Electrical question - whereabouts of diode.

[Tony Smith]

A few weeks ago the wife's R65/80 got hard to start.

I wasn't surprised as she didn't ride it much for a long time and (as I may have mentioned previously) the poor thing tends to suck hind tit when it comes to maintenance hours.

it occurred to me that seeing as I didn't bother checking the Carbs I bought when the R80 engine went in (and I never have since), that some time between approximately 1987 and now they might have injected a bit of crud. Cue guilt music.
 
As I had carb overhaul kits in stock I pulled the carbs off, noting in passing that whilst they are R80 carbs, the left and right come from different year models (Phillips headed screws on one and blade on the other). The second thing I noticed was that there wasn't too much crud in them at all. one float bowl jet was a little iffy, both atomizer chambers had a fair bit of crud, but that was it, and what was there I didn't think would have affected matter too much.

Anyway, since I was there, the carbs were there, an di had an overhaul kit in my hand,  I changed all "o" rings, checked all jets and airways and then put them back on the bike.

It was still hard to start. Bugger!
 
Then a penny dropped. This "hard to start" malarky coincided with "something" going funny with the the starter motor lockout a few weeks back.
 
The symptom of this is simply that in neutral you get a green neutral light, but the starter will not engage until the clutch is pulled in.
 
The penny didn't fully drop until I read Snowbum's electrical FAQ and then looked at the wiring diagram.
 
Somewhere on 1979 R65 wiring harness is a diode that bridges the starter relay power circuit and the neutral light circuit.

According to Snowbum, if the diode fails shot-circuit then the neutral light comes on when you pull the clutch in. If the diode fails open circuit then you have to pull the clutch in to get a start.
 
Add to the mix that this bike was a "pojnts in a bean can model to which I fitted a Boyer Bransden Digital Ignition system and I seem to remember pinching power for it from a spare terminal on a relay and suddenly I know why the old girl is hard to start, either the Boyer-Bransden0 or the coils are being voltage robbed due to the failed diode.
 
 
OK, so where is the blasted thing?
 
To answer this question you have to be familiar with or own a 1979 - 1981 R65 as the later models do this differently.
 
I picked up a few 5 amp, 80 volt power diodes on my way home (all of 25c each) so if I knew where the failed one is I could fix it in a few minutes.
 
If you do know where it lurks, does it look like moto bins Part No.73220?  and please, please, where is the bloody thing hiding?

[AlfromNH]

That's a good question, Tony. I had my entire wire harness out and I spent some time cleaning it up, along with every other electrical component, and I don't remember anything that looked like a single diode.

But according to the wiring diagram, it's fed by two brown/green wires, one from the starter relay, and one from the clutch switch.  

The other side of the diode is a brown/black wire which goes to one of the main connector housings that feeds the tach. It also splits off the connector with a brown/black wire to the neutral switch.

Hopefully that will help you track it down, I'll be interested to learn where it is.

[Barry]

It's buried in the harness but exactly where I don't know. The harness sheath would have to be stripped back to find it and it should show as a bump in the harness as the diode is enclosed in a plastic tube. You would expect it to be close to the starter relay as there is no harness plug and socket connector shown between the diode and the relay.


Here you go I remembered someone had found it in a previous thread.

http://www.bmwr65.org/cgi-bin/yabb2/YaBB.pl?num=1323697033/0#7

[Tony Smith]

Here you go I remembered someone had found it in a previous thread.

http://www.bmwr65.org/cgi-bin/yabb2/YaBB.pl?num=1323697033/0#7

Thank you , thank you , thank you.
Like the poster in the thread you kindly provided the link to I was think I was going mad. I could see on the wiring diagram that there were no harness plugs/sockets nearby.
 
I just could not bring myself to believe that BMW would bury a component that might conceivably fail and require replacement under the harness wrapping, and I was not about to start cutting wrapping just to find it, my 'plan B' was to hard wire a new one into the harness as at least i was lucky to have this one fail open circuit.
 
Now that I know where it is I will fix it properly. I will also take photos so that the next poor bugger who has this happen gets a running start.

[Tony Smith]

Ok, so finding the diode wasn't a problem once I knew what i was looking for. Not even wrapped into the harness.
 
I broke open the plastic case hoping to just put my new diode inside, but I seriously overestimated the power requirement and the ones I bought were way too big.
 
I did intend to cover the replacement I made with shrink tube, but I forgot to put the shrink tube on before soldering the second terminal. Too bad, I'll wrap with canary yellow tape to ease finding it again.
 
Put everything back in place and pressed the start button - starter motor now works as advertised and the old girl is much easier to start

Photo story here:-
https://plus.google.com/photos/104425121947497605748/albums/6032488857364431745