Electrical

Dome Light Short Circuit

Here's one for the record books!

On an '86.5 928 (mine's a Euro 5sp), if the front center dome light ever shorts out killing the fuse
in the fuse panel, you can expect the following weirdness:

1) The digital clock in the center console will fade out and loose time.

2) Liters/KM or Mi/Gal gauge in the instrument cluster will stop working.

3) Door locks will not work right and exhibit VERY weird behavior (up down up down up down,
etc.)

4) Most likely, your car alarm will stop working correctly as well.

5) Of course, no other interior lights in the car will work.

All because of an intermittent Dome Light short that eventually led to the blown fuse!

I had an electrical short in the front dome light that killed the fuse and kept killing it. I couldn't
figure out what the deal was until I stuck a 30 amp fuse in the panel and watched the small puffs
of smoke emanating from between the sun visors (not a debugging technique, I just ran out of
the proper size fuse). I quickly removed the fuse and them removed the dome light to find that
the plastics had broken allowing a ground wire to touch the hot wire in the light, BUT ONLY
INTERMITTENTLY(mainly when I accelerated)!

I repaired the wiring, replaced the dome light fixture, replaced the fuse (with the proper size) and
all is well with the car now!

I would have NEVER thought that these items would be related!

Regards,

Bert Garcia
'86.5 928 Euro, 5sp



I recently replaced my interior ceiling lights, which had been bashed in by the PO. After putting
them in I tried them out, and quickly blew the interior light fuse. Upon checking the fuse box,
the fuse I blew was in fact a 15 amp fuse, while the manual calls for only a 5 amp. So I assume
this fuse had blown a few times before and the PO kept putting in higher current ones
(incidentally, for those who didn't know - this is a really bad idea!).

After some poking around and a few sparks flying, I think I now know why the fuse blew. There
is a rather easy way for the ceiling lights to short out. The ceiling lights have three wires
connected to them - brown, brown/white stripe, and red. The red one (permanent battery
voltage) connects at one end of the light, and part of the metal connection extends out through
the plastic of the light on that end. This is just above the metal spike on that end (one on each
end) that holds the light into the ceiling. It appears to me that these spikes are supposed to
push up through part of the metal ceiling, "locking" the light in place. The ceiling of course is
grounded. Now, if your light is not pushed all the way in (mine weren't when I bought the car,
and I couldn't quite push one of them all the way in, still haven't figured out how to get it up
there), then it is possible for the metal that connects to the red wire (and therefore the battery)
to touch up against the grounded metal ceiling, giving a rather inconvenient short to ground.


So, if your interior light fuse blows, this would be a good place to look. Also, when replacing
ceiling bulbs or light fixtures, make sure you push them in all the way. Incidentally, I haven't yet
figured out the trick to pushing them in, as brute force (grunt) doesn't seem to be the answer. At
least on mine, although that could be due to the ceiling leather/vinyl/whatever shrinking
around the edges.

Mike
86 928S 5-spd