Cruise Control Troubleshooting — Later Cars
Tempostat troubleshooting for later 944s (model year 1985½ onward): the first checks, a symptom-to-cause guide, and the factory voltmeter and ohmmeter values for the two control-unit connectors (instrument plug and front-end/drive plug).
CRUISE CONTROL TROUBLESHOOTING
PORSCHE 944 · 1985½ ONWARD
Pick the symptom you have to start the diagnosis.
How the system works
From model year 1985½ the road-speed signal comes from the speedometer signal rather than a separate wheel pulse generator. The control unit now uses two connectors: Plug 1 carries the instrument-cable signals (speed, switches, brake lights, clutch), and Plug 2 carries the front-end cable to the throttle actuator (motor, feedback potentiometer, and clutch coil). Plug 2 is keyed with a coding pin so it cannot be connected the wrong way.
The cruise circuit shares fuse 19 with the brake lights, and it borrows car ground through the brake-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 19 in the central electric console. The brake lights use the same fuse, so a cruise system that is dead together with the brake lights points here first.
Central electric panel — later cars (1985.5+), by fuse number
| № | Rating | Protected circuit |
|---|---|---|
| 19 | 15 A | Brake lights |
Numbered by official fuse number 1–41 (the late fuse list). The factory does not publish a 1:1 map between these numbers and the physical carrier positions, so this logical view is ordered by number; see the physical Type-2 carrier for the as-installed layout. Fuses 12, 26, 27, 29 and 35 are unpopulated.
Brake-light ground fault
Terminal 2 of Plug 1 takes ground from the brake-light bulb filaments whenever the brake pedal is not pressed. If both brake-light bulbs are burned out at the same time, the control unit loses its ground connection to car body and cannot function. Confirm the brake 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 7 of Plug 1. 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 two connectors on the control unit. Check Plug 1 with a voltmeter and Plug 2 with an ohmmeter. Start with Plug 1.
Tools needed:
- Voltmeter and ohmmeter
- A shop-made flat-plug test lead for back-probing the connectors
- A lifting platform (for the speedometer-signal test)
Procedure — with the ignition off, pull off both multi-pin connectors. On Plug 1:
- Terminal 4 (+) to terminal 8 (ground), ignition on — reading: battery voltage.
- Terminal 2 to terminal 8 — reading: 0 V. Operate the brakes — reading: battery voltage.
- Terminal 3 to terminal 8 — reading: battery voltage. Operate the Off switch — reading: 0 V.
- Terminal 5 to terminal 8 — reading: 0 V. Operate Resume — reading: battery voltage.
- Terminal 6 to terminal 8 — reading: 0 V. Operate On/Accelerate — reading: battery voltage.
- Terminal 4 to terminal 7 — reading: battery voltage. Operate the clutch — reading: 0 V.
- Speedometer signal. Place the vehicle on a lifting platform and raise it until the wheels turn freely. Connect the testers to battery (+) and terminal 1. With the ignition off, the reading is approx. 1 V below battery voltage; with the ignition on, 0 V. Start the engine, engage second gear, and accelerate to 50 km/h31 mph — the reading should swing about every 4 V, increasing with speed.
Note: On cars with a lifting roof the speedometer signal can be checked more simply — unlock the roof and accelerate above 5 km/h3 mph; the roof should lock automatically. Then use an ohmmeter to check continuity from lifting-roof relay terminal A to control-unit Plug 1 terminal 1.
Testing the actuator connector
The throttle actuator is reached through Plug 2 (the front-end cable). Check it with an ohmmeter:
- Terminal 1 to terminal 2 (clutch coil) — reading: 30 to 40 Ω.
- Terminal 4 to terminal 7 (potentiometer) — reading: 2 to 4 kΩ.
- Terminal 4 to terminal 8 (potentiometer) — reading: 2 to 4 kΩ.
- Terminal 5 to terminal 6 (motor) — reading: 20 to 30 Ω.
Replacing the control unit
If no defect can be located based on these tests, replace the control unit.