Testing Diode Boards

[Tony Smith]

My experience over the weekend leads me to suggest that diode boards should be tested under load and not with a multimeter.
 
I have a diode board that tests fine with a meter, but when subjected to load (using a car headlight with high and low beams in parallel) has a diode that goes open circuit.


Of course if you have a specialized diode tester, take this with a grain of salt. But courtesy of an odd failure mode I have 12 hours I will never get back systematically chasing a fault that was eventually found by the Sherlock Holmes principle "that if you eliminate all the probable causes, whatever is left, no matter how improbable, is the cause".

Had I load-tested the diode board at the get-go I could have done a whole mess of more interesting things this weekend past.

[nhmaf]

Well, heat and vibration do alot to stress marginal solder joints and stress fractures - that is why many engineering quality tests uses those factors as accelerants to failure.      Generally, once a diode fails it can be detected with even a fairly budget multimeter, either open or short, using the diode test function can be even better.  But, if there is a cracked solder join in the diode or between the diode and the board, it might not show up sitting quietly on a bench.

But more oten than not, I'd say this isn't how many of them fail - you just got (un)lucky!    There is a range of diode board sets that BMW had initiated a program on for recalling/replacing due to poor solder joints - IIRC, they were painted solid gray (conformal coating color) on the circuit board.   The "good" ones were either all black conformal coat, or gray with a black line across.

[Tony Smith]

There is a range of diode board sets that BMW had initiated a program on for recalling/replacing due to poor solder joints - IIRC, they were painted solid gray (conformal coating color) on the circuit board.   The "good" ones were either all black conformal coat, or gray with a black line across.

I have three diode boards immediately available to me. The one that is now in the R65 - came form the R100, been there since 1978, has a Bosch sticker on it and a grey conformal coat. The one that failed is the one from a 1979 R65 - Bosch sticker and grey conformal. The last is the "freebie" that was thrown in when I bought a spare stator/rotor combination - it is a werle manufactured item and APPEARS to have been re-worked in the past in that the conformal coating (grey) as been removed around the soldered joints and been retouched with something "green". I suspect that there is nothing wrong with this one, it certainly tested Ok on the multimeter, but it was not carefully packed when sent to me and the circuit board has one "corner" broken with the small diode now connected solely by the copper track that detached from the circuit board material.
 
When I get over my crankiness at the lost time on the weekend I will put a bit of "bondo" or something similar on it to stabilize the circuit track and diode and put in a bike to text, it is much younger than any of the other diode boards.

And I do agree, I was/am incredibly unlucky to have a semi-conductor that tests OK on a meter but dies under load, still by the time I solved it I was doubting my knowledge and my sanity.