After repairing the cruise control on my Disco 1 I decided to put all the information in one place...! This is what I did to repair my cruise control, use the information contained at your own will. I cannot be held responsible.
Cruise control system layout:
The mechanical side of the cruise control uses vacuum operated 'bellows' to pull open the throttle and adjust your speed. It does not run off engine vacuum, it has a separate pump, located behind the near side headlight below the jack. From there a pipe runs down the near side inner wing to a 'T' behind the washer bottle. 1 port on the 'T' goes to the bellows on the throttle body, the other leg of the 'T' then runs along the firewall and into the cabin just above the pedal box to the brake pedal switch.
The electronic module is located behind the glove box, next to the ABS module.
The cruise control system is a Hella system and was fitted to many cars, if your searching for parts in a scrap yard also look at 80's & 90's Audi's Volvo's VW's Jag's Citroen's Merc's & BMW's! The module part numbers are Landrover AVR 1173 Hella 5GA-004-397-**
95% of all cruise control problems are due to a vacuum leak somewhere in the system, usually on one of the joints, the rubber pipes perish and crack. Either cut the ends off or just replace them all, the pipe is cheap enough. Also check the bellows on the throttle body, they can crack and leak. The brake pedal valve also has been known to leak. A classic sign of leaky pipes or bellows is the cruise control gets progressively lazy at holding speed until you get to a point when it just won't hold speed at all. I saw one that would hold speed on the flat but on a slight incline would slow down, the bellows had a pin hole in it, we repaired it with a bike puncture repair kit.
If your engine check light is on then cruise control is disabled so check that before proceeding! The cruise control circuit diagram is in the workshop manual but split across many pages so here is a combined, complete cruise control circuit diagram:
Open the glove box, then push up the catches either side to fully open, like in the pic above. Remove the wiring connector on the cruise control module, you may need to prise off the wire clip first. Using a testmeter perform the following tests :
Ground Test - From pin 8 (black wire) on the cruise control module plug perform a continuity test to good chassis ground.
Main voltage test - With ignition & cruise control system on, transfer in high (non difflock) & auto box in 'drive' connect volt meter between pin 8 (black wire) and pin 10 (white & yellow wire) you should have battery voltage if not check fuse.
Secondary voltage test - Connect volt meter between pin 8 (black wire) and pin 9 (Purple & Green wire) you should have battery voltage if not check lock out relay or gear selection switch.
Brake Pedal Test - Connect volt meter between pin 8 (black wire) & pin 5 (green & purple wire). Press the brake pedal, you should get battery voltage.
Steering Wheel Set / Accel Test - Connect volt meter between pin 8 (black wire) & pin 3 (red & white wire). Press the Set / Accel button, you should get battery voltage.
Steering Wheel Res / Decel Test - Connect volt meter between pin 8 (black wire) & pin 4 (blue & white wire). Press the Set / Accel button, you should get battery voltage.
Vacuum Pump Test - Using a jumper wire connect pin 10 (white & yellow wire) to pin 1 (white & blue) this provides power to the pump. Then using a jumper wire connect pin 6 (black & yellow) AND pin 7 (black & red) to pin 8 (black wire). This should energise the pump & solenoid and the bellows should contract.
Speed Sensor Test - Connect a volt meter between pin 8 (black) and pin 11 (yellow & pink), drive the motor around the block, voltage should increase as speed increases.
If any on the above electrical tests fail you will need to check the associated wires/relays/fuses. If all the tests pass it is likely the cruise control module has failed. The same module on my Audi failed in the same way. You will now need to remove the module from behind the glove box held in by 2x8mm nuts, its a bit awkward to remove them but this will differ depending on what tools you have available.
Once the module is removed, using a flat bladed screw driver prise the metal tabs away from the plastic end-case. Gently pull out the plastic end-case then gently slide out the PCB. If your pretty handy with a soldering iron you will need to repair any dry joints. The usual suspects are the capacitors marked here:
Best way I have found is to gently scratch off the varnish, add a bit more solder to the joint, use a solder sucker to remove all the solder.
Then using a little flux re-flow the joint. I also re-flowed the connector and the mini relay solder pads. Remember to clean the flux off the board
when complete and also clean the connector. Reinstall and enjoy your cruise control...!!
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Disco 1 Cruise Control Repair.