Ford Galaxy Ghia 2005. Rear left hand brake light had blown. Fine. Took it out while rear running (rear “headlight”) lights were on, the brake light bulb is a double filament which is also the running light. As I was taking out the assembly the rear running light went out. The new bulb is in, it works as a brake light… But the rear running light does not come on. Swapped the new bulb with the same bulb from the right hand side, it works on the right but the bulb from the right does not work as a running light on the left. Not sure what to do about this, cursing for taking it out while it is on.
Could the computer have disabled the bulb to prevent a short or something like that?
I’ve read something to that effect but how to remedy such a thing if true.
You need an inexpensive 12 volt test light or a multimeter, See if a grounded test light lights at the parking light contact or if you get 12 volts there with a multimeter . If your test light lights or you get 12v, you have a ground problem. If not, you have a wiring or socket problem.
I never use a multimeter for this, but most younger people do.
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Sounds like a loose/corroded connection in the socket, or a break inside the wire going to the socket.
Try wiggling the wires to the socket (with the socket and bulb in place).
If the light flickers on and off you know you’re getting warm.
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A test light has the advantage of drawing a load.
A multimeter can be fooled by reading voltage that’s being fed through a high resistance loose or corroded connection.
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With all the bulbs working fine on the right, and working fine on the left for the brakes, but when any of them are put on the left they stop working for the tail-lights, seems you got a problem with the left tail light socket. Unlikely to be a ground problem b/c that ass’y most likely shares the same ground, and it works for the brake lights.
If I had this problem, first thing I’d do is remove the negative from the battery, and remove the left bulb from its socket. Then I’d see if I could clean or file the inside of the socket an rust and oxidation where the contacts are made. If that didn’t work, and the test light showed I was getting power ok to the socket, I’d probably just replace the socket with a new one.
BTW, not a good idea to replace bulbs when the system is powered up. That job is best left for when the engine is off, key out of the ignition, and everything in the car turned off.
So I took people’s advice and got a multimeter, no voltage going to the runing light but got voltage to the brake light. Tested the fuses and found one of them to be blown, I thought I tested the relevant ones yesterday but the fuses image description which is misleading. I did not think that the rear left hand running light would be on one fuse while the other side would be a different one. I’ve replace the fuse and it is now working again. Thanks for everyone’s help.