Getting my M3 mirrors working

[Cheeseburger]

I bought an E36 328i with M3 style mirrors installed a couple of months back. The mirrors have never worked. Pulling back the door trim I can immediately see the problem - the factory loom comes with a four-pin header, the mirrors come with a three-pin Molex style clip... unconnected. Based on this i'm guessing they're el-cheapo Ebay mirrors and not OEM.

I'm guessing the three mirror wires are left/right trim, up/down trim, and ground. The factory loom must be the same but also a spare wire for the mirror heater. Is my logic on the money here? And can any damage be done with a trial and error approach to determine which wire controls which mirror motor? The three wire colours of the mirror are black, white and red. I assume I can work out the wires of the car loom with the mirror controller joystick, but if someone knows the actual colour codes i'm all ears.

[johnf]

Find a copy of the BMW E36 wiring diagrams (not the Bentley manual) and look at section 5116.0, POWER MIRRORS. The wiring provides up/left and down/right, and includes a wire that operates a clutch that chooses the axis. The mirror heating is mixed in with the motor control and is wired so that the heaters get power when the mirror switch is at rest.

It is probably easiest to use some relays to demultiplex BMW's four-wire scheme and remultiplex it for the three-wire aftermarket mirrors: some good old relay logic of the sort that was common before microprocessors.

[Cheeseburger]

Thanks for the reply JohnF. Here's the core of the schematic in question:





I build guitar amps as a hobby so I understand electronics to a point, but as soon as you use the word demultiplex my confidence goes out the window

The schematic is simple enough, but i've never worked with an axis switching circuit. And i'm not familiar with the two component symbols between wires #2 and #3... i'm guessing the first is a switch of some kind, but no idea of the rectangular symbol... nor is Google enlightening me.

Grasshopper has a lot to learn.

[johnf]

... I build guitar amps as a hobby so I understand electronics to a point, but as soon as you use the word demultiplex my confidence goes out the window....

Even so, you gave exactly the right word to solve the problem: demultiplex.

I assume your three-wire mirror has two motors with a commonly connected side. It that be so, I believe you will just need a DPST relay on each side between the BMW plug and your three wire motor (to replace the up/down clutch in the BMW mirror unit). The relay will choose which motor the switch should control. Here is a schematic of what I have in mind:



If this works, I think you'll owe me a draught of the local life giving product.

[Cheeseburger]

Where is the like button?

Thanks so much JohnF. I think that diagram has earned you a few pints at least. I'm very appreciative

I'll report back on the weekend when I have a shot at the fix.

[johnf]

Thanks so much JohnF. I think that diagram has earned you a few pints at least. I'm very appreciative

You are quite welcome. It will be fun to see how this turns out.

I was just thinking, you probably want to put a reverse-connected diode across each relay coil to absorb the back voltage when you switch it off, or use a relay with such a diode. I have updated the drawing accordingly. I am pretty sure BMW uses a diode to keep from wearing out the mirror switch. A diode is more effective than the resistor I had drawn, but slows the relay contact's release and worsens any contact arcing. To keep from bumping the mirror left or right, I suspect the switch contacts are arranged to first break power to the motors and then the pin 3, up/down clutch (which would prevent the arcing).

[Cheeseburger]

Well, I tried to get this done today, but finding a DPST relay proved a hurdle. I also ordered some convex mirrors which might allow enough visual range to simply set and forget. Well see. I'll get back in another week or so.

[johnf]

Well, I tried to get this done today, but finding a DPST relay proved a hurdle....

BMW sells this one but you might decide it is too big and expensive when you throw in a socket and contacts.

I had OSH Park make up a printed circuit board to mount the last relay I put in my car. They are quite economical for small boards but you may have to wait a bit if you choose their free shipping.