944 S2 Heat on issue

[Outlaw944]

Hi All,

New to the carpokes forum and first post. Appreciate this is probably the 1000th time on this subject but I am pretty stumped on what the driving issue is for this.

I have the common always hot issue with my climate control (non air conditioning). The standard arm clip issue is fine with a new clip, new heater control valve too. I have checked the heater arm motor by applying 12v power and it moves the full range fine. However, when selecting hot/cold via the knob the arm does not move at all. The only time it moves is when ignition switches on and it move about 1/10th of the movement and settles in the hot position.

Appreciate any help on this.

Anthony

[Tom]

Hi All,

New to the carpokes forum and first post. Appreciate this is probably the 1000th time on this subject but I am pretty stumped on what the driving issue is for this.

I have the common always hot issue with my climate control (non air conditioning). The standard arm clip issue is fine with a new clip, new heater control valve too. I have checked the heater arm motor by applying 12v power and it moves the full range fine. However, when selecting hot/cold via the knob the arm does not move at all. The only time it moves is when ignition switches on and it move about 1/10th of the movement and settles in the hot position.

Appreciate any help on this.

Anthony

Welcome aboard Anthony! Odds point to the climate control box itself, but there are a few more things you can test first. You might just want to run through all the tests in our diagnostic guide found in the link below. Be sure to test the solenoids and temp sensors. They don't fail often but when they do they can cause the dreaded always-hot condition. (Actually, just about all failures default to heat-on by design, since Porsche was apparently more worried about hypothermia than heat stroke ).

viewtopic.php?p=2876&hilit=HVAC+diagnostic#p2876


I'd also check the heater valve to make sure it holds vacuum and, assuming it does, check the solenoid that controls it and the vacuum source to that line. See more on that here:



Let us know what you find and we can go from there!

[Outlaw944]

Thanks Tom and appreciate the notes left on an FB post. That is what led me to joining here.

The heater control valve is brand new and holds vacuum. I tested when manually closing and hold a finger over the vacuum hole. Releasing allowed the valve to open again. I hadn't tested live so I just did that and the valve operates as intended when applying 12v to pin A4.

The sensors appeared normal results but I will retest those again. Looking now for a UK company that can test the unit itself. Assume some issue with the temp control knob and hardware behind it maybe.

Thanks,

Anthony

[Outlaw944]

Hi @Tom I went back through the car today and checked all the points in your very handy pdf. Supe helpful! I have the following results. Does anything stand out that could be the issue? Thanks

All flaps operating when apllying 12v to them
Solenoid works when applying 12v and closes the heater control valve
HVAC Tem sensor ranges @46F: Outside 20550ohm / Inside 34420ohm/ Mix 6180ohm
Footwell 160-2368ohm
Defrost 101-2294ohm
Heater mix 750-2303ohm
Solenoids Recirc 112ohm / Heater 117ohm

[Tom]

Hi @Tom I went back through the car today and checked all the points in your very handy pdf. Supe helpful! I have the following results. Does anything stand out that could be the issue? Thanks

All flaps operating when apllying 12v to them
Solenoid works when applying 12v and closes the heater control valve
HVAC Tem sensor ranges @46F: Outside 20550ohm / Inside 34420ohm/ Mix 6180ohm
Footwell 160-2368ohm
Defrost 101-2294ohm
Heater mix 750-2303ohm
Solenoids Recirc 112ohm / Heater 117ohm

The temp sensors should all read about the same if you let the car sit long enough to ensure all sensors are seeing the same temperature. Looks like those readings were taken with the heater recently on? If so, I'd try again after letting it sit all night without running the motor.

The heater mix flap in theory should go lower than 750ohms, but my guess is that's a function of the linkage restricting it's full range.

You say the solenoid works when applying 12 volts and you can see the heater valve under the hood close when 12 volts is applied? I trust the heater valve is nonetheless opening when you plug in the climate control unit?

It's looking like a bad climate control box, unless further testing/clarification uncovers something off with your temp sensors. I'd offer to look at it for you, since sometimes its as easy as the little pins that hold the PCBs together -- but suspect shipping back and forth to the UK would cost more than it's worth -- especially since my ability to fix them is about 50/50 depending on the fault.

[Outlaw944]

Actually the car has been sat for a few weeks whilst doing some winter maintenance. I start it yesterday so for todays testing it has been sat without running for 36hrs ish.

The heater control valve opens back up once I remove the 12v power. It will only operate whilst I apply the 12v

Hopefully I can find a unit from a local 944 owner to swap in and see if it's that or not. I can't find anyone local at the moment that looks at these units. Appreciate the offer. I am in the US a bit for work but only a few days at a time so it probably wouldnt work out.

[Tom]

Actually the car has been sat for a few weeks whilst doing some winter maintenance. I start it yesterday so for todays testing it has been sat without running for 36hrs ish.

