Help with Radio Dimmer problem

Hi gang, I'm new and I was wondering if anyone can help me out with a wiring problem. Let me preface this by saying that this is a car that had been totalled and restored. It is a '96 Cutlass Supreme and the entire dash had to get ripped out to do some bodywork. My Dad did the work for my Mom (the car is hers) and I'm trying to make things work the way they should. The problem is that the radio lights shut off when the lights are put on. They are there when the headlights are turned off though. I've done some work already and I need some wiring diagrams to figure this out to see how everything is wired. Here's what I've done so far... 1) checked the wiring harness to make sure that power was there when the lights are off. Check! 2) found a brown wire that changes from about 5v to 12v as the dimmer switch is moved up and down. Check! 3) Used the same ground wire of the plug when doing the testing. Check! This made me believe that the radio was the problem. I replaced the radio with one bought from Ebay and the exact same problem is there. No, it IS possible that the radio from Ebay is also faulty but I highly doubt that even I could be that unlucky. Anyhow, I don't know how to track it down from here because it is hard to find wiring diagrams that cover the power distribution to things like the headlight switch to the dimmer and back to the radio. Any help would be greatly appreciated and feel free to PM me. TIA Nino

Run a new ground wire direct to chassis. If you have a meter set it to dc volts. Disconnect the radio. Meter the ground wire that you are connected to. Turn on the lights, meter the line again to see if it changes. Do not assume that a brown or black wire is always a ground wire, I just fixed one of those assumptions yesterday. It sounds as though the lighting control is completing the power supply circuit for the radio.Top Secret, I can tell you but then my wife will kill me.

Hi Forbidden, Thanks for the reply. Let me provide some more details. I found a black wire that was, what I thought, a ground and I used that wire (back of connector) to check the yellow & orange wires which showed 12v. Then I checked the brown wire which showed 0v. I then turned on the lights and checked the brown wire, using the same ground wire, and measured a varying voltage from 5v to 12v so I assumed that the ground must be o.k. along with the brown. I've heard/read somewhere that GM DOES switch polarity on these wires on some cars and I haven't found anyone that has told me for sure whether this car does it or not and if so, what wire and how. I'm still thinking that the black was the ground based on my testing. The lights on the radio work fine with the headlights off but don't work when the headlights/parking lights are turned on so I'm a little confused by... " It sounds as though the lighting control is completing the power supply circuit for the radio. " Could you clarify that a little more for me?

Have a look at www.installdr.com for a wiring color code for most GM vehicles. Compare it to what you have. To complete a circuit something must change, either it completes a circuit by adding a ground or voltage to a wire. Something works fine, like your lights on the radio, the circuit may be complete here, when you pull out the lightning control, the circuit path may be incomplete. It does not necessarily have to be on a ground wire, it may be on an igniton line, or variable voltage line like the lightning control circuit. Some vehicles that have daytime running lights or a lighting sensor circuit also provide a voltage on wires as well, it really depends on the vehicleTop Secret, I can tell you but then my wife will kill me.

Nah, I didn't mean "how does a circuit complete" but more specifically what part of turning the lights on completes what part of which circuit. I agree with what you said but I've checked the wires and they "seem" alright even though they aren't and it isn't the radio. I'm hoping someone can get me in touch with the wiring schematics for the power to the lights, radio, and how it all ties into the dimmer. That way, I can figure out what is SUPPOSED to happen when the dimmer comes on and why the lights don't. I should also mention that the radio DOES work when the headlights are turned on but all the lights shut off so you can't see what's going on with the channel/clock. So I know that the bulbs work but there is some switch that isn't switching or something like that.

Is it an aftermarket radio? did you try disconnecting the dimmer wire to the radio? Buy it, install it, play with it. Then take it back out, read the instructions, install it better!

No, stock radio.

The dark brown wire is the parking light circuit in the car. There should be a 2 wire plug on the back of your radio with a brown and orange wire in it. The brown goes to the brown and the orange goes to orange (12 volt + battery for radio's memory) The brown wire shouldn't vary in voltage when the parking lights are turned on. It should go to 12 volts + and not vary with the dash lamp dimmer. Check carefully as there are two dark brown wires in the radio's harness. The other dark brown wire is the left rear speaker's positive wire. There are two gray wires in the radio haraness. One is for the left front speaker negative circuit and the other is for the dash lamps. This lamp circuit shows 12 volts + when the parking lamps are turned on and varies between 0 and 12 volts depending on the position of the dash dimmer. The gray lamp circuit wire goes into the black plug with the yellow ignition power and the black ground wire. The way the circuit works is the radio detects 12 volts on the dark brown wire when the exterior lights are turned on. The radio then partially dims the digital display and references the voltage on the gray wire as to how dim the digital dispaly should get. The backlighting bulbs inside the radio use the gray wire for power to illuminate the radio's faceplate labels. If the radio gets power on the brown and none on the gray wire then the radio's display will blank out when the lights are turned on. Check the wiring and the headlight switch if your results are wrong.sparky

Thanks guys and thanks akimball442! I'll look into this the next time I'm home which is March for spring break and hopefully that is it. That would be great! I didn't check the grey wire but the brown wire did vary it's voltage as the dimmer moved. Can I ask you to send me the scanned images? That would be awesome!