To Repair the 944 Cruise Control (as posted on the Rennlist.com forums)

Here is some info on fixing what seems to be the most common cruise control problem with 944's, from my own experience with my '86 with 230K miles.. This info generally applies to '85.5 and newer 944's. I believe pre '85.5 vehicles used the same servo, but different control electronics which used RPM's instead of vehicle speed as a feedback parameter.

Symptoms:
· Cruise Control works intermittently
· Surging
· Full or partial throttle acceleration above pre set speed

Component identification:
· The Cruise Control Electronics are located under the dash on the left side of the vehicle behind and above the hood release lever.
· The Cruise Control Servo is located in the Engine compartment in front of the Battery.

The Cruise Control electronics compare vehicle speed to set speed, and adjust servo position accordingly. I am not sure if a Proportional Integral Derivative (PID) controller is used, or some other combination thereof. It really isn't too important for our purposes. The important thing to understand is how the servo needs to operate for correct cruise control functionality. The controller needs to set the position of the servo (which in turn moves the gas pedal, and throttle). Since the servo uses a DC motor, which produces a force somewhat proportional to the current supplied to it, an additional positional feedback loop is required. A variable resistor is used in this case, which is attached to a shaft inside the servo. If this variable resistor fails due to contamination, oxidization, etc, this feedback loop is broken, and the cruise control electronics can not position the servo correctly, causing surging and/or related symptoms. In my case, I was able to disassemble the servo, and clean the variable resistor parts to restore operation of this feedback loop, and therefor my cruise control. If you are experiencing similar cruise control problems, this may be worth trying.

You will need:
· Goggles
· Phillips head screwdriver
· assorted sockets,wrenches (10mm,12mm?)
· Spray on contact cleaner (Radio Shack, local electronics supply, or mail order). Look for something that also contains a protectant to minimize future oxidization. This is typically some sort of mineral oil.

Time
· about an hour

Procedure
· Remove cable from servo. It is a small ball joint which is held on by a metal clip. The metal clip has to be slid off to remove the cable.
· Remove the Electrical Connector from its housing/mount, and disconnect.
· Take note as to how the servo and related brackets are attached, unbolt, and remove from vehicle. Be careful not to loose any parts. The metal spacers inside the rubber grommets fall out easily.
· Mark the relationship of the lever on the servo shaft (the one you just disconnected the cable from), and take note as to which side the spring returns it to. If the shaft doesn't feel like it has a spring attached to it, this is another big problem with your servo.
· Remove the bolt holding this lever on by holding the lever with a pair of pliers.

(Gregor adds/clarifies) It is OK for the shaft with the ball end to move freely (at least the 1989 version is..yours may be different.) It is only engaged when the solenoid inside engages the gears when the cruise is activated. The pie shaped plastic gear inside is the spring loadedcomponent.

· Use a high quality phillips head screwdriver to remove the cover. Be careful, the spring may come flying out. You may need to gently tap the sides of the case to open it.
· Observe how things are assembled. The variable resistor consists of two carbon traces on the PCB board, and a wiper attached to the internal shaft. You will notice that the internal shaft is not directly attached to the shaft which exits the case. It is connected in such a way as to only allow the servo to pull on the cable in a manner as to increase throttle. You will also note that you can not rotate the internal shaft by rotating the external shaft while the unit is assembled. This is why we couldn't test for correct variable resistor operation while the unit is assembled.
· Inspect the carbon traces. They should not look burnt, or otherwise damaged. Mine had some grease on it.
· Use spray on contact cleaner to clean the carbon traces, and the wiper on the inner shaft. Make sure you hold the servo so that the liquid (cleaner and dirt) can run out of the case.
· Insert the inner shaft and check for adequate wiper pressure. If the wiper has deflected over time, you may want to bend it a bit to restore contact pressure with the traces. You don't want too much pressure or it will wear out pre-maturely.
· Now would be a good time to check operation with an ohm meter. Connect to the wiper, and the end of one of the traces. Check for smooth readings as you rotate the shaft (must be perpendicular to the PCB). I believe the resistance should vary from several kOhms to close to Zero. Check both traces.
· Once you are certain the traces are clean enough, re-assemble. I re-greased appropriate areas, just make sure you don't get any on the variable resistor contact surfaces.
· Once the cover is back on, check that the shaft returns to its original counter-clockwise position via operation of the internal spring. If so, re-assemble, and re-install in your vehicle. Now take it out on the highway and see if it works!