Rear fog light not working....but i seem to have 11 volts

dickster · 06:06 AM

Hi there,

I have a lhd 86 coupe, and it had the harness extended (before my time) to put the rear fog on the rh side of the car. The fog stopped working recently and on inspection the wires were oxidised, and as the extension was only joined by crimping, I decided to remove it and solder it properly.

I got the lh rear light unit removed and stripped back the fog cables to the originals ready to solder new extension cables. The original wires were also oxidised so I tried cleaning them in vinegar! I tested the voltage at the bare cables with the fogs turned on and I am getting around 11 volts - even with the engine running - is this enough to light a bulb? As I put a bulb onto the bare cables and it will not illuminate.

I unplugged the lh side harness in the engine compartment and there appeared to be some corrosion in there which I have tried to clean up. i then removed the harness earth which is bolted to the bumper shock - and broke the bolt in the process - how do I fix that?

So, I am at a loss, i have all but removed the lh side harness and still get 11 volts to the foglight, but still the bulb will not light - i have tried different bulbs. should i just look for a replacement harness? With the wires oxidised are they no longer any use?

Cheers

john walker's workshop · 02-02-2014, 06:20 AM

should be the same voltage as everything else. bad connection somewhere upstream.

dickster · 02-02-2014, 06:23 AM

Thanks John, that's what I thought. Could it be the oxidised wire causing the problem? Electrics really aren't my thing!

dickster · 02-02-2014, 06:41 AM

With the rear light harness disconnected I tested the voltage at the pins on the plug in the engine compartment, and sure enough 13+ volts. So the problem is with the light unit/harness, are these repairable? I see now that the harness is part of the light unit and rather expensive!

I may try and strip back the harness and splice a new wire in for the fog light..

Cheers

john walker's workshop · 02-02-2014, 09:15 AM

that should help. that's what i'd do.

porwolf · 02-02-2014, 03:50 PM

Quote:

Originally Posted by john walker's workshop

should be the same voltage as everything else. bad connection somewhere upstream.

11 Volts is certainly a sign of a bad contact in the wire somewhere. But voltage alone does not indicate a corroded connection somewhere. Even with a very bad connection a wire that has no current flowing can show full battery voltage at the end. A bad contact along the wiring can cause that voltage to drop to zero when a lamp is attached and current is flowing. A voltage check should always be done with a working lamp in the circuit.

porwolf · 02-02-2014, 03:52 PM

Quote:

Originally Posted by dickster

With the rear light harness disconnected I tested the voltage at the pins on the plug in the engine compartment, and sure enough 13+ volts. So the problem is with the light unit/harness, are these repairable? I see now that the harness is part of the light unit and rather expensive!

I may try and strip back the harness and splice a new wire in for the fog light..

Cheers

To install a new wire as far back to the battery as possible is usually the fastest and safest way to correct a weak link.

dickster · 02-02-2014, 11:23 PM

thanks, i've established that the weak link is the tail light harness, so I am going to try and rewire the fog back to the plug on the harness....

dickster · 02-03-2014, 09:51 AM

the licence plate light is stuck fast - screw heads rounded off. the unit is broken anyway and I have another one, so i'm thinking grind the old screws down, and then drill and tap..

any alternative ideas?

thanks

911pcars · 02-03-2014, 10:10 AM

An 11 volt source should provide sufficient voltage to make the bulb workable if not at full efficiency. However, the path to ground (after the load) must be complete for current to flow back to the battery and allow the load (bulb) to operate.

Assuming source voltage is available at the bulb socket contact and the bulb is good, attach a test wire from the ground circuit at the bulb holder to a clean metal ground path to confirm a working current path up to that point. If not, the normal harnessed ground path is compromised (open circuit or thereabouts). The typical malfunction is the bulb, but an open can be anywhere between the last source signal and the ground path.

One does not need a working bulb (or load) to identify source voltage which is an independent element of a circuit.

Sherwood

dickster · 10:40 AM

Thanks for the help guys. It's now working, I managed to remove the female part of the 6 pin plug without wrecking the rubber and soldered in a new wire.

Now I need to sort out the sheared earth bolt.....

Cheers