How to diagnose TGK valve

The TGK valve is controlled directly by the HVAC ECU. The ECU controls the direction of a "stepper" motor inside the valve. The stepper motor allows fine control of the valve. The valve reports its position to the ECU via an internal potentiometer. The ECU controls the valve according to AC settings and feedback from various sensors.

I don't know if the design of the HVAC ECU has been improved over the years, but the valve driver chips in the ECU fail on cars like the F355, either due to heat stress or failure of the valve it's trying to control. I think it would be hard to diagnose which has broken. The ECU is probably the easiest (but most expensive) to replace, but as failures of the valve can damage the ECUs... and you may end up damaging a new ECU without first checking the operation of the valve.

You could send the ECU to a specialist to check the operation of the ECU, and if no problems are found, change the valve prior to reinstalling the ECU.

Stepper motors are usually controlled by a pulsed signal which may require an oscilloscope to check, not just a regular voltmeter. You could, however, check the valve's basic operation by putting a small dc voltage on the motor to make it run back and forth... and also check the changing resistance of the internal potentiometer at various valve openings.

If you could get access to the TGK valve connector, I could give you the pins to apply power to to check the motor, but I'm not sure of the way the internal potentiometer is hooked up. I've tentatively drawn a possible configuration in the diagram below:

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The ticking noise may be broken gears or perhaps the motor is running continuously because the potentiometer is giving false position information.

Experts?

 

You may be able to access the TGK valve connector via the access port in front of the windshield.

The plug marked could be for the valve?

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Thanks to the FChatter who posted this photo.

 

I have found on a couple of occasions that the TGK valve is bad and has damaged the ECU. I will replace a TGK and then see if it works. If it is clicking, the valve is most likely bad as it is getting the signal but is usually stuck. I have had one with a damaged ECU that the client didn't want to replace. If this happens, I will leave the TGK disconnected in order to prevent damaging a second valve.
Most often, the TGK valve will restore function.