For between 2 & 3 years I have been dogged with a failing speedometer in my 1992 740. A year ago it stopped working completely. The speedometer, trip gauge and odometer all stopped working at the same time (and pretty much in tandem). Since then, I have spent quite a few hours troubleshooting the problem. The FAQs on this site have been very helpful in leading me to the conclusion that the problem lay in the speedometer, not the sender in the differential, and not the wiring.

There is a separate section in the FAQs here entitled "Speedometer Repair". Gory details of speedometer disassembly, circuit board cleaning and capacitor replacement are described. Intimidated, I decided to throw money at the problem. First I got a 1989 740 not realizing it is completely incompatible with the 1992. Then I spent $140 at Everdrive.com to get the right one. 3 weeks later a speedometer cluster arrived in the mail -- sadly as nonfunctional as my own. I called Everdrive and they seemed quite accommodating (at that time) and said a replacement had been located and was on its way. Put the junky thing back in the mail and didn't hear anything more for 2 more weeks. Then, smelling a rat when the Everdrive telephone line did not work and the Everdrive website was down I googled around and discovered they had indeed gone out of business. Hmmmm....

So then I revisited the "Speedometer Repair" FAQ, went down to Radio Shack and got an electronics toolkit for $16, a desoldering tool ($10), and some De-OxIt ($15). I used method 2 (involving careful removal of the speedometer needle and de-soldering 2 rather easy blue wires from the microswitch to the board) to separate the circuit board from the mechanical works and expose the troublesome capacitors which tend to leak and eventually need to be replaced. (It's all explained in the FAQ). Sure enough, between capacitors C6, C12 and C7 and the 2 integrated circuits the board was discolored and some of the pins on the chips were black and gooey. Sprayed on the De-OxIt a few times and rubbed off with Q-Tips to clean the area. Also used toothpicks to scratch off the crap.

Anyway, long story long, I reassembled -- and the speedo, trip and odometer all appear to be working. Not sure what the moral of the story is, but I've been wrestling with this for a long while, it seems to be a very common problem and I thought others might be interested to know that all that might be necessary is a bit of De-OxIt and some patience (I didn't even need to use the de-soldering iron and I reassembled without re-soldering the blue wires). And the FAQ, of course -- Many Thanks to those that contributed that:

http://www.brickboard.com/FAQ/700-900/ElectricalSpeedometerRepair.htm