Starter control circuit

[Dizerens5]

I've been quite intrigued about how the starter control circuit works on my 79 and have just taken some time over the wiring diagram. It's pretty interesting, but difficult to work out. The starter control relay under the tank has its solenoid energised when you push the starter button, but it obviously will not work until it is earthed (grounded), which happens in either of two ways: when the neutral switch operates or when the clutch is pulled in. Either switch gives a direct path to ground. So far so good, but it got more interesting when I realised that the green neutral light works in exactly the same way: it's energised from the kill switch (terminal 4 at the back of the tacho) but only lights up when grounded -- by the clutch switch or the neutral switch. Clever stuff. I'm left with one mystery: what's the purpose of the diode in the circuit just after the control relay? It's the one numbered 33 on the diagram in the handbook. I know what a diode does, it allows electron flow in one direction only -- but how does that affect the circuit? Or let's say, what would happen if I bypassed it? Anybody know?

[Bob_Roller]

With multiple ways to activate the starter circuit, the diode is put in there to prevent the neutral indication light to come on .

If you ever have the situation where the neutral light comes on every time you pull the clutch lever in, the diode is faulty, it's allowing electrons to flow 'backwards' through the light circuit to illuminate the neutral light .

This was the first real electrical problem I had with my '81 R65 .

If you bypass the diode, the neutral light will come on every time you pull the clutch lever in .

[Dizerens5]

Well, butter my bum and call me a biscuit! All this time and I never realised I had a duff diode. Thanks Bob. When I pulled the clutch and the little green light came on, I just saw it as a convenient reminder that it was ok to use the starter. Except of course that the good R65 never, ever stalls its engine!

[Bob_Roller]

All you need to do to replace the diode, is firest find it, if you have a '78-'80 model year bike, it'd located somewhere under the fuel tank .

It's inside of a round  black plastic holder about 1 cm in diameter and about 3 cm long .

It's a common  diode, I'm doing this from memory, so I might not be correct, but I think the number is 1N4001 .

The '81 and later year bikes, have the diode in the starter relay .

[Dizerens5]

Bob will the later relay including diode retrofit to the earlier bikes? It would be a lot easier than locating the diode, hacking open the wiring harness and soldering in a new one without frying the wiring. Just would mean a bypass wire.

[Bob_Roller]

From the looks of the wiring diagram, the '78-'80 model bikes have a starter relay with four pins or contacts on the bottom, the '81 and later bikes have a five pins, so I don't think it will work .

[Dizerens5]

As usual this site has come up with the information! This little diode does not seem to appear on any parts lists and isn't mentioned in the handbook or the Haynes manual. The function does appear in the Haynes Motorcycle Electrical Manual but you have to search, it's not in the index. And even there it does not say which type of diode is used. Evidently it's a part which can fail so it's strange that it's not a listed spare and is so well hidden away that it's difficult to replace. Anyway, thanks Bob for all your help!

[Bob_Roller]

The diode is listed, if you know where to look .

This is the 4 way hazard light system, the diode used in this application, is the same as the diode in the neutral light circuit .

http://www.realoem.com/bmw/showparts.do?model=0364&mospid=51897&btnr=61_0205&hg=61&fg=11

Item # 3 .

[Dizerens5]

Found it thx. But US price for the BMW diode is over $14 while a single 1N4001 diode should cost well under $1 or equivalent from an electronic parts place! However internet seems to show they are the kind of part easy to get if you want 500, not so easy for just one.

[Barry]

And even there it does not say which type of diode is used.

If you can get access to solder in a new diode the very common 1N4001 diode that Bob mentioned will work fine. They cost pence from any electronics shop.

[Bob_Roller]

If you do decide to replace/install a new diode, the are polarity specific .


The 1N4001, looks like a small black cylinder a few mm in diameter and about 5 mm long with a wire lead out of each end .

It should have a thin silver band at one end, if you are replacing the one in the plastic holder, be mindful of which way it was installed .

If you don't, you will have the same problem with the light that you have now .

If you splice it in somewhere in the wire harness, don't solder it in permanently, until you test it, to be sure it's correct .

[Dizerens5]

To wrap this one up, at least for those in UK, Motorworks can get the BMW diode, cost about £13. You need to quote the BMW part number, 6131 1358 268. Now I have to locate the thing under the tank! Thanks everyone.