911 automatic heater control problem

[WCBUD25]

I guess everyone has a problem sooner or later with the AUTOMATIC HEATER CONTROL UNIT. When I click the unit on - no matter what number I choose - I get full heat (I can crawl under and see the butterflies closing) but NO FAN of any speed and NO HEAT REGULATION of any kind - just killer hot air at traffic speed. I have taken the control unit out of the car and traced everthing I possibly can - all looks good - but I have no idea what to check for proper operation.

I then took the control unit completely apart. The motor portion works well but I have no way of knowing if the circuit board is working properly. Everything looks good inside the switch and motor area. All wires are connected and none have been butchered. I am puzzled. I am open to suggestion.

Thanks, eric

[Mysterytrain]

What year car

[MBEngineering]

HI i have the test info for the heating system if you require it, PM me with your ADD and i will send it over.

regards mike

[porschenut]

Have you tested the potentiometer? The Bentley manual describes this test with a multimeter and the adjustment is simple. Mine was slightly out of adjustment, causing the heat regulation to be quirky.

It would help if you told us the year and type of your car. Better yet, put it in your signature.

[WCBUD25]

The car is a 1989 930 and it is very low miles. I can barely work this forum and unforturnately do not know how to PM?? Just another note, I thought that if I switched it all the way to defrost it would by-pass the auto system and turn on the fans but still nothing! Thanks for the responses. eric

[Mysterytrain]

Here is some info for you get get you going until you buy yourself the bentley service manual for your car..

You are going to need a Multimeter for these simple tests and maybe someone that knows how to use it if you don't. Here goes.......

If you flip the control box over and view its guts you will see 2 white connectors on what was the drivers side { towards the front big opening}.
1) The first connector [white wire] is from the sensor on the heat exchanger. You can test the sensor by reading its resistance at 20 degrees C or 68 degrees F. Remove the connector from the assembly and insert the meter probes into the connector. A good working sensor should read 1.7K ohms [1700 ohms].
2) The next connector is for the sensor in the cabin. That bugger is the phallic shaped thing hanging between the visors or the little grille in the dash to the right of the steering column . Same deal with removal and testing, it should read 1.9K ohms at 20 degrees C.
3) The potentiometer can be checked and recalibrated. Turn the heater dial to the number 5 position. Flip the controller over and clip your meter's test leads on the two contacts with wires soldered to them. You should get a reading of 950 ohms +/- 20 ohms. [ no temperature requirement for this test].
Bentley gets a little vague with the rest of the testing telling you to make sure you have power to terminal 15 on connector A and a good ground connection on terminal 31 on connector A.
You might just want to start with test 3 and at least make sure the fuse [number 5 up front, I think] for the system is not blown before you get into the ambient temperature dependent test 1 & 2.
Here is the link to a very lengthy thread on the operation of the engine compartment controller and how it triggers the fans to work :
Help! Does anyone understand the footwell blower circuit?

Keep us posted!

[WCBUD25]

Ron, Just read your response - GREAT! I will head out and start checking. The temp reading might be a trick due to the unusually cold weather here in So Cal ('bout to freeze my shift fingers off) I cannot thank you enough for taking the time to prepare this response - do you need any NASCAR parts?? I will let you know what I find out immediately. Regards, eric

[WCBUD25]

Ok, all I could check out was the pot and it looks good at 1.9. Is there any way of tricking this thing or bypassing it to get adjustable heat? I am not real particular about the gizmo that works the whole mess - just make it happen, you know? Any suggestions would be appreciated. Thanks again in advance. eric

[javadog]

Eric,

The car has two fans. They are located behind carpet panels in front of each door. They have a tendency to fail, particularily if they are not used. Remove the panels, check the in-line fuses and check that you are getting power to the fans. If the fans have power and a good ground, they are probably bad.

As for the temperature problem, you need to check the sensors as Ron suggested previously.

As for the link to the earlier thread posted above, be aware that the automatic heat systems in the 930 and the Carrera or 911SC are somewhat different. The Carreras also had an option of manual heat with a fan assist, which is also completely different.

If you want, you could always remove the auto heat controls and go to a fully manual system by obtaining the necessary parts from another donor car.

