teardowns, reverse engineering and custom projects
Howdy fellas! Welcome and thanks for visiting the DIY Engineering Blog. This is our first post and we are glad to start off with reverse engineering this interesting part.
Today I want to give you some detailed information, especially photos and schematics of an old VAG low-beam height control module (part no. 4A0 941 295), which I salvaged from a first generation Audi A6 Type 4A (C4). It was made for the german market in 1995, so it might differ in other countries. Feel free to leave a comment below, if you happen to have a different version in your car.
To get these modules out of the headlight housing, you just twist the whole thing after removing the electrical connection. Then you can pull them out of the housing. To take it apart, just use a flathead screwdriver to open the clips. Now you can pull it apart gently. Be careful, there is three rubber spacers in between the housing and the circuit board, which tend to get lost (as they already are in the picture below).
Before removing the circuit board, you want to make sure to carefully remove the copper retainer which you also can see in the picture above. I do it utilizing the small flathead screwdriver again as a lever against the PCB.
The copper retainer moves up with the white cogwheel, which it is attached to. It is on the same axis as the metal shaft who is responsible for adjusting the height of your headlamp. So, when the cogwheel moves up, it will push the feedback potentiometer upwards. When it moves down, the copper retainer will pull it back down. When you reassemble the device, you have to make sure the tiny plastic potentiometer slider is placed in between the cogwheel and the retainer. Therefore I push the slider down all the way and then put the retainer on top of it.
Once you got the retainer out, you can just pull out the PCB.
Since in my device the control chip was broken, I desoldered it and put a 16-pin socket in there. I ordered a pack of two chips from china, of which unfortunately just one was working, the other one got incredibly hot. Probably not as genuine as advertised… so be careful what you buy.
Furthermore, I took apart the potentiometer and cleaned it with alcohol. Afterwards I gave it a little spray of contact oil to keep it from wearing out. Also I replaced all the caps in the first device, but after measuring them I realized you can leave the yellow and grey ones inside, since they are still ok. If anyone knows what kind of caps the yellow ones are, I would love to hear in the comments. The electrolytic ones of course had to go.
The motor and gears looked alright and I just gave them some new lubrication after cleaning. Since I didn’t have to look for replacement, I don’t know about suitable motors which are still available, but for this kind of small motors Mabuchi often offers something similar. Feel free to leave a comment if you found something.
Of course I also took the time to reverse engineer the circuit. There’s one for the selection potentiometer, and one for the motor control circuitry. They used quite odd resistor values. I assume, that’s for copyright reasons to immediately spot someone copying the exact same design. For replacement, similar standard type values will be ok aswell.
At first, here’s the height selection potentiometer:
And here the motor control circuitry: