cb500 dead short

[RGP750]

Hi All
I have a problem with my 72 500/4.Went out on it last year and everything perfect.
This year turn on ignition and dead short somewhere, frustrating nightmare.
Gradually disconnected areas of wiring, handlebar switches, rear light, horn regulator, etc and finally every connector in the headlight.
Thought it could be solenoid so replaced that, bypassed ignition switch , still doing it . any thoughts pleeeeeeeese!
Rich

[McCabe-Thiele (Ted)]

Difficult one to diagnose online.
So does it blow the fuse as soon as you switch the ignition on?
Have you done any work on the bike since you lay her up for the winter?
Is the regulator/ rectifier standard?

[paul G]

I would start by disconnecting the red and white wires to the fuse box and rectifier and all the live feeds (anything with a black wire).
Reconnect the red and white wires one at a time and then the black wires one at a time until you find the problematic item section.
This will help to narrow it down.
You can also check the other side of the black wire connections for continuity to earth.

[RGP750]

Not done any work since last ride.
I have disconnected every wire I can think of and it still blows as soon as I turn the key, or cut out the switch and " hot wire-"
If I got to the situation when the fuse didn't blow I could start adding wires until it went again but not there yet.
What else can I disconnect?

[Nurse Julie]

I had the same problem last year with my 500K1 Rich. Graham spent hours and hours trying to sort the problem, he checked and changed connectors and wires, he fitted new parts which may have been causing the issue, but still didn't find the problem. In the end I bought a new loom from Electrix World, he swapped the loom over and the problem was cured instantly and she's been tickity boo ever since. .

[Toko_Jo]

Are you using the correct size fuse and not a car type 25 or 30A? If a  large fuse has been fitted at any time, a wire may have fried internally in the loom itself and melted the insulation of one of the black wires. Only a single fuse on those bikes which makes diagnosis hard. If you have isolated all of the black wires (and also the black/white one if the kill switch is in the 'run' position), I can only think it's a loom problem.

[RGP750]

Brand new loom fitted last year when I gave it a tidy up .
Yes correct fuse fitted .
As I have said it ran perfectly last year and something has gone ary while standing .
Just don't know where to look now.
Nbefore getting silly and replacing everything again

[Oddjob]

New loom? well I think we can discount that then, for the time being. It blows as soon as you turn the ignition switch? all black wires disconnected? You've hot wired it to isolate the ignition switch and it still blows? so not the ignition switch. I'd check the sub loom first Rich. It's one of the 500s major problems in that it only has one fuse, anything wrong and the whole bike dies, not a good idea. Check the wiring on the sub loom to see if any have shorted out. Disconnect every wire in the headlight first off. The reconnect systematically, do the indicators first, most of the problems seem to be caused by them in my experience.

[Bryanj]

Have a good look at the loom under the tank by the coils, i have seen the loom melt there on the green wire if the main ground contact is not 100%
It melts inside the loom all the way along destroying other wires.
If thats ok first things i would disconnect are rectifier and regulator

[Oddjob]

Good shout with the reg and rec Bryan, especially if Rich has had the side cover off, a lot trap wires putting it back on.

[Toko_Jo]

Sounds like you have nearly disconnected everything on the main loom. If so do a continuity check between a black wire and a green wire. and if there is a short (other than the loom chafing on the frame somewhere) then it's the loom.

[RGP750]

Cheers guys
Already removed the regulator and andrectifier.
It really is looking at taking the loom off and seeing if I can find anything. Cheers all

[Nurse Julie]

Brand new loom fitted last year when I gave it a tidy up .
Yes correct fuse fitted .
As I have said it ran perfectly last year and something has gone ary while standing .
Just don't know where to look now.
Nbefore getting silly and replacing everything again

Mine was originally a new aftermarket loom from DSS the year before Rich. Graham put another new loom on, changed the ignition switch and the sub loom. Never did find what part was at fault because i just wanted to ride her once she was running.

[Laverdaroo]

Sounds silly but it sounds like you’ve got two wires connected that shouldn’t be, make sure when everything is back in place that all the connectors are separated as if it ran ok but doesn’t now has something moved and started touching? If it has you won’t find the fault. I’d plug it all back in and start one at a time again if you can as per suggested. I do think it sounds like a crap earth or a loom off job sadly


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[RGP750]

Nothing is left connected so my thoughts are melted loom for some reason. To me it's the only logical answer now. I'll try and get time in the next week to take it to bits I'll report back when I have done it .cheers all