heater stuck on full heat has me stumped..

[maritimeblue911]

I''m not sure what's left to check..

I have followed the excellent guide on http://www.clarks-garage.com/shop-manual/hvac-01.htm

I cannot get nr 3, control motor for temperature mix flap, to operate; it should move with the temperature control dial - I know the motor itself is fine as when I connect it to the plug for nr 4, it moves with the slider on the Temperature Control Unit. I also know it can get power as when I detach it from its linkage, it twitches for a moment when the ignition is switched on/off.

Meanwhile I know the solenoid for the heating valve (nr 3 in the other picture) switches the vacuum line ok as I can see the heating valve move under the bonnet when I connect +12V to the solenoid (but the valve stays always open otherwise).

I have changed the Temperature Control Unit in the dash for another -admittedly second hand - one, no difference.

I have replaced the interior temperature sensor for another -admittedly second hand- one, no difference.

One thing that's confused me a bit is that the under bonnet heater control valve is located at the front of my engine above the oil filter, when all the references online mention it as being at the back of the engine by the firewall.

 

[Waylander]

The S2 and 968 are at the front

have you checked the air sample fan for fluff, the clip on the glove box side of the heater box,

 

[maritimeblue911]

The S2 and 968 are at the front

Ah thanks

have you checked the air sample fan for fluff, the clip on the glove box side of the heater box,

yes, removed the fluff first - and when that made no difference replaced with with another second hand one.

I'm wondering if I should try another temperature control unit - but, £££

 

[944Turbo]

I believe there is a third temp sensor in the vicinity of the blower motor inlet in the scuttle area - in addition to the one behind the centre of the dash and the one by the air sample fan,
Not sure on failure symptoms, but I guess it depends if it fails giving high or low resistance.
Tony

 

[maritimeblue911]

I wasn't aware of these - I've only addressed the fan/sensor that leads to the small round grille in the dash. Would love to hear more about where these others are - and indeed I wonder what their fail symptoms are

 

[944Turbo]

Try https://www.porscheclubgb.com/forum/tm.aspx?m=515165&high=heater+
for some more info

Tony

 

[maritimeblue911]

Thanks 944T

 

[Waylander]

Following on from that update and link, the fan motor is nla, but you can replace the thermistor, in fact you can do a bit of surgery and replace all 3 I have the part number for just the tiny component, bought a bag of 10 for about £4

 

[maritimeblue911]

Waylander can you expand on your post in layman's language - I struggle to make sense of it.! Would love to understand all the details as I won't rest until this is fixed.

 

[John Sims]

Back in the mists of time I remember one of mine did this. It turned out to be a simple clip to one of the actuator arms, accessible after removing the glove box ... if I remember correctly.

 

[Waylander]

John is absolutely correct about the plastic clip on the left hand side of the heater box up beside the glove box, about £1.50 I will find the part number

as to the explanation of my post above

there are 3 sensors
the outside sensor which is in the airflow into the RH side of the air intake next to the wiper motor, it is in a plastic thing on a thin bracket with a 2 pin plug

inside sensor which is
in the end of the fan unit behind the glove box, pulling the tube off and looking down the end you should see it suspended in the middle

third sensor is in the mixing chamber of the heater box
they look different but the thermistor inside is the same

i have sent these thermistors to 968 owners in a few different countries

let me find the number and some pictures

 

[maritimeblue911]

thanks so much - that would be super helpful.

Do you happen to know if failure of one of these sensors can trigger my full-heat-on symptoms?

And yes, my plastic clips that connect to the actuator arms that move the flaps are all good, they were the first thing I checked

 

[Waylander]

thermistors

They are 3mm across,

Locations
Outside

Cabin Sensor, located behind the glovebox, the two black wires are connected to the thermistor, that end gets full of fluff

Heater Mixer Chamber, its in there somewhere

you need to check for resistance on these connections
if your a bit electrical this should help
https://porschedocs.s3.eu-west-2.amazonaws.com/pdf/Heater-Schematics.v2.pdf

 

[John Sims]

...

And yes, my plastic clips that connect to the actuator arms that move the flaps are all good, they were the first thing I checked

I'd check them again as they are cunning little devils and do a good impression of being connected when sometimes they aren't. Mine fooled me first time around.

 

[pse_SC]

I'd check them again as they are cunning little devils and do a good impression of being connected when sometimes they aren't. Mine fooled me first time around.

Agree. That was the was the cause of my ‘full on’ heater on 944 Turbo... []
You can operate and watch it working (or not) when glovebox removed (easier).

 

[Waylander]

I was assuming that this had been checked when all extra information I supplied, if it’s just a clip brilliant, if not the information is here

 

[pse_SC]

All info is useful, Waylander... the best thing about this forum (and particularly the 944 section, it seems) is that people can help others such as you have. Long may it continue []

 

[maritimeblue911]

Thanks Waylander - I will go check my thermistors for resistance. Do we know if a failure of these can cause the heat-always-on issue?

 

[maritimeblue911]

The footwell resistor reads about 56K ohm (and it's working, the resistance reduces when I breathe through the connecting tube) yet the outside sensor reads 10k ohm (also working, it reduces when I warm it in my hand) - are they supposed to be the same?

The one by the heater mixer chamber is faulty, no reading at all - I have ordered a new one (at embarrassingly great expense given that I could have soldered in a new thermistor)

 

[maritimeblue911]

Finally got to the bottom of it - it was the Temperature Control Unit - which I had already replaced, but clearly the 2nd hand unit was anything but 'fully tested', as it claimed to be.. I took the car to Peter @ Motortune in Shotts (nr Glasgow) for a service and he figured it out. He is excellent for classic Porsche btw, highly recommended. Unfortunately his 2nd hand TCU has a different problem in that the delmister switch on it keeps erratically triggering one of the solenoids behind the dash - so I have bitten the bullet and I'm getting a new TCU to be done with it all. Pricey pricey