washer, then the nut, to their fingers (Crazy Glue).  I sometimes have used 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. 

When trying to install the nuts & washers for the solid mounts, they tend to fall down into engine cavities.  They do NOT fall into the real insides 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.  You can get one at Harbor Freight.  This is one of the reasons for folks gluing washer & nuts to fingertips, tools, using of small magnetized screwdrivers....ETC!  BUT, there is always the chance of some hardware falling....and the magnetic wand is VERY, get one!

For the METAL mounts, I recommend that you NOT ONLY use shiny metal waverly washers on the INside starter motor side, but ALSO have a wee drop of Loctite blue on the threads.


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.
05/28/2016:  Update metacodes, scripts, H.L., layout, lamp schematic, expand information.

copyright, 2014, R. Fleischer

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Last check/ edit: Saturday, May 28, 2016