I have for a fairly short time had an XJ40 MY 1992 that runs without problems.
4-speed ZF 4HP24 VIN SAJDKALD4AK657378
Last week however, there was a shift disorder. Suddenly, while driving on the highway, the transmission began to shift gears up and down without noticeable logic. No warning light came on.
The behavior was the same regardless of speed but with short periods of normal operation.
I went down to a lower speed, about 70 km / h and went to a workshop where the gearbox was flushed and a new filter was fitted.
During the test run thereafter, the fault persisted and did not change with the gearbox temperature. The same unexpected up and down shifting at all different speeds and without noticeable logic regardless of the position of the gear selector: D, D, 2 or 3.
Based on how the fault occurred, I guessed an electrical fault or a fault in some control unit.
I disconnected the battery and test drove again and the error remained. I then took out the fuse for the cruise control and then the transmission worked completely normally again. When I put the fuse back, the error returned after about 10 minutes of normal driving at about 70 km / h.
Removed the fuse again and the transmission shifted normally. The only time it temporarily resumed “up and down” shifting was after a strong throttle but only for about 5-10 seconds - then normal shifting.
How is the transmission affected by other systems in the car?
Where should I look for the error?
