Dowst

Trying to diagnose non-operational cruise control in a '90 or '91 964. I have done the following:

Verified, by way of multimeter at control unit plug (under passenger seat):

• Ground and power at plug (across pins 12 and 1)
• Brake pedal switch signal (across pins 12 and 8)
• Clutch pedal switch signal (across pins 1 and 14)
• Steering column stalk functionality ("Off", pins 12 and 3 "Accl" pins 12 and 4 and "Reset" pins 12 and 6)

Additionally, I have attempted to test speed input across control unit plug 11 and 12 while simultaneously spinning the left front wheel (as I am under the impression that impulse is provided by ABS left front sensor). I have not gotten the signal I expect (rectangular pulse of less than 1v to 6v as throughout wheel revolution, as outlined in the 944 factory service manual--the systems are very similar if not the same in my observation), but instead around 3v with ignition off, and then battery voltage with ignition on--the voltage does not fluctuate while wheel is turned at all.

I will confirm year of manufacture, as I am aware that prior to '91 cars received their speed impulse from a hall sensor mounted on the transmission (I do not have car at my location at the moment, not sure if '90 or '91), and did not derive signal through front left ABS sensor to FCU.

Additional to the tests conducted at the control unit plug (all of which checked out fine, except for the speed impluse which may not be present due to the possibility that the car is an earlier model and therefore cannot be tested by turning front left wheel), I also pulled the rear blower and tested the throttle actuator (in engine compartment) at the plug connector to control unit (under seat).

I followed the test procedure that I had found within the 944 factory service manual, and all tests yielded results as they should (via plug in engine compartment):

• Terminal 2 to 4 (potentiometer positive): between 2 and 4 kohm
• Terminal 2 to 3 (potentiometer sliding contact): between 2 and 4 kohm
• Terminal 5 to 6 (clutch actuator): between 30 and 40 ohm

To summarize, all inputs to cruise controller (brake, clutch, on/off, accl. etc) have been verified to exist, except speed input (which I may have been testing incorrectly?) and all listed resistances across pins to throttle actuator seem to check out fine.

Does anyone have any idea where I should go next? Can the output of the control unit to throttle actuator be tested? Can the servo throttle actuator be tested? The speedometer works fine, perhaps the impulse to the control unit is not present?

Any insight or suggestions would be very much appriciated

Laker

From Adrian's book, excluding things you checked...

-Fuse #12
-Internal failure of steering wheel control switch
-CC actuator (drive unit) failure
-CC Control Unit failure
-Speedometer failure

Dowst

Fuse #12 is good, all rear brake lights functional and power is present at pin #1 on the connector.

Steering wheel control switch tests fully functional at various pins on connector.

Bad CC Control Unit/ CC throttle actuator is a possibility, but problem really must be known for sure before replacement parts are purchased.

Speedometer failure is something I am very interested in looking further into, as well as the speed input to the CCU in general.

Does anyone know the path the speed impulse travels to get to CCU? My understanding is from hall sensor (either at ABS or transaxle depending on vintage) to FCU to Speedometer to CCU, is this accurate?

Many thanks to all--

Greg_V

LED bulb fitment in the tail lights can cause the CC to not function properly. I had to add some load resistors to solve that problem. Here is a thread: https://rennlist.com/forums/964-foru...e-control.html