Mark944na86

And before you say "damper linkage in driver's side footwell", I've checked that... (in fact, I replaced the clip when it did break a couple of years back.)

So here are the symptoms: Pervasive warm air from the vents, even with HVAC setting to cold. The only way I can get cool air is to turn on the a/c, which works, but I suspect I'm making the a/c simply overpower the air warmed by the heater this way.

So, I had a look at the heater valve (which is between the PS reservoir and oil filter on an S2.) I was thinking a vacuum leak may have stopped it from closing (valve closes under vacuum.) What I was looking for was whether the arm on the heater valve moved on start-up.

It does. Which (I assume) means the vacuum system is working properly. And the heater solenoid in the footwell. So where does that leave things?

Is it possible the heater valve itself is not sealing properly, and therefore not closing off flow? If I remove it, can it be tested simply?

This is a fairly new heater valve, replaced only a few months ago, after an unseemly failure saw me lose all my coolant on the highway. (At least with the S2 design it didn't dump coolant all over the clutch.)

Is it somehow possible to be getting all this warm air without the heater being on (i.e. while the heater valve is closed)?

I was trying to think of an easy way to block flow to the heater to simulate the heater valve closed, taking the heater valve out of the equation. Pinching one of the short hoses one either side of the heater valves? I'm a bit worried about splitting one, though... any other suggestions?

hobiecat

I went through this EXACT problem. Check the cabin sensor. It's behind the glovebox. There's a little fan
on the sensor that sucks air in from the cabin. Mine had seized up.....it wasn't even turning...so
the sensor wasn't working anymore. Once I replaced it, everything worked like it should. You
can get them from DC Auto....

If that's not it, there's another sensor in under the hood near the fan intake on the passenger side.
That one could be bad as well. Both are a relatively cheap fix.

Mark944na86

Is the part on the diagram here? What part # would it correspond to (below is the S2 system):



(The full DC auto web page with this diagram on it is http://dcauto.gotdns.com/illustration/index/161433554)

JohnKoaWood

#36

Mark944na86

Thanks John -- that's 944.624.013.00 -- described as "Interior sensor interior sensor" on the DC auto description listing below.

Does "Interior sensor interior sensor" have a fan built into it?

(FWIW, Pelicanparts describe this as a "heater temp sensor" -- and their price is almost $100 more than DC Auto! (Looks like Porsche genuine price vs OEM.)

Mark944na86

OK, I've got 944.624.013.00 out of the car (Bosch # 0 130 002 809 in case that's ever useful to anyone).

I can see the little fan on it. I'll do some testing...

fwb42

Keep this up!!!
I've got the same thing going on.

Mark944na86

Well, "interior sensor interior sensor" seems to be working.

It's basically a thermistor and a fan. The fan drags cabin air over the thermistor to measure cabin air temp. Seems a bit overengineered for the job, but whatever...

The fan spins when 9V is applied.

The thermistor changes resistance from about 10K at room temp, to about 8K when I breathe hot 'n' heavy on it. Which proves a) the thermistor is still working, and more importantly b) I'm still alive.

OK, so I'm going to declare "interior sensor interior sensor" tested OK.

Mark944na86

Any other ideas of things to check?

eniac

Possibly one of the flap inside the air box is broken or no longer closing off the heater core? I can tell you that it is not the heater valve(#27). My heater valve on my turbo cracked many years ago(destroying my clutch in the process). As a quick fix I bypassed it until I got a new one. Well 8 years later it's still bypassed. I saw no ill effects from not having it. There is cold A/C in summer, cool air when I turn the temp down when it's cool outside, and nice hot air when I need it.

The only purpose of the valve is to block off the heater core to allow the engine to warm up faster.

Mark944na86

Thanks Eniac -- that's very useful to know. And your idea re the internal flaps is logical, given what you report -- I'm not sure how I could test this easily, however. I'll have to study some diagrams.

Interestingly, Clark's write-up (and some others I've looked at) assumes that a failure to close the heater valve _willl_ result in a "constant heat on" condition -- but what you report shows that (plausible sounding) assumption to be incorrect.

I wonder if the converse is true, however -- that blocking the heater valve would results in "constant heat off"? Given I don't use the heater for 10 months of the year, that could be a useful temporary work around...

Maybe just replace the heater valve with a manual valve of some description?

Mark944na86

This diagram from Clark's site is the best I've seen so far explaining the flap system:



The role of flap 4 is not entirely clear to me. However, it looks like the temp mixing flap 3 is the critical one in this situation.

Interesting how it looks like all air goes over the a/c evaporator 7 before going over the heater 8.

Mark944na86

According to this picture, also from Clark's site, flaps 3 and 4 are both controlled by one electric motor (labled 3 in this picture). So that might be worth checking next for operation, if it's accessable.

Jfrahm

The heater valve can leak internally and look OK, appear to operate. I had this problem with the OEM valve (same part) in my 928.

If it's closing and you get hot air, it's bad. Try direct manifold vacuum to it or wiring it shut and see if you get cool air. If you do, then the valve is OK. IF you use manifold vacuum you will get warm air under heavy throttle but for testing this should work.

-Joel.

Mello

You spoke of the clip under the dash... Is that rod moving when you turn the temperature ****? I've had one get stuck before. I had to give it a nudge and it started working normally again.