850 Dimmer Switch

[scot850]

I will chirp in here to add to what the guys are trying to say to you.

Do the dimmer switches fail? Yes all parts can fail in an older vehicle, but the likelihood of you pulling 6 bad ones in a row is highly un-likely. Personally I have never had one fail, so the fact that you have either 6 fail once you installed them then means one of 2 things:

1) You have a wiring fault, and the point the guys are saying that your vehicle appears to have experienced more than what is normal. This is possibly due to someone jumping the car, but possibly reversing the polarity of the cables. This has been known to fry many items in the electrical circuit, from melting wires to shorting relays out. Based on the assumed failure rate of replacement parts I'd suggest this is where you have an issue.

2) How did you test the parts prior to installing to ensure they were good? To add to that, have you re-tested them following installing into your car and found that they have failed or shorted internally? Assuming this has been done, it would indicate an issue further up the circuit causing the issues like a wiring short.

3) Have you metered the wiring to the dimmer switch to make sure you have even got power going to the switch in the first place?

This is the information we need to be able to try to help you to diagnose the car from afar. Electrical repairs are not my strength by a long shot, but diagnosing issues is what as an engineer I did for many years.

So what we now need is more detail on what you have done, and your test methods so we can help you move on from the frustrating point you have gotten to. We have probably all been there at some point on our own cars so have the bumps on the foreheads from banging the heads against the wall!

Good luck!

Neil.

[BEJinFbk]

Here's the deal. Most cars have a simple rheostat for a dash light dimmer.
A power wire in, a variable resistor with a wiper and variable voltage out.
Your 850 dimmer is a much more sophisticated electronic voltage regulator.

In either case, they both get their marching orders from the headlight switch.
You turn on the lights and the headlight switch passes power off to the dimmer.
If there's no hand off from the headlight switch to the dimmer - No Dash Lights.

In the case of the Volvo dimmer, sometimes the regulator inside dies, but to
plug in that many more and STILL not find a good one? You are running waaaaay
up against the law of averages. You've got another problem farther up the line...
I'm 99.9% sure that your "Bad Dimmer" is just a symptom.

Considering the laundry list of electrical problems that you've posted in the last week
or so, it may be time to pony up some bucks at a good shop. An older stereo shop in
the Bay area may be your best bet. Find one with an old guy that's been around the
block with P80 Volvos. Maybe check with the guys over in Alameda at C-5 for a referral.

[erikv11]

What they said, Neil and BEJ.

no, you misunderstood. My car had a bad dimmer switch, apparently they fail after some years of use. I harvested another five from pic n pull cars -these too were bad -

No misunderstanding, what you describe is 6 bad ones in a row, that is exactly how we are interpreting it. We know they sometimes fail, but not 6 out of 6. The majority of the pick-n-pull ones will be just fine.