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I posted this elsewhere (ahem) recently, but want to get it posted here so I could continuing developing the info over time. When I had climate control problems (no heat) I ended up testing all the control motors under the dash, along with their position-sensors (potentiometers); the HVAC temp sensors; and the heater valve and recirc solenoids.
All all these tests can be done in comfort by removing the climate control unit and testing at the connectors that plugged into it. I did these charts to help decipher the factory wiring diagrams and and translate them into something more user-friendly.
Definitely start by checking your heater valve and associated vacuum lines, and the linkage on the control motors, as those are more likely culprits. But if all that checks out, these charts show how to test all the electrical parts behind the dash.
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This tests the main hot/cold flap and related position sensor.
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This tests the defrost motor and related positions sensor.
I posted this on "another forum" some time ago but thought it might be helpful to someone sometime:
Many thanks to Tom M'Guinn for the effort he’s put into HVAC diagnostics. As many have stated before what an asset to this 944 community he is. Thanks to his sleuthing and his guidance on testing I was able to track down the causes for a constant heat condition in my system. Testing the three temperature sensors it was clear that the outside air temp sensor was faulty and appeared to be all burned out as you can see in the picture. Who even knew an electrical component like this existed in the car and why would it burn out?
Not only does Tom have a full HVAC unit on his bench to test and study how the blooming thing works he tracks down work around solutions for components that are no longer available. I put his theory to work and ordered a 10 K 3950 thermistor to repair my original unit. Even though the top portion had melted off there was still a good connection between the remaining wiring and the terminals so I thought why not try to reuse it. I soldered it in and immediately had normal resistance readings again. Glued on a plastic nozzle from a Walmart air pump adapter to protect the very fine thermistor and problem solved for literally three dollars. Further testing revealed the heater mix flap servo motor did not move when powered up which was also evidenced by no change in resistance readings with alteration in polarity. These are readily available on eBay for 30 or $40 and in short order my servo was working again. Put it all back together fired it up and the HVAC is working perfectly once again!
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After spending hours studying the system here are a couple of pearls of wisdom when you have to deal with this:
Of course check the basics first which include testing vacuum to the heater control valve and assessing the little white plastic clips that hold on the actuator rods at the servos. But if you’re not that lucky don’t just immediately assume the solenoids are bad. There are many things that have to go right before a signal is sent to the solenoids. Perform all of the tests in this excellent post because if any one component is not working correctly it seems to throw a wrench in the entire logic of the control unit. According to Clarks Garage the default is a full heat condition. I’m not sure about a no heat condition perhaps somebody else could chime in on that. Second pearl is if you’re working on those servos I found it much easier to pull the driver seat out unless you’re an awesome contortionist. It’s important when replacing the heater mix flap servo that the linkages are connected when the system is in full heat mode. In other words the temperature control dial must be dialed to maximum heat when you connect up the control rods. This is the default position for logic to the control unit. I hope you find this helpful.
This is a great list of what is what. I've spent countless hours swapping parts out and after doing this I realize that the first test "Heater flap control motor and position potentiometer" I'm getting 1300 and 3500, so that sounds like the position sensor has gone bad. ( I tested everything else and they fall into the specs listed, which is great)
I've tried to find a post here and other places to learn where the position sensors are, what the part number is, and how to replace them.
Anyone have a link or an idea how to do these things?
So glad the charts helped. Without ever confirming, I have always assumed and believe the position sensors are an integral part of the servo motors. You can see the servo you need to address in the video below. Much easier to see and access on the bench unfortunately. In the car, you'll need to wedge yourself in the footwell to get it out. You might remove yours and look for bad connections internally before replacing. Can't help but notice your readings are roughly 1200 ohms high on both ends of the range, so maybe there is just a bad connection somewhere adding that much resistance?
air pump adapter to protect the very fine thermistor and problem solved for literally three dollars. Further testing revealed the heater mix flap servo motor did not move when powered up which was also evidenced by no change in resistance readings with alteration in polarity. These are readily available on eBay for 30 or $40 and in short order my servo was working again. Put it all back together fired it up and the HVAC is working perfectly once again!