Advice on fixing stop lights 740 GL

It is an 89 740 GL. I got a haynes manual that has a diagram
(although the last year shown is only 87). Ha! European diagrams make
little sense! Anyway, the fuse is good. THe brake light in the rear
window works fine, but the left and right main brake lights do not
work. Actually they are intermittent, or were.... now they are inop
all the time it seems, and the "bulb out" indicator is illuminated on
the inst. panel. I see that the right and left brake lights branch at
the bulb failure sensor. (Could not see where the rear window brake
light branches off, unfortunately.) Seems like the bulb failure
sensor is the thing to check next, could be a loose connector? But
where is this module located? Don't have a clue.
I checked the bulbs, connectors, and wire harness in the trunk, it
all seems OK, nothing obvious.
If I could get into the harness near the rear window brake light, I
wold just run a new wire to the main brake lights, but I don't see how
you get to it. Thanks.

 

The most likely culprit at this point is the bulb failure sensor, it's a
round can plugged into the relay tray. If you pop it open you'll see two
circuit boards stacked, and probably some cracked soldering. Heat up the
solder joints and flow a bit of new rosin core solder into any that look
suspect and you should be good to go. If you or someone you doesn't know how
to solder, you can replace the sensor but you should at least look inside to
verify the problem.

 

I was going to suggest the same thing James. But, I'll add that the red
"can" relay is a pricey item new. There are often relay sets on eBay for a
fraction of the cost of new.

RS

 

Yeah they are pricey! I would definitly suggest trying to fix the existing
one first, or even take it to a TV repair shop, any tech at one of those
places could fix it up in a few minutes. Pretty clever design really, the
old ones use two coils wound around a reed switch and wired out of phase. So
long as the current is equal, the magnetic field cancells out but any
imballance results in enough magnetism to trigger the reed switch and turn
on the bulb failure warning. Of course any bad connection in the whole mess
will kill the lights and the sensor is very sensitive to corroded sockets
and such.

 

I've only ever seen the relay make all 3 bulbs fail, not to say that it
can't fail to allow only the 3rd light to work. I'd first look at the
condition of the bulb holders, circuit cards and the condition of the
plastic of the taillight itself where the bulb holders sit as it often
melts there.

--
Mike F.
Thornhill (near Toronto), Ont.

Replace tt with t (twice!) and remove parentheses to email me directly.
(But I check the newsgroup more often than this email address.)

 

Didn't he say he already checked that?

Both times I've resoldered a bulb failure sensor the center brakelight still
worked fine but the main brakelights and in one case, one of the taillights
were out.

 

Yeah, but often when there's a problem with melting it's not obvious
until you're looking for it.

--
Mike F.
Thornhill (near Toronto), Ont.

Replace tt with t (twice!) and remove parentheses to email me directly.
(But I check the newsgroup more often than this email address.)