Day I dread follow up and taillight issue

As I posted earlier, my 86 740 GLE, which I've owned since new, (350k miles)
was rear ended. From insurance standpoint, totaled. After much back and
forth, got enough to repaint the car (only MAACO), and buy a porta-power to
do the body work prior to painting. (And to cover the costs of replacement
taillights both sides, which I had on the shelf from an earlier e-bay
purchase). About six hours work for son-in-law and I, and we were done. Car
is finished, and looks quite nice.

However, the car failed state light inspection (required when reregistering
a "totaled" car). The brake-taillights, passenger side, where intermittent.
No problem, thinks I. Probably too hasty in my reinstallation following the
painting. After 2 hours, I couldn't isolate a ghost ground. The brake
lights were grounding through the taillights. After fighting the problem, I
threw a diode in the taillight lead, and VIOLA!!, problem solved.

So now Victoria, our 740, will be good for another 350k (or at least outlast
our S80!!)>

 

Maaco paint turned out looking nice? That's a shocker, I've heard nothing
but horror stories about them. Paint is one area where I absolutely will not
cut corners, a cheap job that turns out crappy or doesn't hold up well will
only cost more in the long run to clean up and repaint than using good paint
in the first place.

 

Run dedicated ground wires for each light then, if you can... I've looked at
the taillights on my 960 maybe twice, and I didn't get a good look at how
the power rails were done, but it looked like it was possible from what I
can remember... A 960 wagon has the same taillights as a 740, so it should
be similar. I remember metal posts, where spade connectors could just plug
right into (had to fix the taps for our trailers wiring harness...
indicators weren't working, one lead fell off after 11 years) - you might
have to cut the common negative rail, if there is one... Check connections,
and check for appropriate signals from the positive wires first though,
could just be a bad connection. (Check for a constant 12V for the running
lights, a flashing 12V for the turn signal, a solid 12V for the brakes, when
applied, a solid 12V for the reverse, when in gear, and a solid 12V for the
fog lamp/s when on).

I wouldn't use a diode to prevent a problem like that - because it means
there is an actual problem with the wiring - Sounds like one wire is
damaged, so the current is tracking back thru another one of your lights to
find a path to "ground" (there's no real ground on a car... it's all direct
current... positive and negative... Volvo's should be chassis <ground>
negative)

 

In a 740 sedan (is this a sedan or a wagon?) the taillights ground with a
ground lug stuck on one of the studs that holds the taillights in. Oxidation
here is common with an old car, it's certainly possible to ground each light
straight to a point on the frame inside the trunk or you can get some of
those washers that bite into the metal a bit to improve the connection. The
plug and connector where all the wires plug in is a bit of a weak spot too.

 

Those are some good idea's (washers, cleaning the plug connectors).

Dirt and corrosion aren't good conductors, so i'd eliminate those problems
first... Usually it doesn't take much to fix an electrical problem.

 

Thanks. I actually did the "normal" troubleshooting (i.e. cleaned
connections, etc.). I even swapped out the wire membrane on the taillight
assembly. After 2 1/2 hours, I took the coward's way out, and did the
work-around with the diode.

Thanks for the tips.

 

At least it's a clever solution that took knowledge of electricity .