Good luck,
JR

[NoLift911]

Originally posted by javadog
Eric,
If you want, you could always remove the auto heat controls and go to a fully manual system by obtaining the necessary parts from another donor car.

Good luck,
JR

Thats the ticket - the auto temp control was archaic at best when working properly back in the day. I have removed mine long.

-Jeff

[emcon5]

You may have more than one problem. The full-heat-regardless-of-setting thing can be caused by the cabin temp sensor. My 82 did the same thing, the fix was to remove the sensor (located between the visors at the top of the windshield on my car) clean the connectors and reassemble.

This wouldn't have any effect on the fan though.

Tom

[WCBUD25]

Again, great info! I will dive in tonight after work. I am thinking that just sitting around has contributed a lot to things not working. The car for some reason just sat in like a time capsule in FL. It seems to have a correct and documentable 10K miles on it. It runs very well, I have documented that it has the correct engine and I have ordered a birth cert from PCA. I have inspected the car from top to bottom, front to back and there is no rotting, corrosion, leaks, binding or dried seals or parts (like it sat outside or something) it did have a bad RR wheel bearing that I replaced and otherwise I just drive it every now and then. I will let you know what I find. Thanks again to everyone who has responded. Very kind regards, eric 805.377.2232

[dirkvanderveer]

Hello, I'm just a new member , living in Holland , with an 911 1977 imported from the USA. I have managed to resolve all the problems with the Porsche sofar, except the reason of the use of the black/blue wire connected to the 3e contact of the potentiometer on the automatic heater controlknop between the seats. The controller is working without this wire connected , but on all the pictures I have seen, there is only a blue and a grey wire connected to the potentiometer.
So, for me it is still a mistury ? And detailed diagrams are not to find sofar.

[tirwin]

Originally Posted by dirkvanderveer

Hello, I'm just a new member , living in Holland , with an 911 1977 imported from the USA. I have managed to resolve all the problems with the Porsche sofar, except the reason of the use of the black/blue wire connected to the 3e contact of the potentiometer on the automatic heater controlknop between the seats. The controller is working without this wire connected , but on all the pictures I have seen, there is only a blue and a grey wire connected to the potentiometer.
So, for me it is still a mistury ? And detailed diagrams are not to find sofar.

There is a thermistor that is in the roof. It is behind the rubber nipple looking thing that hangs down between the sun visors. It can either go bad or become disconnected. Sometimes they get deleted by accident when people replace the headliner.

If the thermistor fails, it causes the autoheat to go into a mode when it is either all on or all off. When I replaced my headliner I was happy to find that the thermistor had simply become disconnected and reconnecting it completely fixed my autoheat problem.

It is an easy check and you may get lucky like I did!

[Peabo]

On my 84 Targa, mine would fail and it was always the sensor being disconnected. Remove two screws. Sensor falls on the floor. Pick up sensor. Re-insert wire. Replace two screws to remount the sensor.

[gazzerr]

Apologies for the hijack. The dial between the seats - is it supposed to be indexed and "click" when you turn it? Mine just turns smooth.

[tirwin]

Originally Posted by gazzerr

Apologies for the hijack. The dial between the seats - is it supposed to be indexed and "click" when you turn it? Mine just turns smooth.

Mine clicks.

[gazzerr]

Thanks. Mine must be busted then. Added to list of projects .

[T77911S]

auto heat- foot well blowers part deux.

verify power is getting to the board

there is a temp sensor in the dash, it has a small fan on it.

if you have power to the board and the sensor is good the board is bad.,

with that said, I don't know if I have heard of anyone saying that their heat actually "regulates" cabin temp. that would be interesting to know.

I just turn mine on an off and don't bother to see if it regulates it.

[dobryan]

Originally Posted by T77911S

with that said, I don't know if I have heard of anyone saying that their heat actually "regulates" cabin temp. that would be interesting to know.

I just turn mine on an off and don't bother to see if it regulates it.

The temp control in my '77 Carrera 3.0 does regulate cabin temp. The dial between the seats is a relative temp setting from 1 to 10. 1 is less heat, 10 is max heat. It raises and lowers the heater knob depending on input from the temp sensor.