The stereo's connected to the... taillight?

My 1990 240 has been acting oddly.

Specifically I had an aftermarket stereo installed at Best Buy about 3
1/2 years ago. A couple of winters ago it started cutting out- the
sound would stop, the display would go out and eventually it would
come back on. It would lose the radio presets and the clock would not
keep good time. It seemed to be random. It would happen if the car
was parked, sometimes would happen if I turned right, sometimes
happened if I hit a bump.

I haven't had time to take the car back to Best Buy to have them check
it out, but have assumed it's a problem with the wiring, specifically
the power supply. (I know what you're thinking- "two years you've put
up with that? You slacker!" True.)

Three days ago I noticed an intermittent "light out" indicator on the
dash. I traced this to the right rear taillight. Wiggling the socket
rendered the light operative and I noticed as I got back in the car
that the stereo had cut out and was just coming back on. "Huh"
thought I. Well, ever since then the stereo has been performing
perfectly. The clock keeps time, it doesn't cut out.

My question is: WTF? How does an intermittent connection in the
taillight affect the stereo? Does this indicate bigger problems?

Thanks in advance to people who actually understand car wiring and
elecctricity... I am not one of them.

 

If you have corroded fuses in the fusebox you'll get all sorts of weird
electrical gremlins since voltage will sometimes feed back through
unexpected paths. Loose or corroded ground connections can cause wierdness
too.

 

Some stereos have a connection to the taillights for dimming the display
when the lights are on (on the theory it will be dark when your lights are
on). Exactly how this relates to your problem I sure don't know. You slacker
;-)

Mike

 

I'll check the fuses more closely- they looked OK when I looked at
them but it was fairly cursory for the most part. I really only
checked the one for the stereo closely.

Hmmm, ground connections... I popped the stereo out of the dash to
check to see if there were obvious problems with the wiring of that,
and I did see that the negative ground wire for the stereo wasn't
connected to anything. Could that be the culprit? What should it be
attached to?

However, why would the taillight affect it (assuming that's the
cause and it's not just coincidence)? Nothing else in the car seemed
affected.

 

Interesting. How common is this?

Sad but true...

 

The backlighting on the radio in my S70 is bright when the headlight
switch is all the way to the left (daytime). Any other position, and
it adjusts to the bright...dim dash light switch.

 

Yes that most certainly will cause a problem, assuming you're *sure* that
wire is the negative, connect it to ground, there's some screws with other
ground wires on them behind the center console with the switches and heater
controls, just put a crimp-on hoop connector and stick it on one of those
screws.

 

The ground should definitely be stuck under a screw in the metal part of the
body. Relying on the mounting to provide a ground is bad form.

Mike

 

Pretty common - I'd guess about half the models available do that.

We should form a club, maybe have jackets!

Mike

 

Well, the wire has a pre-printed label on it that says "negative
ground" so I assume that's what it is. It's got a "C" terminal on the
end of it.

How do I tell which is the screw with ground wires attached to it
(just so I don't screw it down to something else and fry things)?

 

Wow. Who knew? Not me, obviously!

I'll get around to that when I have time.

 

If it is the body of the car (usually painted the same color as the
exterior) you have the right place. Scrape away any paint under the screw
first, even if there are other wires already there.

Mike

 

Any screw that goes into metal attached to the body of the car will work,
it'll be fairly obvious when you get in there.

 

Thanks!

 

Simple enough. Thanks!

 

My "new" 89 780 has the old Sony cassette/equalizer setup, and just
has a couple of indicator lights on, very faint high frequency noise,
thought i actually heard a radio station it today, but have a wiring
issue some where in the setup.
(Car had water damage previously, and just now getting into the stereo
issue, since I need sound other than the usual and unusual mechanical
sounds I here)

I did notice that the wiring diagram for mine shows this dimmer wire
connection, too.

Although I don't think that's my particular problem, it is interesting
so see that feature.

Alas, I will probably opt for a good aftermarket stero even if I get
this one working, since my cassettes are pretty dusty these days and
cd's are so much better sounding.

Wlil keep the original stereo and 2 Sony amps in case I want to
replace them as original equipment in the far off day I actually sell
the black beauty of a Bertone.

This vehicle definately has some extensive wiring harnesses running
all through it I am discovering!


Good luck with yours. I'd always check the voltages into the radio
before going further, should be I think a constant 12V. line (green),
another 12v. line that's on when the key is on (red) and of course the
grounds, which are usually black in most 12volt negative ground
systems.

A cheap volt-meter can save you some money over the long haul.

Of course a really good meter will last a good long time....