help me find my short: convert

[DavidF]

My 78 convert is coming along nicely. Every time I fix a problem I find a new problem. It's kinda fun until the snow melts here in Vermont.
My current (pun!) problem is a short. The fuse controlling the brake light blows when I apply the brakes, front or back. I replaced and rewired the headlight/directionals, replaced starter switch with Yamaha, rebuilt all the brake cylinders and pistons, and the weird thing, I found a lot of wires that go no where. All other electrics work fine, including the horn, (which is also new), which is on the same fuse.
I have hunted this  and I am running out of fuses to test my hypothesis. Any ideas?

[fotoguzzi]

unplug either the front or rear brake light switch to isolate if it's in the F or R ?

[rodekyll]

Did I see that the rear lights wiring was fixed?

If something got miswired in the rear you could be sending juice from the switch to the ground, bypassing the brake light bulb.  Pull the plugs on the rear wiring and see if it still happens.

[John A]

Sometimes the plastic sockets for the bulb melt. If you are clever you can replace the socket with a metal one from the auto parts store. I like to add a relay to the brake light just for the hell of it.

[Chuck in Indiana]

If you solder a couple of leads on a tail light bulb and install that in place of the fuse, it will light up instead of blowing the fuse.  Saves on fuse expense. (Guzzi content)  As an added benefit, you can troubleshoot to your hearts content..

[Groover]

Great tips here, I like the bulb as fuse one! If I remember right, the rear brake circuit actually goes through the headlight interconnect, thus the left side switch. Sounds like there were some changes made there? I'd check that direction as well. Any difference if you have the high beams on? I know, doesn't make sense, but neither does that headlight interconnect. A lot is happening in there...

Greg Bender's new harness to the rescue?

Here is a wiring diagram.. have fun! (it's for a G5, should be close if not the same as the Convert)

[Huzo]

Short what..?

[DavidF]

Thanks for the tips but after a few days of troubleshooting, I am still confounded. I rigged up a light bulb to the fuse. I have disconnected wires methodically. Both left and right brakes cause the light to glow. When I remove the black wire going to the tail light assembly the fuse light does not glow when the brakes are applied. When I splice in a new wire using aligator clips, the bulb glows. When I removed the tail lights, the fuse bulb still glows.
Any thoughts?

[Chuck in Indiana]

Maybe your light socket is grounded?

[BikeMike]

David,

If you have an Ohmeter you could  remove the fuse and the rear brake light bulbs and check the continuity to ground on the load side of the fuse.   There should be no pathway to ground with the brake light bulbs removed and the Ohmeter should show an open circuit.  If the Ohmeter shows a resistance to ground or dead short with the light bulb removed then you have a short.   In this case you would then trace back each wire in this circuit to isolate / disconnect each to find the source of the problem.

Hope this helps..

Mike

[Idontwantapickle]

It sounds like you have isolated the problem to the tail light or the wires to it. As BikeMike says you need to check each wire. The tail light wires go through the fender twice and can wear through the insulation at those points. Since it blows the fuse with no bulbs in place then it is before that point but after the switches. Do the parking lights operate correctly? That would indicate that the tail light is probably ok and that the wires for that are intact. It is possible to connect the tail light incorrrectly on some types and cause a short. Disconnect it entirely to eliminate or confirm the tail light as the source.
It seems to me you've narrowed it down to the wire for the brake light beyond the rear brake switch or the tail light assy.

Hunter

[DavidF]

I think you have the right idea and I need to accept it. The wrinkle is that I have found maybe 10 unconnected wires around the bike. A PO must have added extra lights, stereo, trailer. There are a lot of dead ends.

[Kiwi_Roy]

I use the bulb in place of fuse method as well, its an old Sparkies trick.
The fault is on the load side of the brake switches (either one) to the stop light so you only have about 3 connections to check.
The diagram someone posted shows a connector near the tail light, pull that apart to eliminate the light itself.
Take the bulb out to make sure it's not that.
Inspect where the wires pass through the fender.

Try jamming the brake on then wiggle every connection to see if it effects the light.