PROBLEM My cruise control does not work.
Possible Causes Bad brake light connection, defective clutch and brake pedal switches, bad connection to instrument cluster, defective cruise control brain, or defective cruise control servo motor. In some cases the control stalk can become loose, in which case it is difficult to initiate cruise. Automatic transmissions are slightly more complicated. The cruise control takes input from the transmission range display so that the cruise will not function with the transmission in neutral*.
What can I do? First check the easy things:
Check under the hood to see that the cruise control bowden cable is properly fastened to the throttle body, next to the throttle cable. If it is broken, the cable is available from the BMW dealer, although replacement may be a more difficult procedure than the rest of this writeup.
Confirm that your brake lights are working. Since the brake lights are known to be problematic on their own, take some time to clean the contacts for the bulbs in the rear tail light housings. You can access them by removing the trunk lining along the tail edge of the trunk. The cruise control is grounded through the brake lights.
Remove the trim panel below the instrument cluster. Check both pedal switches with a DVOM or multimeter. The cruise control switch should be closed without the pedal depressed. The brake light switch should be open without the pedal depressed. Replace any defective switches, or adjust the mounting height of switches with their threaded nuts, to make sure that the pedals make full contact with them. The clutch pedal over-center spring is also sometimes necessary to keep the pedal pushed against the switch.
If the pedal switches test properly, and they do not bind, move on to the instrument cluster connections. For cruise control to work, you must have a large blue plug installed in the back of the cluster. It is upright, and next to a yellow socket of the same variety. There are also two smaller connectors, which should be located at the cluster's markings: "blau" and "weiss", which are the German words for blue and white. The connectors will be these colors. The trick to connecting them is that they should be installed so that the wires point towards the drivers seat, not the front of the car.
Next, be sure to check the cruise control stalk. Check that it does not have a lot of free play. The mounting screw is accessed only with the steering wheel removed. The yellow wire should see power when "off" is selected. The green wire should see power when "resume" is selected. The blue wire should see power when "accelerate/set" is selected. You can check these with a test light clamped to a solid ground, such as the ground behind the steering column.
If you don't run into any problems so far, then you can bet that the brain or servo is defective. The brain is by far easier and cheaper to acquire and replace: Mine was $5 or $10. It is a medium size silver box with a blue connector on it, which is the same shape as the large blue connector on the instrument cluster. It is located beneath the dash board and steering wheel, in a black holder, which also holds the gong and OBC box assuming you have them. If you are making a trip to the junk yard anyway, it may be worth your time to grab both the brain and the servo (located in the engine bay, on the driver side fender) so you won't have to go back. When my cruise control brain was defective, it caused the servo to spin, making a grinding noise, whenever the key was in the run position. I can't say whether this is the only mode of failure, but it might be a useful clue. Upon replacement of the brain, the servo worked fine and did not require replacement.
If you replaced the brain with a known good unit, the only thing left to chance is the servo. If you reach this point feel free to post a question in tech talk. I'll update this when I know more.
A diagram of most cruise control components, along with their part numbers can be found at realoem.com
*If you have an automatic transmission, you should check that your transmission range display is connected and functioning properly. In these cars the cruise "enable" circuit needs to see ground, which is provided through a blue/brown wire from the neutral light in the transmission range within the instrument cluster.
PM me to suggest changes or ask questions.
Thanks.