Motosport

I have a heating issue on my '89 951S I just cant figure out..

The heat only comes on when the temp dial is turned almost to full heat, and even then it only blows luke warm air.. When the car is started during warm up, and I have the heat fully turned up, it does blow reasonable hot air. But when the car is up to full operating temp, the air temp goes luke warm even with the temp dial turned to full heat. The temp range on the dial goes from 18 - 30 degrees C, with 22-24 being the normal comfortable temperature, and if I turn the temp dial down to 27 C. from full heat, the air temp already starts going down to barely luke warm. And anything lower on the temp dial just results in cold air. It basically acts like the temp control unit needs to be calibrated..

I did some searching, but most problems seems to be to much heat or no heat at all, and I didn't really find any treads with the same problems as mine. I checked the heater control valve, and it seems to work fine. Its fully open with the engine off, and closes when the engine is started and the temp dial set to cold. If I turn up the temp dial up to 22-24 C, the HCV starts to open and if i turn the temp up further, the HCV goes to full open. All the vacuum lines looks good and are properly connected in the right locations. With the engine at full operating temp, the coolant hoses to and from the HCV are equally warm and so hot that I can only hold them for a few seconds. So it seems that there is plenty of hot coolant going to the heater core, but still only luke warm air coming out of the vents..

I also tried bleeding the coolant system with the engine warm and the front raised, but as soon as I opened the bleed valve there was coolant coming out, so I assume there's no air in the system.

Can it be the temp control unit in the dash thats faulty, has anyone experienced this.? What else should I look for.? Any advice is much appreciated so I don't have to face the upcoming Scandinavian winter with anything less then a fully functioning heater..

Alex.

User 52121

Did you check under the dash to make sure the clips that hold the rod which controls the flapper valve is still connected?

They tend to break. I just had to replace mine.

Just look under the driver's side dash, on the side of the center console. There was a thread up here not too long ago that had pictures of what to look for.

Motosport

Yes, I did check the clips and they were all good, and the flapper valve moved accordingly when I turned the temp dial.. I was hoping it would be an easy fix like this, but I guess not..

Alex.

gruhsy

Your issue sounds like your heater core could be full of scale. I had to flush mine when I did my new build. When I pulled the 2.5L out, the cooling system was full of scale from the previous owner not using demineralized water.

Try flushing your cooling system with a descaling product for cars. It wouldn't hurt. Mine was doing the exact same thing you describe when I first bought it.

The car engine was hidding a surprising amount of scale.

Your in and out lines for the heater core will be hot regardless of weather it's plugged or not.

Hope that helps.

Jason

Bri Bro

Temp controller or the air temp sensors.

Motosport

I dont think its debris in the cooling system causing this, since my car has always been serviced at an authorized shop and the last time the coolant was replaced it was filled with distilled water. But if nothing else cures my heater problem, I'll pull the heater core and flush it.

Is there any to test the temp control unit, or is the only option to try installing another unit.?

Where are the interior air temp sensors located.? I looked at Clarks Garage, but the only thing I found was that there is one located somewhere on the passenger side/behind the HVAC unit.. Can anyone tell me the exact location and how it looks, or even better has a pic of it.. I know how they look on VW cars, but I'm not sure if its the same on our cars...

Thanks, Alex.

Motosport

I did a little more testing today, and when I turn on the defroster it produces plenty of real hot air from the windshield and side vents, regardless of the temp dial position.. So that should confirm that the heater control valve, heater core, flaps etc. is working correctly. But as soon as the defroster is turned off, it goes back to blowing luke warm air even on full heat, and as soon as the temp dial is turned down a few degrees from full heat it just blows outside temp air.

I guess that just leaves the temp control unit and interior temp sensors, or could there be other things causing this behavior.?

Thanks - Alex.

Bri Bro

The factory manual has a good section on it and shows were are the bits a pieces are located. Be careful to get the correct year as thing did chamge I would start by swapping out the controller since it is the easiest to do and is the most likely to fail.

Motosport

Problem is that I don't have a factory service manual, but I think I'm going to buy one in near future as I've got the feeling its going to be very useful many times over..

I'll try and borrow a temp control unit for testing before I buy a new one, since they seem pretty expensive. And it that doesn't help, hopefully someone can help direct me to the location of the temp sensors..

Thanks - Alex.

Bri Bro

Maybe this will help.
http://the944.com/manuals.htm

Motosport

Brian, thanks a lot for that great link... I cant believe I haven't found this before..

All sorts of great info on our cars, but the best part is all the shop manuals ready to download.. I guess my printer will work overtime tonight..

Alex.