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Headlight circuit getting hot, hot, hot....(CP TR6 1970)


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Ok just replaced my front loom with a new one, last one went up in smoke.

 

From whats left appears the headlights were the issue.

 

Rewired the car everything now working again. However when headlights on wiring under the column getting hot. Its the Brown/blue to brown/white cable which is the one which provides power thru the ignition switch to the headlight circuit. I do intend to fir relays to the headlights but would like to get to the bottom of this before I do so. Headlights are halogen replacements ones so that shouldnt be an issue. Wiring is getting hot after a few mins so that cant be correct. Whats the best way to diagnose this please.(I dont want to fry my new loom)

 

Once I have eliminated I intend to fit relays to the headlights probably powered from the alternator spare terminal.

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Hi Robin,

if it was just a connector getting hot then it would be corrosion at the connector joint.

However as the wiring is getting hot then that suggests higher than normal current.

Have you disconnected the bulbs from the loom and checked to see if/what current is flowing - it should be zero amps.

The normal 60watt bulbs take apprx 5 amps each - 10 amps for main bulbs total (are the dips on when the mains are on, if so add another 8amps for these)

 

Roger

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Roger,

 

ok tried and failed, can measure volts and continuity however when I try amps no reading. Looked it up on google but no reading on my multimeter also lights not on indicating I've got something wrong. In case it helps its seems that only the cable from the switch to the connector which houses the cable to the ignition switch is getting hot

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Hi Robin,

if the lights are not on (when switched on) then you have a serious short. The problem lies after the hot wire but before the lamp loom.

Disconnect the hot wire from whatever it is attached to down stream and do a resistance check - low resistance = not good.

 

Roger

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OK thanks, just tested multimeter fuse...its blown, so I'll get another. I

 

If I connect the headlamp circuit direct from a 12v supply (in the case the one from the ignition that powers the column headlight switch) to the power lead under dash for the headlights all work no probs, not hot.

 

So its presumably something to do with the column switch and or wiring..... Is there some test I can do on this as it appears to work fine (apart from getting very hot). Just to be clear the headlights/ dipped and full beam work fine ....ie when column switch operated they come on as they should, and when dip switch is depressed they dip.

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At the risk of stating the bleedin' obvious, please remember that this circuit is not fused so if you leave it switched on for long enough, the loom will fry and possibly cause a major conflagration.

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David, yep thanks, I'm only putting it on when I am holding the offending wire so I can feel it warming up, as soon as it does its turned off (and battery disconnected), looking after the new loom.

 

I'll take the switch out and see if the wires are all OK.....any other thoughts appreciated.

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Almost certainly the contacts in the switch have high resistance and this is heating the wire.

 

Dismantle switch and clean maybe, dont know these switches very well.

 

Bridge out the switch with a short section of wire with the same kind of terminals on it and see if wire stays cool

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Thanks. Will have a go tomorrow. How do I test for high resistance with a multimeter and what would the reading be vs should be.

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Switch resistance must be less than 0.1 Ohm. You will find it hard to measure this with an ordinary meter.

 

So run the lights and measure the voltage across the closed switch contacts. Every volt lost here is heat not light.

Should be less than 0.2V or so.

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Sh1t, thanks Neil, Alan, measured volts against closed contacts, its about 1.2 volts less than the feed when the switch is open.....guess then trays the problem. Is it work removing the switch and cleaning...have squirted it with contact cleaner no improvement. New ones seem pricey and just forked out for the fried loom.

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Robin, is it a column switch? If so the wires might be rubbing on the metal at the top of the column and shorting out. This was a persistent problem on my car until I lined the top of the column where the switch wires go into the dash with a section of plastic milk carton so now they rub against that (non conductive) surface instead.

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David, thanks I'll have another look. I took out the mounting screws and eased the switch away from the column to do the test so may not be the problem. Certainly worth another look, in the daylight. Thanks again.

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You measured the voltage AT the switch with respect to ground it seems.

 

Its the volage with both test probes on the switch, One probe on the swich-contact that feeds into the switch, the other probe on the switch-contact that feeds the lights.

 

You are lookig for voltage lost as the curent goes through the switch.

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Alan

 

indeed I did but I also measured it between the contacts (closed) and thats where I detected the 1.2 volt drop.

 

Have removed it today and given a good clean. However the resistance between the live to headlight and live to sidelights varies considerably with the headlights some 3 or 4 times higher. So guess its new switch time..

 

PS thanks for all help

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Alan

 

indeed I did but I also measured it between the contacts (closed) and thats where I detected the 1.2 volt drop.

 

Have removed it today and given a good clean. However the resistance between the live to headlight and live to sidelights varies considerably with the headlights some 3 or 4 times higher. So guess its new switch time..

 

PS thanks for all help

fit a relay where the bullet connector goes to the column switch.you will need a few female spade connectors to fit onto relay.

wire it so the column switch only powers the coil in the relay.there will only be 1.5 w current going through the headlight switch.

and save you buying a new one.

do the same for the side light/dash bulbs.using a mini relay.

all tucked up behind dash/speedo

heres a crappy diagram

richard

post-2190-0-51952500-1372778950_thumb.jpg

post-2190-0-51952500-1372778950_thumb.jpg

Edited by rpurchon
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1.2V across the switch is 12W of heat! No wonder the wire feels hot.

 

At some stage I think you had overcurrent in the headlight circuit. Most likely this has heated the switch-contacts and the temper has gone out of them so they are no longer springy.

 

I am a believer in fuses in the headlight despite howls of protest on here about the fuse failing and the lights going out suddenly.

 

If there is a short you will be in the dark anyway but in the dark with burnt out wiring.

Only if the fuse decides to spontaneously quit will I be worse off.

 

The situation wont change much if you fit a relay. Only that the column switch will get an easier life.

Edited by AlanT
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Only that the column switch will get an easier life.

 

 

a lot easier life. 1 w instead of 110w or 9 amps

it a worth while and simple upgrade even if you have a good switch and wiring..

saves running live wires to relays behind the headlights under the bonnet .

richard

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Wired in relay as advised and unfortunately not hot but warm feed from switch. Bit the bullet and bought a new switch....ouch. Hopefully that will sort it, iLl try without relay just in case, then relay the headlights anyways. Don't want another cooked loom or worse Fire...thanks all.

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Fitted new switch and now all wires ate cool so hopefully all sorted. Will add the relays now as a belt and braces solution. Worth noting a dodgy switch can cost you a loom if your lucky or worse a fire . Amusingly haven't used the lights much so they must have fried the loom over time and then on last trip gave up the ghost. Loom burnt through in 4 separate places.

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