I'm going to copy what I said in another thread from here: http://www.vwwatercooled.org.au/f47/...tml#post528302
Hopefully that helps out.

Quote Originally Posted by sortofbigbj View Post
The cruise control doesn't seem to work, not that it bothers me, I've never used it and around town i cant see much point, but if it was an easy fix it wouldnt hurt to do.
First things first, take the car for a drive and turn the cruise control on. (If you need the manual you can grab it here: http://www.mooshen.com/vwwc/golf/golf_mk3_manual.pdf ). What you want to do is set the cruise and hopefully hear the relay's ticking trying to turn it on when you hit set and off when you tap the brake. If you hear the relays ticking it's likely to something wrong with the vacuum pump or the lines (which are things that always breaks )

The cruise control is a vacuum operated system, the pump sits behind the washer bottle on the passengers side front (basically in the front guard). This vacuum does nothing more complex than pull the accelerator pedal down. The line runs into the front passenger side tunnel/frame bit then across to the pedals. The high tech way of testing it is to grab the pipe and suck on it. If you can feel pressure get someone to feel if the pedal is moving, if not, it's time to hunt down the broken pipe. The pump itself can be tested by grabbing some small alligator clamps and running some power through it to see if it pumps.

In my case I found that the vacuum pipe was cracked *just* where it exited the frame at the front of the car, which was damn lucky since I'd hate to have to try and run all new piping.

Bottom line, it's a pretty simple system and it's likely that the pump or the line is busted.

For reference the system diagram looks like this: