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Is it possible to test for a short circuit?
Irony - 18/7/12 at 07:54 AM

I am just wiring my headlights up and getting mildly confused with the pin outs of the sierra stalk. After every stage I test it with a bulb connecter to one corner of the car. I wired up the side, dip and then the main beams. Sides and Dip were okay but when I first switched on the mains I could have swore I saw and heard a little flash of a arc. I wasn't looking at the column (I was looking at the headlight) so I don't know where exactly it came from. I spent ages poking around trying to recreate the problem to no avail. Also all the lights are working exactly as expected.

Admittedly there are wires hanging all over the place from parts of the loom not yet wired up so it could be that. But now I am worried. I have a 10amp fuse in the main beam (with relay) circuit. Is there anyway I can test for a problem?

snowy2 - 18/7/12 at 08:04 AM

put a multi meter on resistance measurement, connect on one end of the wire you wish to test (the copper bit not the insulation) and the other to earth if it gives a reading 0 or a few ohms it is likely to be shorted to earth.

NOTE: if your looking for a problem on the steering column switch pick the steering column it's self to be the earth for testing purposes.

[Edited on 18/7/12 by snowy2]

r1_pete - 18/7/12 at 08:05 AM

Yes, you need to use an ammeter in series in each circuit, I'd go before and after the switch for each circuit in turn.

For one 55w headlamp bulb you'd expect to see a current flow of about 4.5 Amps, a 5w sidelight about 0.5 Amps, remember to double it for two bulbs.

ETA,

If you come across a short the meter will swing off the scale, but as said below you would expect a fuse to have blown.

[Edited on 18/7/12 by r1_pete]

big-vee-twin - 18/7/12 at 08:13 AM

If you have a short. It is highly likely that a fuse would have blown due to the high currents involved. Switches do spark when being operated it is a normal thing, they are usually designed to cope with this

MakeEverything - 18/7/12 at 08:51 AM

quote:
Originally posted by big-vee-twin
If you have a short. It is highly likely that a fuse would have blown due to the high currents involved. Switches do spark when being operated it is a normal thing, they are usually designed to cope with this


Also, fuses flash when they blow too, so may be worth looking under the dash for an in line fuse.

Personally, i would put a new fuse in and see if it goes again whilst jiggling the wiring loom. If it doesnt happen again, then it is likely that a failing headlamp bulb caused a high enough resistance to blow the fuse. Have you checked the lamps?

Irony - 18/7/12 at 09:16 AM

Everything seems to be working fine. I cannot seem to recreate it. If I put the smallest blade fuses I can find into the system and a tiny bulb then will the fuse be more likely to blow if there is a problem?

big-vee-twin - 18/7/12 at 09:48 AM

Take out the Bulb leave in the fuse and test the wire from the switch output to earth with a meter, if there is no resistance there is no short if it reads zero there is.

If everything is working ok ie your lamps come on when you switch them on, then there is no short, you may have just seen the switch sparking.

Quick addition - leave light switch turned on and disconnect battery then test.


[Edited on 18/7/12 by big-vee-twin]

MakeEverything - 18/7/12 at 09:31 PM

If you cant recreate it, then replace the fuse as was and crack on. The fuse did its job, and i reckon it was a dodgy lamp (did the lamp blow??) but wouldnt worry about it.