Thread: TIPS on replacing Disco 2 Auto Trans Switch

I know that there have been plenty of posts about replacing the "XYZ" or starter-inhibitor switch on the ZF 4HP22 automatic transmission gearbox, but I think I have come up with some useful tips for this job. The tips relate to aligning the replacement (or repaired) switch to match the position of the gear selector lever inside the cabin.

The Discovery Workshop Manual and RAVE CD show a diagram of the switch in position on the auto trans box. In this diagram, use of Land Rover special tool LRT-44-011 to align the switch is also shown. The Haynes manual for the Discovery reproduces the same diagram.The special tool just serves to establish a line that is at right angles to the flats on the gearbox selector shaft (and with which the selector lever engages), and point that line at the hole. See Figure 1.

Hardly anyone would have the LR Tool, and in any case it's not quite that simple when you are trying to do this job without removing the front exhaust-pipe section, because it is impossible to see the hole in the switch body that the setting pin is supposed to go into. It's hard to see past the exhaust pipe, and the hole is hidden by the switch body itself when you are underneath the car and looking up.

The first part of the gearbox selector shaft (close to the auto gearbox) has both a spline and a flat on it. This section engages with the hub of the inhibitor switch. The outer end of the shaft has 2 flats, with thread between them. The flats are on opposite sides of the shaft, and so are parallel to each other. These two flats engage with the cable-operated gear-selector lever. The flats are used to turn the selector shaft to gearbox position "N" before sliding the switch onto the shaft.

TIP No. 1
Two 10mm bolts secure the switch body to the side of the auto gearbox. The bolt holes in the switch body are elongated to provide for adjustment of the switch position. They are not shown like that in Figure 1, but that is how they actually are. These slots are located in "feet" that raise the switch body off the gearbox surface. Applying a smear of grease to the underside of the "feet" containing the bolt slots makes slight movement of the switch position easier, if it is required later.

TIP No. 2
The alignment of the inhibitor switch as described in RAVE is done with the gearbox selector shaft in gearbox position "N". Before (or after) sliding the switch onto the gearbox selector shaft, insert a nail, thin bolt, short piece of drinking straw, or thick matchstick into the round hole located under the start of the flexible section of the wiring that exits the switch body. The hole is the size of a 4mm (5/32") drill bit. Having something poking out of the hole enables you to see where it is. It can easily be taken out when the job is finished. See Figure 2. Do not tighten the bolts, but get their threads started.

TIP No. 3
Use a clout or box-nail (having a wide, flat head) as a needle to point at the pin previously inserted into the alignment hole in the top of the switch body. Hold the flat head of the clout onto the upper flat on the end of the gearbox selector shaft with a finger nail. You could put a tiny amount of grease or "blu-tac" on the nail-head to make it stay there. If the nail points straight at the pin, tighten the securing bolts (but not completely). If necessary, firstly rotate the switch body slightly until the nail points at the pin. See Figure 3.
Return gearbox selector shaft to position "P" (fully clockwise) and re-connect the selector lever.
Make sure the electrical multi-plug is also connected!

Inside the vehicle, check that "P", "R", "N", "D", "3", "2", "1" all appear next to the odometer as the gear lever is moved through those positions. Also the red indicator lights on the gear selector panel. If all OK, then finally tighten the switch-securing bolts, and the job is finished.
If not, check whether a very slight pull or push on the gear lever makes a missing one appear. A pull on the lever means the switch body needs to rotate slightly CLOCKWISE, a push on the lever means it needs to be turned slightly ANTI-CLOCKWISE. Refer to Tip No. 4.

TIP No. 4
Place cabin gear lever in position "D" (because it is the central one, with 3 other positions on each side of it). Under the vehicle, slightly loosen the two bolts securing the inhibitor switch, and very slightly rotate the switch body one way or the other. Nip up the bolts, and check the gear position indicator. If "D" is displayed, check the other positions.

Repeat until it's right.
Fully tighten the switch-securing bolts.