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RE: 4hp22 automatic transmissions

The following is an very abbreviated compilation of an extensive thread 
on the subject recently on the 7-series digest (BMW-750 @domain.elided).
Most agree that the same design flaws apply to the more recent ZF 4HP24
as well.   If anyone would like the entire (6 or so pages) I'd be glad to 
E-mail it to them. 

The consensus seems to be  BEWARE of ANY prolonged idling, and 
especially high-speed idling during emissions testing or AC recharging.

The bulk of the following was shamelessly plagiarized from Jeff Moser 
and/or Chris Koch:

 -- begin--
The basic gist of the (BMW Service) bulletin, is that it is permissible to 
operate the engine at speeds up to 2000 RPM for up to 90 seconds with
 the transmission in park (not neutral) ONLY if the engine has not been
 running for at least 30 seconds, and ONLY if the transmission has not 
been shifted into any gear other than park.

Once the transmission is shifted into any gear (forward or reverse) the 
clutches are pressurized, and stay pressurized until the engine is turned 
off for at least 30 seconds......

I recently purchased an ATSG ZF 4HP22 rebuild manual from the 
Transmission Exchange (1-800-776-1191) for $12.00.  Of course, the 
book describes how to rebuild the beast, but more importantly, it describes 
some modifications that must be made to these transmissions to correct 
"Premature Forward Clutch Failure".

The problem is that the seals or the bore of the main input shaft can wear,
and cause fluid to leak from the torque converter directly into the forward
clutch (clutch A), causing the clutch to activate.  Clutch activation in
these circumstances can cause the clutch to fry and/or the car to try to
move forward when in neutral if the engine is accelerated above idle.  To
correct the problem, one of the shaft seals must be changed from steel to
teflon, a 0.045" diameter hole must be drilled into the forward clutch A
piston (to bleed off any misapplied pressure), and the forward clutch
orifice in the valve body must be enlarged (to compensate for the lost
pressure due to the 0.045" hole in the piston).  The manual doesn't say what
years of transmissions are affected by this problem.  I remember someone
from this digest describing a similiar procedure that was done when their
transmission was rebuilt (Chris?).  I also know that BMW had a transmission
recall in the 1986 timeframe for a similiar problem.
- --end--

HTH 
Kevin Heiner
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