Late 85 944 Heat temp control

[Mark911C2]

Turned the cabin temperature control knob up the other day to a higher setting and the heat came on but would not adjust back to a colder setting when I moved the temperature control back to the coldest position. Is there a sensor inside the cockpit that gives feedback to the temperature controller? I noticed a plastic circular insert next to the cigarette lighter (just to the right of it) that has plastic slats, some of which are now missing (broken off). Can't find anything that describes that insert so don't know if that's part of problem or not. Appreciate any thoughts on this!

[944 Ecology]

There is a total of three thermistors which provide input to the hvac controller.

1) At the air intake to the hvac system at the base of the windshield, passenger side.

2) Behind the glovebox, a long tube connects the grille (the circular part you mentioned) to a air sucion motor, the sensor is part of the motor.

3) In the center vent in the dash, the third thermistor.

The best test is to let the car sit overnight and measure the inputs from the sensors to the controller. At the same temperature, all three thermistors should read the same. There is nothing unusual about the thermistors, they can be replaced with generic pieces.

[GregAmy]

Originally Posted by 944 Ecology

The best test is to let the car sit overnight and measure the inputs from the sensors to the controller.

Where/how is that done? I am experiencing similar wonky inconsistent heater temperature control in my '92 968.

- GA, who wonders why we can't just have a slider lever and adjust it manually to suit...?

[Monkey Wrench]

a thermistor is a resistor that varies with temp, I think you need to just disconnect them and do a resistence comparison. if the car sat , in theory they are all the same temp thus should be equal resitance.

If that sensor itself is a fairly common one you can probably just get it through an electronics supplier. if it has a number on it, google it , youll probably see it on sites like digikey , mouser , electrosonic or similar at a reasonable price.

Completely agree, too many things on cars became electrical, and used proprietary parts. The newer the car is, the more unnecessary wiring and proprietary electrical parts were introduced, and the more owners became forced to buy proprietary and unique products.

the less of that proprietary and unique electronic garbage a car has, the simpler and more cost efficient it is for an owner to maintainon his own vehicle, and the less he has to go into a dealer to have his pockets lightened.

The main reason this guy loves old cars ;-)

[GregAmy]

I was involved in a conversation of what's the best "old cars" to daily drive now...and this is the era, mid-90s to early 00s. Better than the older stuff but not as complicated and over-engineered (he says, looking at this heater design) than the new stuff.

So while I know what a thermistor is and does...where are they in this car and to where do they go? I presume they all go to a common computer someplace where it would be easy to test them in one place; if so, where is that and what pins?

I've got the hardcopy wiring diagrams, I'll give it a deep look-see.

[Jfrahm]

First, check the vacuum line from the intake to the vacuum canister near the battery (Left hand drive, USA config). Check the one-way valve. Replace any loose or cracking connectors connectors or line. Then check the line to the heater valve and the connection there.

If your heater valve is old it might be failing. This can either be on the vacuum side, losing the ability to close and stay closed, or on the water side; leaking coolant through when closed.

These heater valves are not great in my opinion and I like to change them to one that won't crack open like an easter egg and dump all your coolant on the RF tire. I usually use Four Seasons Valve 74671 , which is under $20 on the jungle site.

Given your presentation I suspect you lost vacuum to the can and ran out of vacuum to keep the valve closed after you selected heat.

[GregAmy]

Originally Posted by Jfrahm

If your heater valve is old it might be failing.

I think you might be on to something. I went into the garage and checked it out; it's the valve under the PS reservoir, yes? As it sat, the rod was fully extended outward, away from the vac solenoid (as expected, with no vac).

I started the car with the temp set to full cold, the rod did not move. I switched the temp to full hot...and the rod did not move. With the car idling I removed the hose and checked for vacuum with my fingertip, nothing noted (my MityVac es mort).

I manually pushed the rod in and it did not move freely and when I released it it did not return fully, I had to extend it. A Clue.

I then took it for a ride around the neighborhood with the temp set to full hot and within a mile I had solid cabin heat. I stopped in a parking lot and noted the rod was still fully extended. So that tells me that the default "fail" position for the valve is full hot (which makes sense).

While parked I moved the temp to fully cold and I could hear the dash flappers moving around and the output air changed locations (it got higher output on the dash) and it stopped sending out heat. I rechecked the rod position and it was still fully extended.

I played with intermediate temperature settings and it seems that nothing in the middle made a big difference on temperature output, it stayed cool and only changed how much air came out of the dash. And when I moved it to fully hot I again got very hot air coming out.

My thesis is that this "computer" uses a combo of both heater valve and flapper control to decide where and how much heat to put out. And, since my heater valve does not seems to be working properly, that hot water is constantly going through the heater core and all I have controlling cabin heat is the flappers. Somewhere in there it must have the ability to block off airflow through the heater core, instead pulling it from inside, but I lose the granular control it offers that way; the system becomes binary (only ambient or only full heater).

All based on only a 15 minute drive, of course.

check the vacuum line from the intake to the vacuum canister near the battery

I saw that under there; is that flapper control? If it is, I've verified that something in the flapper system is getting vacuum, so that implies there's good vacuum.

And, temp control system is moving those flaps around, so that implies the sensors are probably good.

So we're back to the heater core valve.

These heater valves are not great in my opinion and I like to change them to one that won't crack open like an easter egg and dump all your coolant on the RF tire. I usually use Four Seasons Valve 74671 , which is under $20 on the jungle site.

Direct bolt-on? Same hose sizes? "Jungle" (lol) says it's not a proper part and directs me to a URO plastic version. But if it works, I'll use it.

Looks like quite the PITA to get to and install...

Greg

[GregAmy]

For reference, it appears all three sensors go into the "Heater and A/C Switch".

Outside Temp Sensor is pin A9 to ground
Inside Temp Sensor is pin A2 to pin B4
Output Sensor is pin A1 to pin B4

That unit also controls the Defrost Flap, Footwell Flap, Mixing Flap, and Fresh Air Blower. All so we can have a nifty Celcius dial to use instead of a simple heater lever...

[Jfrahm]

shows the 74671 as a cross to a URO that they is correct for the 968, so it should be OK and should be the one I have used in a bunch of transaxle Porsches.

ÜRO PARTS 0008305784
FOUR SEASONS 74671

Application for the URO:
AUDI 5000 1978-1988
AUDI V8 1990-1991
--snip--
PORSCHE 944 1985-1991
PORSCHE 968 1992-1995

It's a bolt in and the the arrow for flow should be pointing towards the radiator, away from the firewall. I've installed them backwards (oops!) and they work OK that way. You probably will need a ziptie or heat shrink or something on the vacuum line as the barb is a bit small. You can also get some small diameter silicone vacuum tubing that will stretch over the plastic line and the barb tightly.

You might see if the line to the heater valve sucks up some alcohol or WD40 or something when it's supposed to be sucking the valve closed just to be sure you have the appropriate vacuum signal.

> I saw that under there; is that flapper control?

It's the vacuum for various HVAC functions, the check valve and can make for consistent operation at various throttle settings. I do not know offhand what's vacuum controlled and what's solenoid or what.

> If it is, I've verified that something in the flapper system is getting vacuum, so that implies there's good vacuum.

Um, sure. Making assumptions is a good practice when dealing with a puzzling problem. :-)

-Joel.

[GregAmy]

Originally Posted by Jfrahm

Um, sure. Making assumptions is a good practice when dealing with a puzzling problem. :-)

Not "assumes", brother: "implies". I choose my words carefully.

Thanks for the Tips! Next stop, a new heater valve. - GA