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Weirdest Electrical Problem I've Seen


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I've joined here to see if the assembled Ford expertise can make head or tail of this electrical problem I'm having with my daughter's T-reg (99) Focus (1.8 petrol).

If I engage reverse gear, there are volts across the contacts in the reversing light bulb holder... 12-ish with engine off / 14-ish with engine on, as you'd expect.

If I do not engage reverse gear, there are no volts at said contacts. As you'd expect.

So there is nothing wrong with the reversing light switch circuit.

If I remove the bulb holder from the socket on the end of the wires, and attach the bulb holder's terminals to a bench PSU at 12V, the bulb lights.

So there is nothing wrong with the socket, or contacts, or bulb.

It should work!

Yet if I plug the holder in to the wires, and the bulb in to the holder, the light does not come on.

The only thing I can think to explain this is that there might be some kind of protection circuitry that has become over-sensitive. Whilst there is no load across the reversing light circuit the volts appear normal, but as soon as the load is applied, the "protection" is erroneously kicking in because it thinks it's being overloaded, and cuts out the volts altogether... so the bulb doesn't light.

Does that sound feasible? If not, what? And is there a cure?

All fuses in the fuse box are intact.

Thanks in advance for any help or suggestions.


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Does sound wierd. I suppose the next thing is to join the two contacts that go into the switch at the gearbox with a piece of wire with the ignition on. If the bulbs light up, its the switch that is the problem. If they dont light up it is something inbetween. Not sure if there is a relay involved anywhere.

Good luck

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Hi walrus,

Have you checked that the bulb contacts are making a good connection with the holder? A bit of corrosion may prevent the bulb working but the sharp tips of your DMM probes may give a good reading.

The other thing to check is if you have a dodgy connection in the switch or the wiring - this may not be a complete open circuit, but just give a few tens of ohms resistance. Your DMM will have a high impedance so will measure the correct voltage, but you will not get the current to light the bulb.

Good luck

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