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Brake light wiring mystery - any electrical geniuses out there? 700 1991

posted by  Allanwayne on  Mon May 16 07:46 CST 2005
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last visit: Sun Jul 8 15:05 CST 2012
[RELATED]

Hi, I have a '91 740 wagon with a 3rd high level brake light.

Problem: right tail light works, but when brake is applied, it goes out (instead of brighter) and instead the right rear fog light comes on (the highest one).

Also, none of the 4 fog lights work (which I don't really care about), the and there is no current in the fog light switch (not illuminated when on) or after the fog light switch.

After replacing the brake light bulb and cleaning the contacts, I found this at the wires coming into the light assembly with a multimeter:

resistance at common ground bar:
.01 ohms with brake off, .06 ohms with brake pressed

voltage at brake/tail light (as expected):
12 v at tail light wire, 0 v at brake light wire with brakes off
12 v in both tail and brake light wires with brakes pressed


voltage at fog light wires:
hot wire :.14 v brakes off, 12 v with brake pressed (and fog light comes on)
grnd wire: 0 v with brakes off, .44 v with brakes pressed

For the life of me I can't figure this out. How is the resistance increasing in the ground bar with the brakes on? and is this the reason for the brakes lights going out during braking?

How is current being shunted to the fog light during braking?

Any ideas?





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