Cruise Control Troubleshooting — Early Cars
Tempostat troubleshooting for early 944s (up to model year 1985½): the first checks, a symptom-to-cause guide, and the factory voltmeter and ohmmeter values for the single control-unit connector and the throttle actuator.
CRUISE CONTROL TROUBLESHOOTING
PORSCHE 944 · UP TO 1985½
Pick the symptom you have to start the diagnosis.
How the system works
On early cars the road-speed signal comes from a pulse generator at the left front wheel: as the wheel turns, the generator produces a rectangular pulse that the control unit counts to read speed. A single multi-pin connector links the control unit to the car wiring; a separate connector in the engine compartment links it to the throttle actuator (a motor with a feedback potentiometer and a clutch coil).
The cruise circuit shares fuse 8 with the stop lights, and it borrows car ground through the stop-light bulb filaments. Those two facts drive the first checks below.
Troubleshooting
When cruise control misbehaves, work through the first four checks before opening any connector. If none of them find the fault, continue with the control-unit connector test and the actuator connector test.
Note: This guide assumes a single failure is causing the fault. Diagnose one cause at a time.
Cable damage or misadjustment
Check the Bowden cable from the actuator to the throttle linkage for damage and correct adjustment. A stretched, kinked, or wrongly set cable keeps the actuator from controlling the throttle even when the electronics are healthy.
Blown fuse
Check fuse 8 on the central electric board. The stop lights use the same fuse, so a cruise system that is dead together with the stop lights points here first.
Central electric board — early cars
| № | Rating | Protected circuit |
|---|---|---|
| 8 | 8 A | Stop lights |
| Relay carrier (positions 1–11) | |
|---|---|
| Relay | Function |
| 1 | Fan |
| 2 | Fuel pump |
| 3 | Blower |
| 4 | Headlight cleaners |
| 5 | Warning-time relay w/ gong (USA) / cat-converter monitoring (Japan) |
| 6 | Rear-window defogger and seat belts |
| 7 | Plug bridge (term. 56–56b) |
| 8 | Additional high beams |
| 9 | Horns |
| 10 | Intermittent wiper action |
| 11 | Flashing lights |
Stop-light ground fault
Terminal 8 of the control unit takes ground from the stop-light bulb filaments whenever the brake pedal is not pressed. If both stop-light bulbs are burned out at the same time, the control unit loses its ground connection to car body and cannot function. Confirm the stop lights work before suspecting the control unit.
Clutch switch fault
The clutch switch must be closed in the neutral (pedal-up) position, switching ground to control-unit terminal 14. If it is misadjusted or failed open, the control unit behaves as though the clutch is always pressed and never holds speed. Adjust or replace it per the clutch-switch removal procedure.
Testing the control-unit connector
If the first checks are good, test the single multi-pin connector on the control unit with a voltmeter.
Tools needed:
- Voltmeter
- A shop-made flat-plug test lead for back-probing the connector
Procedure — with the ignition off, pull the multi-pin connector off the control unit, then:
- Terminal 1 (+) to terminal 12 (ground), ignition on — reading: battery voltage.
- Terminal 12 to terminal 3 — reading: battery voltage. Operate the Off switch — reading: 0 V.
- Terminal 12 to terminal 4 — reading: 0 V. Operate On/Accelerate — reading: battery voltage.
- Terminal 12 to terminal 6 — reading: 0 V. Press the Memory button — reading: battery voltage.
- Terminal 12 to terminal 8 — reading: 0 V. Operate the brake — reading: battery voltage.
- Terminal 1 (+) to terminal 14 — reading: battery voltage. Operate the clutch (switch opens) — reading: 0 V.
- Terminal 12 to terminal 11 (Hall pulse generator) — turn the left front wheel slowly by hand. The voltage must rise and drop from less than 1 V to more than 6 V per wheel revolution (a rectangular pulse).
If a reading is wrong, the fault is in the corresponding switch, the stop-light circuit, the speed pulse generator, or the wiring to that terminal.
Testing the actuator connector
The throttle actuator (drive) has its own connector in the engine compartment. Check it with an ohmmeter.
Procedure — with the ignition off, disconnect the connector in the engine compartment, then read across the drive half of the plug:
- Terminal 1 to terminal 7 (motor) — reading: 3 to 15 Ω.
- Terminal 2 to terminal 4 (potentiometer +) — reading: 2 to 4 kΩ.
- Terminal 2 to terminal 3 (potentiometer sliding contact) — reading: 2 to 4 kΩ.
- Terminal 5 to terminal 6 (clutch actuator) — reading: 30 to 40 Ω.
Replacing the control unit
If no defect can be detected by the connector tests, check the wiring harness to the control unit for continuity and corrosion. If the harness is sound and every input checks out, replace the control unit.