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Diode Boards & Grounding Wires
On BMW Airhead Motorcycles
((With some information on testing, pertinent to power diodes in general))

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 early Wehrle-manufactured boards have a problem with solder joints and non-folded-over wires on the 6 power diodes, and the Oak-recommended, and frankly difficult to do board modification, may be needed if the solder joints are failing, especially if you have a R100 engine, which develops more heat. This is particularly so if you have a RS or RT.   I've had good luck with just using a higher temperature solder (50-50 plumber's solder), and enlarging the solder pad area, by CAREFULL scraping.  This is vastly easier to do.   Drilling and adding wires and soldering, per Oak's method is quite difficult, and is best done by total disassembly of the printed circuit board from the rest of the diode board, and I can NOT recommend that method.

Over decades of working with these diode boards, I have seen BOSCH-labeled boards with the SAME non-folded-over diode leads.   Thus, no matter the board maker, inspect 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 - lead, mentioning this because some meters have - voltage on the + red 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.  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).
Some meters have an actual DIODE TEST function, and you do not then have to use the OHMS function, unless you want to.   The meter, depending on its function being used, will indicate forward voltage drop (diode test function) or a resistance reading (ohms functions).  In the 'passing current' leads orientation dorectopm on the ohms functions, the actual reading various between meter models and brands.  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 diode, ~0.6 volt.

Re-stating, differently, and 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).  An ohmmeter has one or two small internal batteries that enable "turning on" the diode under test.
Some, usually expensive ohmmeters, do not turn on diodes on their 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 and raising the full scale range setting on the ohmmeter (if it has manually operated ranges), and 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. 

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.  

Many multimeters have DIODE TEST functions.  In nearly all of those multimeters, the indication is, more or less, the forward voltage drop of the diode under a LOW current supplied by the meter's internal battery through internal resistances.  If you reverse the leads, you should not get a reading.  The forward voltage drop of a silicon diode is about half a volt or 0.6 volt perhaps, all on large power diodes.  The test will show a slightly higher voltage, around 3/4 of a volt, on very small diodes.   Your Airhead diode board has 6 large power diodes, and several smallish diodes.  Thus, measuring any of the diodes should indicate somewhat in-between 1/2 and 3/4 volt, if the leads are in the correct direction.

Very special diodes such as "zener diodes"; Schottky diodes; ETC., will indicate differently, and for our motorcycle purposes, are unimportant except as a mention here.

I like either ohmmeter or diode test function AND the following test to be done; 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 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!  

You can purchase a socket with wires attached for tail lamps if you want to.  Makes a neater setup.  Or, just solder wires to the low-power lamp section if the lamp is the two-filament type.  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, 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 either!  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 at a time.

When the wires from your transformer and 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, 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.  It is the 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.

2. 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 and 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 was improperly formed and soldered leads on the Wehrle manufactured boards, with the solder joints failing especially on fairing equipped models, 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, and 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 and 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 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, and thread the inside of the round piece, or, just make the entire piece on the lathe from one piece brass, and 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 and tap it through, and 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 PIA 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, and 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, and 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 rearward area 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 board 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 by 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 and scrape away the paint from all 4 mounts on the timing cover. Rubber mounted boards were to have the existing ground wires replaced if they looked corroded or overheated, and 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 boards 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 been later).

Well...that may be confusing, but OK, as another bulletin supersedes that one! But, you will still need THIS bulletin information... is the information for this October 1993 bulletin:

COMPLAINTS: diode board grounding wires getting hot, stiff, solder melting, melted insulation, and 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 and upper right diode board mounts were each a connection point for this 'wire assembly', and these wire connections were made onto the mounts. NOTE!! This is in ADDITION to the already existing grounding wires at the top mounts! 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.  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 engine case, the TRUE ground. Thus, everything is tied together and 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, however.  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 and 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 and mounts are CLEAN of paint, corrosion, etc., and the timing chest and 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 and 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.

NOTE 4:  See; item #1, for a lot of information on the diode boards.

Problems removing and replacing the diode boards:

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 a 8 mm wrench.  In order to do this job properly, you WILL want the following tools, UNLESS you are removing the inner timing chest casting:

A small combination wrench (that means open wrench at one end, box wrench, preferably 12 point, at the other end) of 8 mm size.  You will want to take an acetylene torch and bend this wrench.  SOME folks have used a small 1/4" drive 8 mm socket, and drive handle, etc., some modify those, some use a 1/4" universal U-joint adapter in using that socket and drive handle.  Whatever YOU want.    The bent wrench is to be used in tightening 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 ones!  An idea for a removal-of-the-diode-board tool, besides the 1/4" 8 mm socket (perhaps ground thinner) is as follows:

Obtain an 8 mm tubular socket wrench....really just a tube incorporating a very small diameter socket at one end and 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 and nut for the solid mounts (or, replacing the rubber ones if you are that stupid).  Some folks glue the washer, then the nut, to their fingers.  I tend to use a steel screwdriver I have temporarily magnetized, to 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, but cuss words help.  NEVER remove the starter cover without disconnecting the battery first....otherwise it is sometimes possible to short the hot starter solenoid terminal to ground, and cause sparks; or, destroy the diode board if removing the front cover with battery still connected.  Check the starter motor and solenoid nuts...they need to be tight and properly positioned.

When you are trying to install the solid mounts nuts in the starter motor cavity area, they, and the waverly washers you should be using, tend to fall down into cavities.  They do NOT fall into the engine interior, but may be out of sight.  Thus, having a magnetic wand is a nice tool to have.

For the METAL mounts, I recommend that you use the waverly washer on the INside area (starter motor side), AND have a wee drop of Loctite on the threads.

Regarding those 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, and 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 behind the nut on the outer side of the board.   You MAY not even want these washers on the lower studs as specified in the kit instructions.  The reason is a bit subtle and not easy to notice.   The diode boards are NOT made the same, although they look similar, and I mean Wehrle versus Bosch.    LOOK at your diode board UNDERside, and 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.    NOTE that none of this discussion has anything to do with the various locking washers used on the starter motor side nor outer, forward, nut side.


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.

copyright, 2014, R. Fleischer

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