Bear with me, this will be one of my lengthier posts, but lots of good info for E28s; for TL;DR folks, just skip to the last photo for what connectors go where.
I've been putting my M5 back together after being in a mostly disassembled state for many years (garaged), and have been tackling getting the instrument cluster cleaned up, installed euro telltale and warning indicators, etc. Of course I neglected to label what went where when I pulled the cluster years ago, so here we are in this thread (among many)
I located a very similar post by the OP over on bimmerforums and pulled his images off that site; these are them.
This last photo of his, based on my hours of research on the wiring diagrams and testing with a multimeter, is mostly incorrect. If you hook up the edge connectors how he has listed, the cruise control will not work. There is only one spot on the US spec M5 (and likely later model 535s) that has terminals to hook up the edge connector with the two blue/yellow (Bu/Yl) wire on terminal 2 and the single blue/brown (Bu/Br) wire on terminal 1; this spot on the back of the speedometer is know as connector C8 and it is shown in the photo below. You will also notice, the other sections of circuit board to the left and right of C8 have no terminals, so even if someone plugged one of the other edge connectors in to one of the bare spots, nothing would happen.
Per the BMW wiring diagrams and my testing, the only edge connector that should be plugged into the back of the speedometer is the one for the cruise control, discussed above. However (this is where it gets weird), on my M5 the cruise control edge connector (2 Bu/Yl wires, 1 Bl/Br wire) was factory cable tied to the main harness "trunk" and there was no way in hell it had enough slack to plug into the C8 connector on the back of the speedometer. But when I last drove it (and I've owned the car since 2008, and made two cross country trips), the cruise control worked perfectly and so did the on-board computer (OBC). Also if you look closely at the photo above, only the number 2 pin of C8 showed any wear (or evidence of the edge terminal sliding across creating the parallel striations in the contacts). That's weird. That would mean one of the other three edge terminals with only contacts on the number 2 side must have been plugged in. So I pulled out my multimeter and started checking continuity with the Bu/Yl wires (terminal 2) of the cruise control edge connector. Turns out the edge connector with the single Brown/Red (Br/Rd) wire has direct continuity with the Bu/Yl of the cruise control edge connector. Pin 2 of C8 is the speed signal (see wiring diagrams below), pin 1 is a ground and goes to the
cruise enable pin of the cruise control computer. What I can't figure out is
how the cruise control worked all these years without it? I thought somehow the cruise control computer must be getting this
cruise enable ground signal from a "back door" source, but none of my testing proved that. So that left me wondering what to do? I've ended up cutting the cable tie to release the cruise control edge connector and plugged it in to C8. Per the BMW wiring diagrams this is how it should have been and in a month or so the car should be back on the road and we can test it out.
So to clarify, for the US spec M5 and likely later model 535s, the only connectors that are plugged into the back of the instrument cluster are the large blue, yellow, and white connectors (should be self explanatory
), the ABS warning light (green wire, gray/red wire, bulb), and the cruise control edge connector (2 Bu/Yl wires, 1 Bu/Br wire). Nothing else gets hooked up.
How I found the cruise control edge connector cable tied to the main harness
Cruise control edge connector and Br/Rd edge connector
Wiring diagrams (there are more, but these show most of the important parts)
Final connections (corrected version of OPs photo above)
Standing by for questions or comments.
Not sure what happened. They worked when I first posted it, and this seems to be my only post with this issue. I'm just going to repost.