X5 transmission control module replacement..

[turquise1]

Originally Posted by X5Matador

Ozzie,

What was the transmission doing that alerted you to change the control module.

Shortly, after driving the car for a while (30-40 mins in city), I would come to a stop and when I step on the gas again the car would not move first, with increasing rpm it would jump eventually causing a TRANS FAIL SAFE mode.

I have changed, transmission cooler, battery (alternator checked ok), transmission speed sensor (because of P0720), transmission valve body, neutral safety switch and nothing solved issue. I also suspected interference from aftermarket sound modules and unplugged everything with no results.

Transmission guy told me there is a problem with the control module, I have just purchased a used module (same serial number etc.) and about to replace it tomorrow to test it (hopefully no programming needed). If that does not solve the issue the only thing left is an internal problem with the transmission, but lets see..

Ozzie

[RRPhil]

Of all the things that can cause issues with a transmission, the ECU is by far the most unlikely.

Are you able to select reverse okay, even when the TRANS FAILSAFE PROG message is displayed?

Phil

[turquise1]

Originally Posted by RRPhil

Of all the things that can cause issues with a transmission, the ECU is by far the most unlikely.

Are you able to select reverse okay, even when the TRANS FAILSAFE PROG message is displayed?

Phil

Yes I am..

Thanks,

Ozzie

[turquise1]

Hey guys,

Well the new/used module is in and the car is the same. So, it is NOT a module issue.

Any other ideas before I &^%^& the transmission guy?

Ozzie

[RRPhil]

Originally Posted by turquise1

Any other ideas before I &^%^& the transmission guy?

A-clutch leakage caused by the axial bearing between the B-clutch hub and the C-clutch drum failing, allowing axial movement of the input shaft assembly within the B-clutch drum, uncovering the O-ring seal between them, causing it to split and provide a leakage path.

Oil pump can keep up with the leakage when the fluid is cold but when fluid becomes warm and less viscous the pump no longer has sufficient flow to maintain clutch pressure and the A-clutch slips and the ECU detects this from the speed sensors and immediately dumps the transmission into failsafe. When the fluid cools down again the transmission performs fine.

A-clutch is used in all forward gears except 5th so ECU engages limp-home (4th gear), still detects slip in A-clutch and has no choice other than to try for 5th. Unlike the A & C clutches which have accumulator circuits for smooth gear engagement from stationary (D & R respectively) 5th gear comes in with a bang.

Reverse uses C & F clutches only so its function is unaffected by the A-clutch leak.

Phil

[turquise1]

So you are saying that its an internal transmission issue. I do agree with you its something internal (although the transmission guy still insisted today that its something electrical) but how do we explain things turning back normal (for a while at least) after I turn the engine off and on?

Ozzie..

[RRPhil]

If it's an A-clutch leak, this is the O-ring seal that fails and the transmission has to be rebuilt to replace it :



If your engine/transmission are at normal operating temperature (i.e. have been running for at least 20 minutes) and you get the failsafe message, key off to reset the ECU, and then restart the engine are you saying that you can then drive off and have no problems for another 30 minutes before the message reappears? If so, it probably isn't the problem I've described above. This issue would only apply if the transmission had to be left to cool before it functioned correctly again.

Phil