bsmutz

Went to start my car the other day and my battery was completely dead. A little investigation turned up a short in the courtesy lamp circuit. When I looked at the circuit in the repair manual, I thought that it was just the lamps and the antenna motor, so I jumped to the conclusion that it must be the antenna even though it seemed to be working fine. I didn't do any repairs or further investigation, just recharged the battery and let it sit for awhile to make sure there wasn't anything else going on. With the fuse out for the courtesy circuit, I hopped in to go to work this morning and found that there are other things on the courtesy lamp circuit that I was unaware of. Apparently the whole temperature control circuit is on there and a couple of other things. Anyway, the gist of this post is that I noticed there was no sound from the stereo which got me to wondering if the amps for the speakers are on the courtesy lamp circuit. I've been having problems with the left side shutting off, squealing, etc. and am now thinking that perhaps the short is in one of the amps. Thanks!

Mike mike

What year C4?

bsmutz

'91

jfb

Shorts blow fuses, fusible links, or cycle circuit breakers. Only one circuit in your 91 has no fuse and that is the wire that goes to the starter. You don't have a short, you more likely have excessive leakage current. Put the courtesy fuse back in and at night check the underhood lights, vanity mirror lights, door map lights, center console light. Don't leave your key in the ignition because it keeps the theft detterent circuit on. Do you have a radar detector, CB transceiver, aftermarket alarm or radio or audio amp? Check em. GM says leakage current should not exceed 50 milliamps. My 87 draws 27 ma. You can measure leakage current by connecting an ammeter in series with the disconnected negative cable to the battery. Wait for the courtesy lights to time out and switch the ammeter down to where you can measure the current. Pull the courtesy fuse so you can keep the passenger door open and start pulling fuses one at a time while watching the ammeter. I believe on the 91 there are some fuses near the battery, pull them too. Let us know what you find.

black96lt4c4

ill take a shot.id like to know how he found a problem in the circuit he described.he evidently has something staying on which is draining the battery voltage.the measurement of current in series with the battery cable is textbook for finding a current drain.if the current drain is outside of specs,pull fuses one at a time to isolate the drain circuit.a blown fuse indicates current above the fixed fuse amperage causing an open in the power supply circuit.simple terms,a blown fuse usually indicates a short to ground,break in circuit,and with no powersource cannot cause a power drain.more info needed.but ill say again the measurement of amps in the batterycable is right on.good luck.junior

bsmutz

Okay, thanks for the replies so far. I measured 0 ohms to ground on two of the wires on the junction block. One went to the underhood lights, which is expected. Couldn't figure out where the other one went, so I started pulling fuses. The short went away when I pulled the courtesy light fuse. I blew the fuse in one of my amp meters when I connected it in series between the battery terminal and the negative lead. I didn't want to blow the 2nd one, so I haven't measured the leakage current yet. There is a 10 amp fuse supplying the current to the courtesy lamp circuit. It is not blown so I think I must be under 10 amps which leads me to believe that there is something limiting the current to under 10 amps on the other side of the fuse. Everything seemed to work normally when I had the car running with the short/current leak doing its thing. I didn't leave it on long, just enough to park it in the shade to work on it. I originally suspected the power antenna motor but ruled that out as it is working correctly. I also suspected the radar detector that I installed getting power from the makeup mirror light, but it is still working fine, too. The only thing that hasn't been working right is the speakers. If I put my left foot forward under the brake pedal or against the speaker housing, the left channel will quit working. Sometimes I'll get the high pitched squealing from that side, but mostly it's either on or off. If I move my foot again or kick the speaker housing, it will come back on. Also hitting a bump or driveway will cause the left side to go off. It almost seems like there is a wire running under the carpet just below the brake pedal that is affected by my foot pressure. I did not see any wires running through that area last time I had the speaker housing off and pulled the carpet back, however.
Good call on checking the various courtesy lights. I'll try that tonight and see if any of them are behaving strangely.