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

 

it's a different circuit. trace it back w/ a VOM.

 

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.