Topic: 1978F 750 - Neutral light comes on with clutch

[Patrick]

As the subject line states, every time I pull in the clutch lever on my 1978 750F the neutral light comes on, creating a whole new world of false neutrals. The light works correctly when I change gears without the clutch.

There is a little plunger behind the lever that extends when I pull the clutch level in. It moves freely. I cannot see a way to take the clutch perch apart to service this little plunger or the wiring from the clutch. Somehow the neutral light circuit grounds out when I pull the handle to disengage the clutch and this little rod pops out.

Can someone tell me how the clutch electrics are supposed to work? I know the clutch ties in with the neutral light so you can't start the bike with the starter unless it is either in neutral or the clutch is disengaged. But I don't think the neutral light is supposed to come on with the clutch. That would make the light a "starter" light, not a neutral light.

If that little plunger is not supposed to light the light when it extends then what is supposed to happen when it extends? How is the clutch wiring serviced? The clutch mount looks like it is one piece to me.

I've never had to fix wiring in a clutch mount before. Any help would be appreciated.

Patrick

[mystic_1]

So, later CB750's had a starter interlock which ties into the neutral switch.  It contains a diode which, as I understand it, when it goes bad causes the symptoms you describe.  Now, I've never worked on a system like this but that's how I understand things.

Look for and test this diode.

mystic_1

[Shenanigans]

If I remember correctly in the wiring harness there is a (one way?) diode with the sole purpose of keeping the neutral light from coming on when you pull the clutch in. I dont know how the electrics on your particular bike are set up but I would guess to say there might be something similar. Maybe its something to check?




Edit: You beat me by seconds Mystic_1 

[Patrick]

Does anyone know where this diode is located? I don't remember seeing it when I was going over the wiring. That was months ago, though, I might be forgetting I saw it. It would help to know where to look.

[mystic_1]

Here's a wiring diagram of the 78F.  Diode is shown near the tail light, but I don't know if that's where it's actually physically located.  Maybe near under the seat near or under the cross member in front of the shocks?  That would put it near the turn signal relay I think.


mystic_1

[Patrick]

OK, I found the diode. How do you check a diode? Alternately, will any diode do as a replacement? I look at the parts fiche and I am not clear which part this is nor whether any are available. Is there are aftermarket diode I can get at my local electronics store?

Thanks,
Patrick

[hapakev]

Diodes are one way circuit devices, allowing voltage to pass only in one direction.  If its bad, you will have voltage on both sides of the diode even if you switch the leads.  The best way I think is to test resistance with an Ohm meter.  You need to isolate the diode or the wiring on either side of the diode and test the resistance through the diode, then switch the leads from the meter  and test again.  One way should read OL or open.  If you get a 0 or some resistance both directions, you have a bad diode.

By the way, diodes are specific to the application. you need to find out the rating on the diode.  It is coded with colored bands on the diode body itself.  And it needs to be hooked up the same way it was origionally.  You should be able to find these at an electronics parts store pretty easily.

[Bodi]

You need a diode capable of handling the lamp current - not much but a "signal diode" would be insufficient. Usually the diode is destroyed by a short circuit in the lamp circuit, even a blowing lamp can cause this (as the filament breaks it can fall across the support wires).
A 1N4000 series diode will be fine, rated at 1 amp I think, the voltage is unimportant since ratings below 100V are quite unusual. This sort of diode or its equivalent is very common. You just have to connect it the right way, one way will work right and the other way will disable the neutral switch and probably light the light via the clutch.

[Patrick]

Thanks, Bodi, that's what I'll look for. Thanks to everyone else for your input.

I suppose it shouldn't surprise me that the diode is bad. Just about all the electrics on this bike were bad when I got it. Now it will be all new.

Patrick