The heater control valve opens back up once I remove the 12v power. It will only operate whilst I apply the 12v

Hopefully I can find a unit from a local 944 owner to swap in and see if its that or not. I can't find anyone local at the moment that looks at these units. Appreciate the offer. I am in the US a bit for work but only a few days at a time so it proabbly wouldnt work out.

hmmm.... if those temp sensor readings were taken after the car sat 36 hours without running, they are curious at best. They should all be reading roughly the same. At 46F, they should all be around 23K ohms +/- 10%. Your outside temp reading seems to be in the right ball park, but the flap sensor seems quite low and the inside seems high. If you can find a known-good cc unit to try, I'd still do for sure. While hunting for one, you might try substituting 20k resistors in for the questionable sensors (or retesting right at the sensors to see if there's a harness issue). Pretty sure you can pull the rear covers off those connectors and remove the pins/cups if you wanted to bypass the sensors/harness and plug in fixed resistors just to see if maybe the cc is working as intended but just being fed bad temps by the sensors. I'd say there is 'some' chance of that, but still more likely the cc unit if the heat stays on even when you crank the temp all the way down to the blue dot. (On the other hand, even with the cc unit fixed, it might have a hard time regulating temps to the desired heat if those sensors are bad.)

If further testing suggests those sensors really are bad, you might want to open the housings and check that they are securely soldered on their wires. If so, and if you need to replace them, these sensors would be a close match for the factory units. (Vishay ACC004)

https://www.digikey.com/en/products/det ... 04/5967514

You could buy the whole sensor assembly, but I find it's much easier (and a whole lot cheaper) to just solder new sensors to the harness wires, assuming the harness is ok. Here are some shots from my car, showing how one of the sensors just broke away from the harness and how easy they are to replace.

sensor 1.webp (209.81 KiB) Viewed 475 times

sensor 2.webp (391.15 KiB) Viewed 475 times

[Outlaw944]

Thank's that's usefull information. I went back to the car to test the three thermistors again and its more interesting.

The outside temp still reads well at 20000ohms, the inside has now settled at 25000-30000ohms. It did drop too 6000 but assume iwas due to the small fan pulling in cold air while i had the ignition on. The mixer chamber however is now not reading at all. I removed from the holder and the wires look okay but there is zero reading either via plug A or directly on the wires.

I also took to inspecting the wiring harness whilst there and have seen that plug C has some exposed wiring. Not sure what this plug does (supply power?) I do not see any evidence of arching and it look fresh so maybe it was a result of pulling the unit out.

Screenshot 2026-01-20 at 17.55.29.png (5.57 MiB) Viewed 454 times

Screenshot 2026-01-20 at 17.55.19.png (6.25 MiB) Viewed 454 times

[Tom]

Those exposed wires are for the blower motor control, so probably not related to your temp control issue. Definitely fix them though. Does you blower fan sometimes have a mind of its own -- if so, that's probably why... I'd also take a very close look at all the wires to make sure there aren't more like that.

The temp sensors are NTC sensors, meaning the hotter they get, the lower the ohms. So when yours when to 6k ohms, that's actually a high temp reading (~92F). It's possible something warmed it up, but also quite possible the sensor is just old and failing. Your outside temp sensor might be drifting to the cold side too. From the manual, the known points on the scale are:

10K Ω @ 77F
20k Ω @ 50F
34K Ω @ 32F

There are math models (Steinhart–Hart) that let you find a best fit curve based on those data points, which is how I arrived at the suggested replacement sensor in the digikey link above. I did the math several years ago and settled on that particular thermistor as the best fit replacement. To my amusement, I just gave those 3 values to ChatGPT, and it calculated a b-value of 3987 and recommended that exact same thermistor! Especially since one of yours seems dead, I'd replace all three and then see where you are. It may or may not fix fix the heat-on problem, but the system will never work as intended with bad or no sensor readings. If you want to really confirm the sensors, de-solder them from their wires and let them sit together somewhere for a few hours. Then check their resistance and compare the results to this chat-generated chart based on the ambient temp when tested.

°F (°C) Ohms
-----------------------
32 ( 0.0) 34000 <-- reference point
34 ( 1.1) 32120
36 ( 2.2) 30360
38 ( 3.3) 28710
40 ( 4.4) 27160
42 ( 5.6) 25700
44 ( 6.7) 24330
46 ( 7.8) 23040
48 ( 8.9) 21830
50 (10.0) 20000 <-- reference point
52 (11.1) 18860
54 (12.2) 17780
56 (13.3) 16760
58 (14.4) 15800
60 (15.6) 14900
62 (16.7) 14040
64 (17.8) 13230
66 (18.9) 12470
68 (20.0) 11750
70 (21.1) 11070
72 (22.2) 10430
74 (23.3) 9820
76 (24.4) 9240
77 (25.0) 10000 <-- reference point
78 (25.6) 8970
80 (26.7) 8460
82 (27.8) 7990
84 (28.9) 7550
86 (30.0) 7140
88 (31.1) 6760
90 (32.2) 6400
92 (33.3) 6070
94 (34.4) 5760
96 (35.6) 5470
98 (36.7) 5210
100 (37.8) 4980

[Outlaw944]

Well, i pick some new thmistors up tomorrow. I'll swap out all three and see what the reads are when done. I'll report back soon! Hopefully they will all read witihn parameters. I'll tidy up those exposed wires. I never had a fan issue until it was rmvoed but still plugged in. Wiggling it made the fan come on so that'll explain that.

I had a service ticket on the CC unit of June 2004 so at some point someone had it looked at or swapped out for a second hand unit. Hard to track down a place to take a look at moine so I am holding out that someone is close by that I can swap out to see if it works in their car or not.

@Tom I assume there are the three temp sensors but only two or replaceable themistors? The one under the bonnet measuring air coming in is a cone shape with a plug end?

Thanks for all the help. Trully appreciated.