Timm's BMW Case Study - E38
728i fails to shift into 4th and 5th gears
|Date of manufacture||Jan 1998|
|Engine||Single Vanos M52|
This was a rather strange one. The autobox operated really well in manual mode, all the gears could be selected with no problems at all. The only problem was that the autobox would refuse to shift out of 3rd gear for no apparent reason. This was an intermittent fault, sometimes the autobox worked perfectly, other times the only way to select 4th and 5th was to go into manual mode and select them manually.
Carsoft showed no autobox faults, as far as the transmission was concerned, everything was lovely. Carsoft did show a fault with the camshaft sensor but this was a common fault that often was often shown erroneously. The engine was running perfectly so this was taken no further.
This problem spanned many months, meanwhile the camshaft sensor was changed as the camshaft sensor fault kept re-appearing. Against all common sense, the replacement of the camshaft sensor cured the problem! I have heard of this happening on one other occasion and am sure the two faults are related to a 'limp home' type ECU mode. This does not seem to happen on any engine other than the 728i.
Recently there has been a spate of similar problems that have been caused by gearbox fluid temperature - the EGS (gearbox computer) sees the temperature is too high and protects the gearbox by reducing torque - and it does this by holding the gears much longer and inhibiting 4th and 5th gears. This might sound strange behaviour, but the lower gears reduce gearbox torque considerably.
The temperature increase can be real (usually caused by a blockage in the heat exchanger on the face-lift models), or imaginary (caused by a faulty temperature sensor within the gearbox) - repair is flushing of the heat exchanger (or replacement), and replacing the temperature sensor which unfortunately is part of the gearbox internal harness - so a second-hand one is often the way to go,