Only cold air from heater - Solved, I hope!

[944_Kev]

1986 944 2.5 Lux
I can only get cold air from the heater on my 944. Ok in the summer but no good when winter arrives!
With the heater control knob turned in the hot position and with the heater fan on, only cold air exists the vents in the footwell and dash mounted vents.
When I move the upper & lower sliding heater control knobs all the way to the right (fully on) the air flow is strong. With I slide them to the left (fully Closed) the air flow stops. So they are working ok.
I've noticed that when I slide the control knobs to open the vacuum heater control valve in the engine bay doesn't move to open. But when I press the dash mounted air circulation switch the vacuum valve opens. Shouldn't this valve open without turning off the air circulation switch? Am I right in thinking the heater should produce hot air just by sliding the dash mounted knobs to the right and the temp control knob to warm/hot?
I've blead the cooling system using the bleed valve at the front/top of the block.
The two water pipes that supply water through the bulkhead get very warm, so water is reaching the heater matrix.
Is there another valve or something I need to check or replace?

Any advice would be most appreciated.

Cheers,
Kev.

 

[Mike_Pollock]

Look at the side of the heater unit in the passenger footwell and check the clips, and then there is the resistor block that can fail - favourite here.

 

[Paul 290T]

Kev, The sliders are for directing the air from different vents. Its only the rotary knob that controls temperature. Have you tried the quick demist button, does this provide hot air?

 

[944_Kev]

Ta for replies above. Clips on nearside footwell look ok. Flap opens when slider is pushed across and air (cold) can be felt, air stops flowing when slider is fully to the left, so that seems ok. Not sure about resistor yet. No warm/hot air when demist button is pressed on. Just plenty of cold air. When turning the heater temp knob should any resistance be felt or a click noise to let you know it's on? Mine just turns around with no resistance at all. It seems that the temp knob isn't recognised by the heater - hence no heat! Any more ideas/advice anyone? Was wondering about the solenoids, I gather there are two of them. Ta, Kev.

 

[robdimond]

I thought 'cold air only' meant it was one of the thermistor temperature sensors (either the intake one under the bonnet or the cabin temperature sensor under the dash). Most other faults, such as the infamous broken clip are designed to fail hot.

 

[944 man]

The first step would be checking the coolant level...

 

[944_Kev]

Still can't get any warm or hot air from my heater. Coolant level is always spot on, no problem there[] I took the glove box out last weekend (yes I took the interior light fuse out first) the little white clip holding the rod is A-Ok!......piccie below. I also took out the rubber tube that goes from the slits in the dash next to the cigarette lighter. At the other end is a little motor, this spins around when the ignition is on. Where do I find the thermistors/heat sensors? .........both under the dash and in the engine bay, and what do they look like? Any help/pics would be most appreciated. It's driving me nuts not getting any heat into the car [] Thanks, Kev.

 

[944_Kev]

Piccie of working little motor under dash at the end of rubber tube........

 

[tref]

There is a valve in the back of the engine bay somewhere - can you see that going one way and the other - i.e., is there coolant flowing into the heater matrix?

 

[944_Kev]

The heater control valve opens / moves in and out when I press in the air recirculation switch. Hot water then goes in the heater matrix. I thought the heater control valve should open no matter what position the air recirculation switch is.......??

 

[944Turbo]

one of the servo motors (beside the glovebox) siezed in my lux, I stripped it down, cleaned the motor and reassembled, I seem to recall while it was seized the valve under the bonnet didn't move. Tony

 

[944_Kev]

ORIGINAL: robdimond I thought 'cold air only' meant it was one of the thermistor temperature sensors (either the intake one under the bonnet or the cabin temperature sensor under the dash). Most other faults, such as the infamous broken clip are designed to fail hot.

I've removed the under dash (behind the glove box) thermistor & fan housing. Piccie below. Is it possible to renew the thermistor / heat sensor that sits at the front with two wires each side. Maplins sell thermistors for a few pennies....http://www.maplin.co.uk/search?text=thermistors&x=15&y=11.........are these any good? if so which one? Any advice much appreciated. [] P.S. the little fan works fine.

 

[Diver944]

If you have a copy of the workshop manual, I'm sure there is a page in there that lists the spec of the thermistors so you might be able to match up a suitable Maplins replacement for pennies. I replaced the thermistor under the bonnet many years ago and it was under £20 IIRC but who knows what they are from Porsche now without calling them. The manuals used to be available on the Promax and Cannell.co.uk websites but they don't seem to be active any more []

 

[944Turbo]

When you move the heater knob does the servo motor move? If it doesn't if you disconnect the white clip you should get full heat.

 

[944_Kev]

Thanks for replies above. I now have HEAT inside the car!! Yes 944Turbo, I was thinking about removing the rod and leaving it in the default hot position. However I sussed out that the motor was only working sometimes (partially seized). I removed the motor and took it apart as far as I could, cleaned it up and gave it a dose of WD40. On putting it back I was relieved and surprised that I could see the motor and rod moving when I turned the heater temp knob. Yippee......I could feel some heat in the footwells & defrost button for the first time in a year! I also found this link very helpful..... http://www.clarks-garage.com/shop-manual/hvac-01.htm Piccie of motor in pieces......

 

[944_Kev]

Piccie of motor removed and white clip with fitted rod......[]