I'm looking everywhere for an answer to a puzzling problem. Please forgive me if you've already read my post on mye28.com. I recently replaced the differential in my '88 535is (5-speed) with a 3.46 LS unit sourced from an '88 528e automatic. If it matters, the original was a 3.25 open differential. After the swap everything worked as expected, except for the speedometer, which didn't register at all. After a series of tests that I won't bother to describe here, I think I have traced the problem to the sensor in the differential. This is supposed to generate a pulse as the teeth of a chopper wheel inside the differential pass between two prongs on the sensor. I have verified that this does happen with the sensor installed in the original differential, but not with the "new" one. Strangely, it appears that the tips of the chopper wheel blades are farther inside the differential in the new unit compared to the old one. That is, measuring from the sensor mounting surface, the depth to the blade is about 12 mm in the original differential and 19 mm in the replacement. I thought all of the e28 differentials were completely interchangeable, and it seems that only one speedometer sensor is available all of them. Given this, I don't know what's needed to fix this problem. New sensors cost $50, so I'm reluctant to try a one unless there is a reason to think it would work differently than the 2 I have. Any suggestions would be appreciated. Dana Loomis Reno NV 89509 88 535is
All speed sensors are interchangeable on the e28 chassis. There is some differential interchange possible with other series. I'm not sure about how the different series pulse sensors interchange, I would assume for inventory purposes they were the same. Perhaps the differential was not original to the 528e? Did the speedometer work in the 528e? There were some 3.46 rear ends in e23 (7 series) that basically fit but there is some difference in the rear cover plate. (the one the pulse sensor fits in) Have you tried putting your old rear plate on the new differential? You can test the pulse sensor by unplugging the harness, attaching a multimeter to the blades then turning the elevated rear wheel while watching for a pulse of voltage across the multimeter. You can test the harness by unhooking it beneath the shift boot and doing the same thing. (plug the harness back into the pulse sensor first). You can repeat the last step again at the plug at the back of the cluster. Pulse sensors don't seem to fail often.
Thanks, that's very helpful. Unfortunately, I don't know whether the speedometer worked in the donor car. I did do the test you described on the pulse sensor and got 1-2 ohms continuously while turning a wheel. I haven't done that test with the harness connected, but I get 8 V across the connectors in the plug when the ignition is on, so it appears the harness is okay. I'm going to try a new sensor and if that doesn't work, I'll open up the differential. The rear covers on the 3.46 and the original differential look identical, but it won't hurt to swap them.
I'm adding to this in case any one is interested. I tried a new sensor with the same result, so I opened up the differential. The rear covers on the new and old differentials are identical. But, as I feared, the bladed wheels inside appear to be different. The blades on the 3.46 LS differential are about 5 mm shorter than on the original 3.25 open differential. As a result, they don't pass deeply enough through the sensor forks to break the circuit. This is completely baffling: this system must have worked on the car it came from, so there must be a way to make it work in a 535is.
