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taillights

Here's a new one for me. My '91 240 hasn't had any problems with the tail lights up until this evening when I got pulled over and the cop telling me I had no tail lights. Luckily he just sent me on my way with the emergency blinkers on.

Anyway, they've been working up until now and in fact I'm pretty sure they were working when I started up the car as I usually check to make sure there is a reflection when I turn the lights on at night. (my old 740 was notorious for having the tailights blink out) I checked the fuses and they seem to be ok. Is there a relay for the tail lights that isn't listed in my Hayne's repair book? Any ideas? Both sides are out. Brake lights work, emergency lights work, front corner lights, turn signals work. Just the tail lights are out.

Please help.

Thanks Denny

Reply to
Denny A
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it's a different circuit. trace it back w/ a VOM.

Reply to
Richard W Langbauer

Pull out the bulb failure sensor, it's a red or black cylindrical device clipped up under the dash on the driver's side plugged into a large wiring harness. On the 740 it's in the fuse box under the radio.

Pop the cover off this and you'll find a stack of circuit boards in there and likely a bunch of nasty cracked solder joints. If you can solder yourself then it's easy to fix, otherwise maybe you've got a geek or engineer in the family who can fix it in a few minutes. Failing that, you can buy a new one but they're not particularly cheap, or go to a junkyard and pull one that may work fine.

All the current to the monitored lamps passes through the failure sensor. It's a clever design, the older ones at least use magnetic reed switches, each with two coils wound around them. Current from the lamp on each side passes through one of these coils but in opposite directions. When the current is balanced, the resulting magnetic field cancels out but an imbalance leaves a net magnetic field which causes the reed switch to close and completes the circuit to the warning light. It's technology that was available 150 years ago and yet it does the job nicely until you get bad connections.

Reply to
James Sweet

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