850 SE saloon: High-level stop lights don't work

I've a '93 850 SE saloon. The two high-level stop lights (the one in
the passenger cabin that shines out the back window, and the strip
along the edge of the boot spoiler) don't work when the brake pedal is
pressed. A bulb warning shows on the dash, but I've changed bulbs all
round (including all of the little ones in the strip), to no avail. The
stop light fuse is sound. The regular stop lights work fine.

Any "bright" ideas about what I should check next? (if you'll give
forgive the pun).

TIA,

Stuart

 

Bulb failure sensor failure perchance?

 

In <>,

I've seen the contacts corrode, which causes the same symptoms. Just remove
the entire unit, use a little piece of emery paper, or a pencil eraser over
the contacts, and fit the unit back in again - perhaps the problem will
disappear.

AC

 

Well, there's the latest ...

Got myself a replacement Lamp Control Unit via eBay -- popped it in --
exactly the same symptoms still

I had a bit more time to play around today, and found that the power
supply to the spoiler lights simply runs off a crude pair of wires that
run from the input to the cabin stop light.

As an experiment, I disconnected these wires, so power was being
supplied to the cabin light only. Made sure there was a good bulb in
there -- but same problem -- no light, plus bulb warning light showing
on the dash when I press the brake pedal.

Both regular stop lights still working fine at this point, I hasten to
add.

So, what gives? Anyone? I guess, in theory, the (used) replacement LCU
could have the same problem as the original, but that's unlikely. I
suspect also that once I've sorted out the power supply to the cabin
light, the spoiler lights will work fine once I wire them back in.

Any pointers, please, on how I proceed from here?

And on a final note, could any owners of Volvo saloons that have a boot
spoiler like mine please confirm whether a) both the spoiler stop
lights AND the cabin stop light come on simultaneously, b) only one or
the other comes on, or c) neither come on!! Thanks.

 

The problem is in the stupid little fingers that press down from the lamp
housing onto the contacts on the rear deck. You'll probably find that the
fingers are burnt due to years of poor contact. You can polish them, you
can bend them, you can add conductive grease, but they'll still fail again.
You can buy a new light assembly from the dealer with shiny new fingers
that'll work for another 5 years for about $100.

....or, you'll find that the wire that come up to the rear deck contacts. has
exactly the same connectors as the ones that connect the lamp to the stupid
little fingers. The wires have just enough length that you can pull them
out a couple of inches and push them right onto the lamp body. Thus, with
no tools, you can bypass the whole stupid little finger assembly and never
have this problem again. Through the stupid little fingers in the trash.

The only downside is that it takes a little more effort to change the bulb
because of the short wires but your transmission will probably fail
catastrophically before you ever have to change the bulb again.

 

The problem is in the stupid little fingers that press down from the lamp
housing onto the contacts on the rear deck. You'll probably find that the
fingers are burnt due to years of poor contact. You can polish them, you
can bend them, you can add conductive grease, but they'll still fail again.
You can buy a new light assembly from the dealer with shiny new fingers
that'll work for another 5 years for about $100.

....or, you'll find that the wires that come up to the rear deck contacts has
exactly the same connectors as the ones that connect the lamp to the stupid
little fingers. The wires have just enough length that you can pull them
out a couple of inches and push them right onto the lamp body. Thus, with
no tools, you can bypass the whole stupid little finger assembly and never
have this problem again. Throw the stupid little fingers in the trash.

The only downside is that it takes a little more effort to change the bulb
because of the short wires but your transmission will probably fail
catastrophically before you ever have to change the bulb again.

 

Robert, you were exactly right. Problem solved. Many thanks for your
timely reply.

Stuart