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Subject: Useful information about our ZF transmissions Author: DavidC (moderator) : member since December, 2003 : 12931 posts Posted on: 2008-04-13 11:41:14

I received this message today on Chris Koch's 7-series-digest email and copied here for future reference. It's very enlightening. I knew about the general problems with our transmmissions (like the 1500-rpm problem while in neutral), but did't understand the why's and wherefores. This explains it nicely.


Subject: 1987 635CSi - actually based on the E28 five series
From: "Christopher P. Koch"
Date: Sat, 12 Apr 2008 13:23:13 -0400

Dear Bob,

Your father's transmission is probably a ZF 4HP22-EH as I believe that was standard fare as of 1986. It was electronically controlled and can be identified by the three position switch "E - S - M" on the console.

This transmission had several serious problems which was caused by several design faults. We discovered a fairly adequate "cure" in the early 1990's - replace the mineral oil based ATF with full synthetic ATF and change some operational habits. The end result is that we could get the transmission to last the life of the vehicle.

The first problem was that once the transmission is placed into reverse or drive, the forward clutches remain engaged. As you can imagine, if you drive somewhere and then sit there with the engine idling in park or neutral and the A/C on, the transmission fluid soon overheats from the clutches generating heat as they slip. The mineral oil based ATF destructs at around 250 - 260º whereas fully synthetic ATF such as Mobil 1 can take temperatures up to 450º. Furthermore, when the car was brought into shops for emissions testing, which required the engine to be run at 1500 rpm, the clutches fried in minutes.

The second problem was that since the transmission was "electronically controlled", hence operated much more precisely than mere hydraulic and vacuum controls permit, the designers opted to cost reduce the transmission and run the power shafts through drilled aluminum bores rather than using expensive roller bearings and seals. Again, as you can imagine, the minute the ATF breaks down, the shafts begin wearing the bores eccentric which then allows the high pressure fluid to leak by the shafts, which lowers the line pressure to the forward clutches and the clutches fry.

When this happens, these (expensive) subassemblies have to be replaced because just replacing clutches will not solve the worn-out bores. In addition, some oil passages were enlarged to supply great fluid volume.

The solution to these problems was switching to full synthetic ATF and always turning off the engine at your destination for 60 seconds, then turning it back on to run the A/C or run the emissions tests, making sure not to leave park therefore ensuring that the clutches don't engage.

In the case of your father's transmission, merely replacing the clutches will not buy you very much. The transmission has to be completely disassembled, all wear items replaced, all bores must be checked to be in spec and all upgrades performed. This should all be done precisely in a clean room, not some transmission shop's grungy old work bench.

ZF has a rebuilding facility in the USA that completely restores the transmission to the latest upgrade revision levels in dust-free clean rooms. They make these transmissions available to BMW Dealerships and Automatic Transmission Distributors at essentially cost. Depending on your father's negotiation skills, you should be able to buy one of these "better than new" transmissions at an attractive price. You know where your local BMW dealership is and the Distributors in your state are listed on ZF's website.

As you can tell, I do not recommend having your local transmission shop rebuild the transmission under any circumstance. However, if you are not able to R&R an automatic transmission yourself, you should be able to negotiate a deal with your local independent BMW service center to perform the transmission swap.

There are five important things to be performed during the swap:

1) replace the rubber transmission mounts
2) replace the rubber guibo (if any cracks are found) and driveshaft lock nuts
3) replace the driveshaft center bearing making sure not to let the center
splines disengage as the driveshaft is precision balanced as a unit
4) preload the center driveshaft bearing to specification
5) use only full synthetic ATF

Yours,
Chris Koch
Buffalo, New York

David Cecil
Lexington, KY
Titanium silver '01 750iL, 92k miles
Oxford green '97 740iL, 184k miles
Alpine white '92 750iL, 95k miles (for sale)
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