Folks, what can you tell me about BMW automatics?: Grassroots Motorsports forum: Grassroots Motorsports Magazine

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Sept. 17, 2010 10:02 p.m. turboswede SuperDork

Specifically the one used in a 94 540il. Girlfriend's Dad is having to push a button on the transmission in order to get the car to start and he thinks the entire transmission needs to be replaced.

I think the electrical issue with whatever sensor is causing the car to fail to start needs to be repaired (along with the short in the A/C system that is blowing fuses when engaged)

I'm thinking neutral safety switch and electrical ground issues.

Then again, I'd rather avoid working on the car at all and would prefer to provide him some information to give to his local shadetree mechanic friends. Especially since it is in Modesto, and I'm in Portland.

Since I don't know BMW's and I know the collective brain trust here know more than enough to be dangerous, I figured I'd ask and see what comes out.
Sept. 18, 2010 12:03 a.m. oldtin HalfDork

Position sensor would be my first guess. Any info on conditions when it failed? Noises, shifting wackiness, slipping? Does it move or shift once started? Trans is an a5s 560z for a 540 I. Basically rule out mechanical vs electronic failure...
Sept. 18, 2010 11:59 a.m. M2Pilot Reader

I had a '94 530i. Never had any transmission problems with it but I sold it at 95K or so because I've always heard that the transmission in those cars is good for around 100K. It's my understanding that the transmission is basically a GM unit. I'll gladly defer to more learned opinion.
Sept. 19, 2010 10:42 p.m. curtis73 HalfDork

94 540 should have the ZF4HP22. Not a forgiving or happy box. Nightmares, if you ask me.

Simply swapping out the tranny for a used one can fry the EPROM in the TCM. The ONLY fix is to upgrade to a flash-type box and completely re-pin the wiring harness. Its something that was pretty common at BMW dealerships, but nobody knows how to do it anymore.

If it needs to be rebuilt, find a ZF specialist. Normal run-of-the mill tranny shops will just mess it up. I manage a chain of 12 transmission shops, and only one of my builders is allowed to touch a ZF.

But... the more common failures are electronic. Position switches are a common problem and its where I would look first. They're in the console on the shifter itself. Probably a yellow box.

Question... is the shift cable getting harder to move? Its another common problem, and the force you have to use on the shifter causes the cable to stretch and/or the position switch to be out of adjustment. The most typical symptom is that you drive for about a mile and then (when its warmed up and the computer tries to take over) it starts slipping or disengages completely. Reverse usually remains with a hard engagement.
Sept. 19, 2010 10:49 p.m. WilberM3 Reader

a lot of the bigger bmw cars with auto 4sp transmissions are GM units with reworked valvebodies and shift program and probably a few other things. they even have a big square GM logo cast into the housing, not sure what yours is specifically. my mom had a '96 740iL with nearly 160k on it with zero issues.... until she high centered the car on the trans.
Aug. 19, 2011 7:37 p.m. fasted58 Dork

canoe