C4 Headlight Module Problems
 

Hi - This is the first thread for me, so I apologize for the length in advance. I have been chasing a problem on my headlights for 2 months now.

My car is a 1992 LT1 auto with out the daytime running lights. 2 months ago my right headlight started acting up, (jerking, part way up or down, etc) I rebuilt the motor as done by many of you here. When I test both headlight motors with jumper leads to the battery, both headlights will rotate up and down, depending on the polarity of the jumpers on the 4 pin plug that plugs into the headlight control module. This tells me that the circuit from the module to the motors and the motors are OK.

Here is the problem: after the repair of the right headlight motor, the only headlight that would rotate up/down was the left light. Both of the lights turn on, but the right one won't rotate.

I have the factory service manual (ST-364-92-2) and ran through the headlight module testing charts with the result that the 5 wires going into the module are operating as they are supposed to. In summary:
1. the black ground wire has continuity and low resistance to ground.
2. Both red wires have 12 volts on them all the time.
3. The white wire coming into the module (from the light switch)has 12 volts on it with the headlights off and no voltage with the lights on.
5. The yellow wire has 12 volts on it when the headlight switch is on, none when off.

So, the manual says the module is bad, and I ordered a new one and installed it. As soon as the new module is plugged into the harness, the headlights come on without turning the headlight switch on. When the headlight switch is turned on, the right headlight rotates up, the left doesn't rotate, and when the HL switch is turned off, neither headlight rotates or turns off. (WTF????)

This was strange, and I rechecked all the wiring tests on the car per the manual, and again all the results were correct for the wiring of the car into and out of the module. I concluded that the module was bad and took it back for a new one.

Long story short, the second module did exactly the same as the first one. I am wondering about something: My manual has a statement in it about connecting the headlight module with the battery connected and the switch in the off position. It says, "CAUTION: Do not plug in headlight control module unless battery is connected and the headlight switch is in the "OFF" position." This warning is repeated at the top of each testing chart page for the HL module.

I read in this forum that other people recommend the battery be DISCONNECTED before plugging the HL module. What is the right way? Did my method of following the manual cause the problem? Are the new parts the same as my old one? Old PN 1651037, this was a different number than on the new module. I am getting a third module tomorrow, and i will connect this one WITHOUT the battery connected. I don't think this will work, but I'll try it anyway.

What is going on here? HELP, my patience is wearing a little thin at this point.

Mike

I replaced mine w/battery connected, switch off and both lights closed and mine works fine.

I had a problem caused by a headlight wire getting pinched between the hood and the headlight cover.

Mike,

My FSM (93LT1) has the same statement about having the battery connected and the switch in the off position.

I replaced mine with the battery connected and the switch off and didn't have any problems. In my case, it was a bad headlamp motor causing the problems. The 1'st module I bought shorted out when I turned on the lights. I bought a second module along with a new right headlamp motor and the problem was solved..

It doesn't make any sense to me either. I don't know why the headlamp would turn on with the switch in the off position....
Maybe a bad headlight switch? Short somewhere?

Man, I don't know...:lurk:

Well I tried a 3rd module today and connected it with the battery disconnected. The result was the same- as soon as I plugged in the module the lights came on with the HL switch off. That means it doesn't matter which way you do it. My HL switch tests good by the book and with the old module plugged in, the right headlight lights, and the left one lights and rotates when the headlight switch is turned on. To me it looks like something has been changed in the new modules that no one at the dealer or the people he called at chevrolet knows about or will admit to. My frustration rises- - - -

I read the codes on the CCM and it said C12 - no problems. Has anyone got any ideas?

Mike

Well, I fixed the headlights with a modification I really did not want to do. I wired both headlight motors to the left headlight module circuit, and both light motors work perfectly. The HL modules have a function that senses the current draw on the motors, and when the motors stop their travel either way, the motors are automatically stopped. I figure that if they work wired this way, the modification is safe and there probably wont be an overload failure of the HL module circuit I am using.
I haven't been able to verify this, but I am positive that there is a difference in the new headlight module and the one that came with my car in 1992. I can't get Chevrolet to admit this, but it is the only thing that makes sense, as all circuits on the car work perfectly into and out of the module. Since the only interaction with the CCM is the control for the high beam indicator, and since there are no codes applicable to the headlight module, (and no codes thrown) the car circuitry is right and the module is different for the 1992 vettes. I hope no one else has to deal with these issues, but if you do, PM me and I'll see if I can help. I suspect the 1993 and later cars will probably work with the current module part number.

Mike