Remember, all this is for a 91, so your system or its component locations might be a bit different.  If you supply me with information on differences for your model year, Iíll include them in the write-up.

The cruise control system consists of an electronic control module, a vacuum pump, a vacuum pot, cutout switches on the brake and clutch pedals, and a few feet of vacuum hose.  Most problems are caused by leaks in the hoses, so are easy to fix.

The vacuum pump is driven by a small electric motor, and also has an electrically operated valve that allows vacuum when energized, or dumps vacuum when de-energized.  Itís located in the aquarium, on the passenger side (US). 

The vacuum pot has a rubber diaphragm connected to a metal rod.  Vacuum pulls the diaphragm back, which causes the rod to retract.  A metal bead chain connects the rod to the accelerator linkage.  The vacuum pot is about 3Ē away from the top of the accelerator pedal (itís not easy to see).

The control module is beneath the dash on the US driver's side.  It's attached to the front of the inverted white 8Ē x 8Ēplastic box that serves as a general attachment and wire routing point (itís secured with a slide-on sheet metal clip).  Itís very simple inside: A memory IC, a resistor (resembles a resistor, anyway; might be a coil or a fuse, who knows?) and a couple of relays.  Thereís not much in the box to go wrong, and it usually doesnít.

The cutout switches not only open the electrical circuit when the pedals are pressed, they also dump vacuum.  I broke my clutch switch when replacing the master cylinder.  I replaced the vacuum/electrical switch with a simple electrical switch (I just plugged the vacuum line that went to the original switch).  It performs just the same as the original switch.  As far as I can tell, the vacuum-dumping switch function is just a redundant safety feature.  NOTE that brake light switches close (make contact) when the brake pedal is pushed, to turn on the brake lights.  The cruise control cutout switches open when the pedal is pushed, so standard brake light switches wonít work as replacements.  I used a very generic momentary pushbutton switch from Radio Shack.  While typical brake switches have long-throw plungers, generic pushbuttons typically have buttons that only move an eighth of an inch or so.  I glued a small square of  ľĒ balsa wood to the clutch pedal where it contacts the switch.  The proper shim thickness will just allow the switch to make when the pedal is released, without restricting the clutch pedal travel or putting too much pressure on the switch.  I didnít get an original replacement switch because the Saab switches appeared to come only in a matched set (I couldnít figure out how to replace only one) and I figured that since itís such a non-standard design, Saab probably wants an arm and a leg for them.

Another NOTE: The brake and clutch cutout switches provide a ground for the controller through the brake light filaments.  I am not sure if a single brake light failure will disable the cruise or not, but it's a good idea to check the bulbs and the sockets to make sure the bulbs can get a good ground.  The filament monitor should alert you to any problems.

Troubleshooting

If the cruise doesn't operate at all:

It appears that the brake light bulbs must be operating in order for the cruise to work.  That's how I interpret what the manual says, anyway.  They're easy to check, so make sure the bulbs and grounds are good first.

Check the fuse first.  If it's OK, check the adjustment and function of the switches.  Bob Davis says they are adjustable.  He says: "Push the pedal in far enough to release the switch, pull the switch plunger all the way out (there should be clicking sounds), then slowly release the pedal back to the resting position."  He also says that it appears that on 95 and later cars, the switches lack the vacuum function (do not know if these are adjustable).  If both switches and fuse are OK, look up under the dash and make sure the bead chain isnít broken.  If that's OK, check for vacuum leaks (see below).

If the cruise operates erratically:

Check for vacuum leaks.  There is a hose that runs a few feet, from the vacuum pump to the vacuum pot.  That hose also tees off and is wyed to the two pedal switches (or just to the brake pedal switch for a car equipped with automatic transmission). The simplest way to check for leaks is to disconnect the hose from the pump, suck on it and see if it holds vacuum.  Remove the aquarium cover.  The pump is on the passenger side (US).  There are two short hoses that run from the front of the pump to the back of the pump, but donít bother with them unless you canít find anything else wrong.  Disconnect the main hose from the lower attachment tube that goes into the front of the pump (the upper one is the air intake for the pump) and then suck on the hose.  If it doesnít hold vacuum, thereís a leak (likely).  The hose could be split at one of the attachment points, one of the switches could be defective, the vacuum pot could be bad or something could have damaged a hose (the long screw at the front center of my aquarium cover chewed a hole in my hose).  To eliminate switches as the culprit, disconnect the hoses from the switches and plug the hoses, then repeat the suck test

No leaks, good switches, good chain?  The pump gets power on pin 3 of its connector, by way of the cruise control switches, and ground on pin 2 by way of the controller.  The valve gets power the same way and ground on pin 1, again by way of the controller.  The controller is permanently grounded by pin 8 of its connector.  It gets operational ground on pin 3 by way of the cutout switches and the brake light bulbs.  It gets permanent power on pin 1.  Speed is sensed on pin 5.  Pins 2 and 6 appear to supply power by way of the cruise controls switches.