I AM GOING BONKERS!!!

[jx2mad]

One would think that having worked in electronics and communications for 34 years I should know my way around a circuit, especially one that consists of a battery, on/off switch and a light bulb. But this one has got me stuffed! My high level aftermarket stoplight on the top of my canopy door is not working. Unplug the light assembly and connect across a battery and the lights work. Depress the brake pedal and measure the voltage at the terminals for the stoplight and I read approx 12.3 volts so all ok there. Reconnect the lights and they don't work. I have checked the crimp connectors and they test ok. What on earth is going on? Jim

[stevo]

sometimes when using a multimeter you can be lead astray if circuit has a high resistance, plug the light in and test the circuit under load see if you still have 12 volts or less

[Ancient Mariner]

Polarity concious LED ?

[bob10]

Probably a foolish answer, but...Earth? Bob

[muddy]

check the circuit with a test lamp and NOT a multimeter ---you will find there is a high resistance fault

[vnx205]

A couple of the answers above are probably on the money.

I had exactly the same problem with a reversing light. The multimeter showed 12 volts or something like that, but the light refused to come on.

It was a very poor connection where the trailer lights had been wired in, so I had plenty of voltage, but almost no current.

A poor earth or a poor connection is probably the most common cause of electrical problems and can give exactly the symptoms you described.

[steveG]

A couple of the answers above are probably on the money.

I had exactly the same problem with a reversing light. The multimeter showed 12 volts or something like that, but the light refused to come on.

It was a very poor connection where the trailer lights had been wired in, so I had plenty of voltage, but almost no current.

A poor earth or a poor connection is probably the most common cause of electrical problems and can give exactly the symptoms you described.

My reverse light had the same symptoms - but the high resistance was in the reverse switch on the gearbox.
To make it even more interesting to troubleshoot, sometimes if you left it sitting in reverse for a few minutes while troubleshooting it would come on.

Another thing that can give a clue with lights down the back of the vehicle is if there is any change in the other rear lights (indicator/tail/brake) when the light in question is turned on. A slight dimming or slight glow in another light is a sure sign of a bad earth in my experience.
A recent one I experienced was RH indicators going funny (fast flash) when I put the brakes on. Turned out to be a bad earth (in the RH brake light IIRC).

Steve

[UncleHo]

If dealing with anything of Lucas origin you will find that each and every lamp or instrument has it's own earth and with the later vehicles and thinner wiring the fault can be very hard to find

cheers

[jx2mad]

Hi all. Looks like I have a high voltage, low current connector at another point. I hate crimp joints. Jim