View Full Version : Electrical fault finding

08-18-2006, 01:31 PM
After a year of messing about with good honest tools to get the car safe, I finally have a driveable car. Woohoo! Sadly now I must enter the mystical world of electrical truobleshooting...

I knew that the car had some "issues" thanks to previous owners messing around with things, but I am a bit stumped to work out what is causing the problem.

Every time I turn the key in the ignition a fuse blows. It is the green circuit (75 TR6) which includes the reverse lights, brake lights, heater, wipers, flashers and hazards. That seems like a lot of circuits to me. My current (no pun intended) thinking is to remove the wires from the switches and so on in turn until I find the one causing the problem. The problem with this is that I only have limited stocks of fuses! Is there an easier way? I have a multimeter, but won't pretend I know much about its use...

Sorry to be so dense - I am really excited to get on the road, and I want to get this sorted out this weekend, if posisble! Thanks in advance for any help.

Geo Hahn
08-18-2006, 03:02 PM
...remove the wires from the switches and so on in turn until I find the one causing the problem. The problem with this is that I only have limited stocks of fuses!...

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Good plan... if only you has a reusable fuse. But hey, you do!

Make a test light from a 12V bulb such as a turn signal bulb or dash light bulb. If you can solder then solder a pair of wires to the bulb, otherwise you'll need a socket.

Connect the test light in series in place of the fuse, i.e. fuse removed, one lead connected to one fuse clip, the other to the other fuse clip. Now instead of 'blowing' the short will just cause the light to glow.

Start undoing your various items (switches and so on) until the light goes out. The one that extinguishes the light is the Bad Boy.

Some folks use a buzzer (an old 'remove key' buzzer would work) so they can just listen for the buzzer to stop without crawling out from under the dash or asking the wife to watch the bulb.

08-18-2006, 03:10 PM
The fuse blows when there is no/little resistance in the circuit.

I'd suggest you get an owner's manual with a schematic diagram of the electrical circuits- any good manual should have one.

Here's one for a Triumph Herald- yours will be a little different but it should be a useful start:


Before you start pulling the wires first check that they're properly connected to the switches and aren't making any contact with metal they should not. Also, if you can make sure that none of the other switch components aren't making contact either. Also check that the wires themselves are sound and don't have any bare spots or places that have rubbed and shorted against metal.

If the switches, connections and wires all look healthy then:

I'd suggest you pull the wire from each switch and check it for a short to ground with your meter. For that you'd use the resistance (Ohms) scale and connect one probe to the connector to the switch and the other to a good ground.

A wire that's shorted will show "0" resistance while one that is good shouldn't move (infinite resistance).

If all the wires seem bad then check their connection at the fuse block for a short.

If none of them seem bad then check the switch internals/bodies. If you've access you might also check the switch bodies for an internal short in the same way- using the probe to the connector that the wire attaches to. If you've no access... find a buyer.... /ubbthreads/images/graemlins/devilgrin.gif

If you've a fuse to spare you might try to "brute force" test. pull all the wires from the switches and keep them from touching any metal. Put the fuse in, switch the ignition on, and then start re-connecting the wires one by one. Ignition off. then then next. Ignition on. Until by a process of elimination you find the switch that blows the fuse. And hope its the only one that's bad. Since you've already got the others disconnected go ahead and test them too, since you're already doing it. If none of the others are bad you've found the one and can pull the switch body and see what's wrong with it.

Geo Hahn
08-18-2006, 03:35 PM
In case you need this, wiring diagrams here:


08-18-2006, 04:01 PM
There is an easier way to start finding which circuit is bad. Buy a test light( they are cheap, and real handy to have), figure out which side of the fusebox is powered,(probably brown or white wires, don't have a diagram handy) and one at a time disconnect the green wires at the fuse box.Most likely only one circuit will be bad, and by seeing what works, it is simple to figure out what doesen't.That way you only have to trace the components on one circuit, not all of them.

08-18-2006, 05:35 PM
If you've a fuse to spare you might try to "brute force" test.

[/ QUOTE ]

On second thought this is a BAD (well, not so good as it should be) idea.

Don't use a fuse.

Use your meter. Connect one probe to the side of the fuse box where all the suspect wires connect. Connect the other probe to ground.

If you've already checked that the wires were OK then connect them one by one to their switches.

The bad one should then show a "short" with "0" resistance.

Disconnect it, and finish testing the other switches.

You can then pull the bad switch and see what's wrong.

08-19-2006, 11:37 PM
Thanks for the ideas, guys. I tracked the problem down to the hazard switch, where the bulb holder had come loose and was contacting the incoming positive wire. Naturally, it was the hardest item under the dash to reach! I spent the evening tracking down the "ingenious" wiring techniques of the DPO - without him it would never have occurred to me to connect the left and right indicators to the same side of the switch!

On a related note, can anyone tell me if I need a whole new hazard switch - I don't have the actual push switch that sticks out of the front of the dash, just the bulky part at the back (sorry to use such technical jargon!). What appears to be the part of the switch which would push in moves very easily with a screwdriver, but doesn't seem to catch. I know this is a bad description, but I hope someone speaks enough "idiot" to understand me!

Thanks again for the advice.

08-20-2006, 08:18 AM
OE style hazard switches are not available. The aftermarket switches seem to work, I just vow to never turn mine on. Another secret area of fuse destruction is the detent circuit in the wiper motor, the little switch that brings your wipers back home when you turn them off. If fuse burning persists, try unplugging the wiper plug at the motor and see if the fuse burning stops. Ask me how I know.


08-20-2006, 10:13 PM
If you are looking for the switch, try Scott Harper at Team Triumph in Warren, OH. (330)392-7176