Dartmoor7 Posted June 13, 2017 Share Posted June 13, 2017 My offside reversing light is not illuminating.I have 11.80 volts at the wires which attach to the bulbholder.The bulb works when swopped to the nearside.The bulbholder works when swopped to the nearside.What am I missing?It's driving me insane! Link to comment Share on other sites More sharing options...
revilla Posted June 13, 2017 Share Posted June 13, 2017 Do you have 11.8 volts with the bulb in place in the holder, or just measuring the voltage with the bulb removed? If you have a high resistance in the circuit (a "bad connection" somewhere), you will still read close to battery voltage when there is no load and no current flowing, but the voltage may fall close to zero as soon as you load it with a bulb. Try to check the voltage with the bulb fitted. Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted June 13, 2017 Member Share Posted June 13, 2017 Crossed with Andrew's.I have 11.80 volts at the wires which attach to the bulbholder.That's between the wires, not between the feed and a good earth?What's the voltage between the lamp contacts with the lamp removed? And again when it's replaced?Incandescants or LEDs?All other lights checked and working normally?JonathanPS: Two reversing lamps? Link to comment Share on other sites More sharing options...
Dartmoor7 Posted June 13, 2017 Author Share Posted June 13, 2017 11.80 between the supply wires when not attached to bulbholder.Between live and as good an earth as I can get on my old rustbucket, 11.94.Between supply and earth wire ends when attached to holder and bulb,46mV.Incandescents.All other lights OK.Two reverse lights standard on a 1994 car. Link to comment Share on other sites More sharing options...
revilla Posted June 13, 2017 Share Posted June 13, 2017 OK the next thing to check is the voltage between the supply wire and a good earth with the bulb and bulbhokder attached. If this is close to 12V you have a high resistance problem with the earth. If it is close to 0V, you have a hugh resistance problem with the supply side. Link to comment Share on other sites More sharing options...
Dartmoor7 Posted June 13, 2017 Author Share Posted June 13, 2017 So, both wires connected to bulbholder, bulb in place, check voltage between earth and the supply wire? Link to comment Share on other sites More sharing options...
Dartmoor7 Posted June 13, 2017 Author Share Posted June 13, 2017 If I've understood correctly, that measurement is 0.33V. Link to comment Share on other sites More sharing options...
revilla Posted June 13, 2017 Share Posted June 13, 2017 Yes, that was the right thing to do.And the voltage is very low, indicating that there is a problem with a connection somewhere between the battery and the supply wire to the lamp. You need to trace the identify that current takes from battery to bulb.Do you have a wiring diagram for your car? So if you start at the battery end of the circuit and measure the voltage from a good earth you will get close to 12 volts. By the time you get to the bulb end that will have gone down to almost 0 volts. It won't go down a little bit at a time as you work along the circuit, there will be a connection somewhere where it just drops from 12V to 0V, and that's where your problem is.You've got one big clue already; if I understand correctly and the reversing light on the other side works correctly, you know the fault must be after the point where they separate. This I think should eliminate everything like the fuses and switches and just leave you looking at the wiring at the back of the car. See if you can trace the wire backwards from the lamp.The fault is less likely to be a damaged wire and more likely to be a plug or connector, or where wires are joined together. Look for anything dirty, corroded, loose or otherwise suspicious. If it is a damaged wire, to be that bad it should be fairly obvious, so looked for a wire that has been trapped, pinched or otherwise physically damaged. And remember, the fault will always be somewhere between the last point in the circuit where you find 12V and the first point in the circuit where you find 0V. So you can slowly narrow in on it.Make sure you do all the testing with the bulb in place; it is only when trying to pass a current through the increased resistance that you will drop a voltage across it, which is why you seemed to be getting almost 12V with the bulb removed. Link to comment Share on other sites More sharing options...
Dartmoor7 Posted June 13, 2017 Author Share Posted June 13, 2017 Brilliant!Thanks everyone, that's my job for the weekend sorted.I'll see if there's a diagram in the old build manual.Need to clean it up a bit first, there's a lot of pony/cow/sheep crap under there. Link to comment Share on other sites More sharing options...
revilla Posted June 13, 2017 Share Posted June 13, 2017 On your build manual? Link to comment Share on other sites More sharing options...
Julian H Posted June 13, 2017 Share Posted June 13, 2017 You have two reversing lights???? One is normally a fog lamp. Even if it has a clear lens, the bulb should be red and be operated by a switch on the dash? Link to comment Share on other sites More sharing options...
Ian B Posted June 13, 2017 Share Posted June 13, 2017 Older cars had two reverse lights Julian Link to comment Share on other sites More sharing options...
Dartmoor7 Posted June 14, 2017 Author Share Posted June 14, 2017 >>On your build manual?Could be, the whole garage smells like the moor after some blats. Link to comment Share on other sites More sharing options...
Leadership Team SLR No.77 Posted June 14, 2017 Leadership Team Share Posted June 14, 2017 1994 car ... the wiring for the rear & reversing lights is fed down the LH side of the car then across the rear IIRC the RH reversing light positive feed is linked from the LH, the LH being fed from the loom both lights have a fused permanent 12v feed, unswitched the switch (on the side of the gearbox) earths the lights when reverse is engaged, rather than the switching being on the live side of the circuit as is usual with the other lights on the car. Link to comment Share on other sites More sharing options...
Dartmoor7 Posted June 14, 2017 Author Share Posted June 14, 2017 Many thanks for that, the diagrams in my build and owners manuals are too small to be of much use. Link to comment Share on other sites More sharing options...
Duckpit Posted June 14, 2017 Share Posted June 14, 2017 I have 2 reversing & 2 fog, so I guess it depends on the car Link to comment Share on other sites More sharing options...
Sussex Mid AR Posted June 14, 2017 Share Posted June 14, 2017 Well you learn something new every day! Thx IanJ Link to comment Share on other sites More sharing options...
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