Car Clinic: Grim Jaguar Gear Problems

Car Clinic: Grim Jaguar Gear Problems
 

Question:

The automatic gearbox on my 1999 Jaguar 3.0 V6 SE doesn’t shift normally into fourth and fifth gear. I have changed the transmission fluid and replaced the fluid filter, but the problem persists. During fast acceleration it also displays the message “Gearbox Fault” on the instrument cluster. What can it be?

Ike

Answer:

Ike, the spectre that we dread when gear shifting on an autobox becomes erratic is a defect inside the ’box — one or more of the multiplate clutches used for gear shifting may be slipping, for instance — which would mean the ’box has to be stripped at great expense. Before you bite that bullet, however, I suggest you first have the following possibilities investigated.

An instance has been recorded (see this thread) where symptoms identical to yours, also on a ’99 S-type, were caused by misfiring traced to a faulty coil pack. The resulting power loss confused the transmission control module, which tried to compensate by adjusting the shift points. This led to erratic shifting which the on-board computer attributed to a “Gearbox Fault”. This may explain why the fault message comes up during high power demand.

The electrical circuits taking signals to and from the model have to be scrutinised for broken wires, bad connections, or water intrusion, all the way from the contacts on the shift lever (behind the “J- gate” on the console) to the solenoid pack controlling the hydraulic pressure for gear shifting. Electrical problems have been known to cause symptoms similar to yours. Everything on these circuits can be checked without removing the gearbox.

It will be considerably less costly to replace some solenoids than to overhaul an autobox, especially if it turns out the fault is not in the ’box!

Gerrit Burger