I have an 87 firebird and my headlight control module went out. I also have a 94 formula, I looked at the control module on the 94 and it looks the same as the one in my 87. So I came across one for a 93-97 for cheap and tried it in my 87, but no go. Does anyone know what the difference is between the two? They look identical from the outside and have the same 5 pin connector on one side and 4 on the other. I know the one I just bought works because I hooked it up in the 94 and it worked fine. But it will not do anything when it's plugged into the 87. I know that it is what actually failed because when it first started messing up I could tap on it and my headlights would go up. But it just stopped working one day, and tapping on it stopped working to. Thanks in advance for your help

As far as I know there is really no different between them at all since the part number for those are the same. Actually it's the same part number all the way up and down the years that use it as far as I can find.

Are you sure though that it's the module itself that's the issue and not the pigtail connectors that hook up into it? I've seen cars where it seemed like it was the HCM that was the problem but when a different one was used the problem was still there and it ended up being the wires where they went into the connector for it that finally ended up being the problem (the wires in the pigtail connector were bad). The person changed the pigtail connector (had to splice the wires in to change it) but after they done that no more problems.

Well, the part that is confusing me about it and why I'm asking the question is that I can take the old one and plug it in and it will lift the driver's side light, and the passenger's side motor will twist a little bit but not turn. I know the motors function fine as I have manually made them work to verify them. But when I plug the one that I bought in, it does nothing at all. Then, like I said I took that module and plugged it into my 94 and it worked fine. Put it back on the 87 and again, nothing at all. I don't hear the relay in there click or anything. I will definately check the wires out tonight after work. I hope that it is that simple, it would make my day because I hate having to drive around with them up. Thanks!

That is weird. Just to verify that they are the same I took the ones from my 97 and 99 and put it into my 87 and they worked fine in it and I took the one out of my 87 and put it into my 97 and 99 and it worked fine in them so I'm not sure why that one is working in your 94 but not in your 87. Have you tried putting the one from your 94 into the 87 and seeing what happens then? But hopefully it is just something with the pigtail connector and you can get it fixed without much trouble.

As far as the drivers side coming up and the passenger side not doing anything but twisting, I'm wondering if there might be a problem with the headlight motor itself on the passenger side, which there could be with the age of the 87. Also as far as checking them out by manually raising them, that really doesn't mean anything since when you manually raise them you're actually bypassing the electrical part of the headlight motor that causes them to raise up when you turn the headlight switch.

I checked the motors by applying power to them and reversing polarity to check them. That was how I ruled that out. But since you are saying that you interchanged yours and they worked, that is leading me more toward the wiring harness now.

Okay, so curiousity got the best of me this evening. I resistance check through both connectors and I have no resistance through them, so they are all good through the connector. It still didn't make sense to me that it wouldn't do anything, so I took both of the modules apart. The one on the 87 uses a relay with a 12V coil and uses 3 standard 3 pin transistors to determine bias in the circuit which is used to stop the motors when the light has bottomed out. Also, the relay is actuated by the yellow wire (which is hot when the headlight switch is on) through ground which is the black wire. The one from the 93-97 era was disassembled and I found that it uses a 9V relay coil. It isn't actuated in the same manner as the other one. No specific pin directly links the coil. Current passes through several resistors and then goes into a chip on the circuit board. Without a schematic of this board I am unable to know how the current is then passed through the chip. So scratch my head on that one. The other thing that stood out in my mind about that board is that it unlike the other one only uses 1 of the 3 pin transistors. At that point, I decided I wasn't going to be able to just look at this circuit and get an idea how it was working without a schematic.

So looking at both of these circuit cards, I guess it is safe to say that at some point GM revamped the headlight control module.

I think instead of spending more money on another one that might not work, or buying a new one for a couple hundred dollars, that 2 5pin relays and a 3 position return to center toggle switch will be controlling my headlight motors. I drew up the schematic for it tonight, and just need to go buy some cheap parts tomorrow.

Cool on how you checked the motors out.

Hum, I'm wondering then why mine worked all around then. Maybe it has something to do with the build date of the cars. I know my 87 has a build date of July 1987 so that makes it one of the very last ones made for that model year, so maybe that makes the difference.

But good luck getting the parts for the schematic you done and getting your lights to work again.

Thanks for all of your help trying to figure this one out! Hope I can help you out the same if you ever need it.

You're welcome. Sorry I wasn't of more help though. Thanks for the offer of help in the future, much appricated.

The 87-89 module is different than the 90-2002 module. I don't know why Sues is working for both cars because it shouldn't. I suspect somone re-pinned the plugs that plug into the module on her 87.

Lon