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Heater Control diagnosis


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Is the problem that the fans don't turn on?

or does it blow cold air?

The fans run strong, but the flappers at the heat exchanger aren't closing. The cables are free and the flappers move freely too. The servo motor is not moving the rod to actuate the flappers. Therefore, I am not getting heat, only fresh air.

If it was up to me I would eliminate all the sophisicated electronics/motors/sensors and just install a lever system like in the older cars. Let's face it, Most people driving this car are driver types and don't mind some road noise, climbing into the seats, knowing how to use the tool kit, and adjusting a manual heater system.

But, I do have a desire to maintain the originality of the car and therefore will attempt to rectify the heater problem.

Thanks for your help

Edited by Rickster2
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  • Admin

Have you had a look at the mechanical cable connections?

Inside the automatic heater, there is a link which operates the lever for the heater cables. It is attached by a small plastic ball end socket that screws on. You can see this by removing the rubber cover on the front of the unit, and looking into the heater unit, on the passenger side. There is also a plastic inspection door on the unit. It is easier to do this if the passenger seat is out. The part number for the socket is 999 168 021 40. Be careful not to pull too hard on the link in the fore-aft direction, as it can damage the motor.

Chris Powell - PCA WebSite - 2/22/2005

Also, ...

There are the two temperature probes. The one in each heater duct and the the unit above the mirror. If either one of these sensors go bad the reference point for the controls is no longer valid. The heater bases its control on resistance of the sensors which is converted to temperature. It then uses the interior temp and the heater duct temp as a delta reference. The sensor in the interior of the car at room temp should be apx 1.5kOhm. The sensor in the duct with the engine warm (212 Fahrenheit) and should be apx 135 ohm. If both of these are bad the unit could have a bad switch or control unit. The control switch can be checked, but must come apart. The control unit should have switched 12 volts to it. If the lines are dead then the control unit is good. The cost of the control unit is very high and should it be bad I would try to search for a good used one or remove it all together.

Stephen Kaspar - PCA WebSite - 9/9/2003

If neither of those help let me know and I'll see if I can find my 930 service manual and lookup the wiring.

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OK, I'm not very knowledgable with electic but can use an Ohm meter. Considering the linkage, I have the control box completely removed from the car. While it was still in the car I worked the controls to see if the servo motor would move. Since my engine is not in the car (top end job) I heated the rear sensor at the heat duct and watched my Ohm meter change as I heated the sensor. I could not get the motor to move. All linkage is connected and cables and heater boxes are free moving.

The only sensor I am aware of that is inside of the car is the dashboard sensor which is connected by a rubber hose to an inlet that is to the right of my steering wheel. I might be missing something there! And, I did not know how to test the dashboard sensor.

I also confirmed that I am getting power to the control box.

I thought I could energize the motor and confirm that it works. But, I could not figure out how to test it.

Otherwise, it sounds like I should wait for my engine and check the system with a warm motor. The Ohm meter values are helpful. I will revisit this problem in a month or so unless someone can suggest how to test with no engine.

Thanks again for all the help


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