Folks,

I have progressed enough in installing a '93 engine and wiring looms, along with SRS, into my '86 245 body with M46, and I have a few problems remaining that I think I can solve, but I have one that I don't know how to deal with. The speedometer does not work.

This is a hybrid setup necessitated by not having a '93 rear end. What I did was take an electronic speedometer marked "9800" from an earlier electronic cluster and put it into the '93 cluster, in place of the one there marked "40800". That way, my 86 rear end should drive the speedo OK, but I would also benefit from the SRS test light and the little set of SRS electronics that are on the cluster board. I also wanted to have the upshift light for my '86 M46 and a light to indicate that the overdrive was engaged, as in the '86 cluster. I discovered hat the '93 FCU provides the upshift signal for manual transmissions, and I tried to wire the ABS light (no ABS on my ’86) to indicate that the M46 overdrive is engaged. That took a little research and rewiring, but nothing complex.

When I started the engine a couple of weeks ago, after more than a year, it turned over and warmed up nicely--to my amazement! The speedo, however, didn't seems to act correctly. When I had turned the ignition on, the speedo rose to indicate a high speed, and when I started the engine, it dropped to zero. That symptom is repeatable.

I disregarded that while I bled the brakes and did the other preliminaries to getting the car rolling, hoping that the speedo would work once the car was underway. Well, it doesn't; it sits at zero regardless of the car's speed. I investigated what was going on at connector 233 (the three-wire thingy that is held in place with a white cover). The blue wire to it (pin 3) reads 12 volts whenever the ignition switch is on, and that is consistent with the ’93 wiring diagram. The black wire (pin 1)shows a few tenths of an ohm resistance to ground, which also seems correct. That leaves the wire to pin 2, which (unless I screwed up earlier) connects through the loom to the speed sensor on the differential case. The speedomoter's behavior indicates that this wire improperly picks up a positive voltage when the ignition is turned on, and that that voltage drops to zero when the engine is started, and that speed sensor pulses never get to the speedo though that wire—or the speedo doesn't recognize them.

In case it isn’t obvious, I'm in over my head on this one, and I'd be grateful for advice from those knowledgeable of electronic speedos. I’m investigating the archives, but I don’t expect I’ll find anything directly relating to this problem—which I am guessing is self-induced.

Thanks for any thoughts.