Short version: The pulse generator in my 4HP22 transmission was broken. The fix was simply to replace the part for about $40 (instead of a $3000 new transmission!).

Long version: jvrx7 contacted me and noted that he had the same problem with his 4HP24 in his 750i. The pulse generator is a small coil hidden deep in the transmission that measures the rotational speed of the transmission. You can see it here: http://www.syne.net/e32/Pulse_Generator.pdf. It sends this information back to the AEGS which determines what gear the transmission should be in. Since my coil was broken, the AEGS thought the car wasn't moving and kept it in first gear.

Here's how I confirmed the problem: jvrx7 pointed out that the coil connects on pins 3 & 4 of the transmission connector (X2005). I didn't want to get under my car right away so using the wiring diagrams, I traced the connection back to the AEGS connector (X2000). This is the GIANT connector that plugs into the AEGS behind the right lower speaker in the passenger side kick panel. There's a tab on the connector towards the front of the car that releases one end of the connector. Just pull it down until the other side pivots out. Once you have the connector out, the speed sensor is on pins 27 and 8 (page 484 of the wiring diags). I used a multimeter to determine the connection between these two pins was open (normally there should be resistance from the coil).

I had my mechanic fix this for me as I was a little wary of opening up the transmission. However, jvrx7 offers his method:

"Very easy, but messy... best to start with a cool engine & tranny, drain the tranny pan, loosen the big nut for the return line, loosen the 6 bolts/clamps that hold the pan to the tranny, then remove the pan. ( you will get ATF on you, so wear old clothes ) loosen the two Torx bolts that hold the bracket that holds the sensor in place at the aft side of the valve body, remove, slide the sensor out of it's "hole", un plug. Reverse order after plugging in the new sensor. Top off with ATF, & enjoy!"

Thanks so much and I'm so relieved to have fourth gear again!

E32 735i 02/88

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