How Does Automatic Heater Function?

73ess · 09-20-2009, 11:13 PM

I've got the owner's manual and Bentley manual for my '88 930, but neither of them goes into much detail in describing just how the Automatic Heater functions.
There is a knob between the seats that has 10 positions, then the temp sensor in the dash and one in the heat exchanger. But not much description on when each of the three blowers (one in engine and two footwell blowers) should be on.

I'm assuming all three turn on when the temp sensor reads a temp lower than what the position on the dial translates too? So let's say position 5 translates to 75degF and the temp sensor in the dash reads the interior temp as 70degF. Shouldn't all three blowers be running until the dash temp sensor reads 75degF?

I can hear the servos closing and opening the flaps in the heater ducts at the rear of the car, when I move the Automatic Heater Control dial to different positions, but I can't get any of the blowers to turn on. Even when the inside temp is around 55degF! All the fuses look fine. What are the typical problems with this system that would cause the blowers to not turn on? I read that the heat exchanger sensor controls power to the footwell blowers. So I'll check the resistance of that sensor to make sure it's within spec. What controls power to the blower in the engine compartment?

73ess · 09-22-2009, 09:37 AM

Anyone know how they function? Maybe most are no longer working at all, eh?

madmmac · madmmac's Garage

Disconnect and put 12 volts to your rear blower, if it does not run, I don't believe your footwells will either.

tomphot · 09-22-2009, 11:06 AM

Mine works fine.
>>>>
Owners manual says...
With the knob you control the temperature in the car automatically.
Pos 1 - 9 - Low to max heat (thermostatically controlled)
Pos DEF - Max heat (not thermostatically controlled)

To increase the hot air supply, 2 additional fans can be switched on by means of the rotary knob.
Pos - 1 - 7 - Additional fans running at low speed.
Pos 8 - 9 - Additional fans running a medium speed
Pos DEF - Additional fans running at max speed<<<<

Canada Kev · 09-22-2009, 01:05 PM

I was always under the impression that the fan speed was controlled by the dash lever, while the knob between the seats only controlled the temperature. Though, I did notice that when on the defrost position, the fan speed seemed to ramp up.

And to be honest, I never really read the manual...

imcarthur · imcarthur's Garage

For more: The Mysteries of Porsche 911 Auto Heat

Ian

RSTarga · 09-22-2009, 05:48 PM

It's been awhile since I had my 930, but that central control has a little servo that moves the original mechanism up and down. I seem to recall that mine disengaged once and stopped actuating the lever. It was a simple fix. You should be able to hear that motor when the temp setting is moved. If you hear the motor but don't get the heat the thing has disengaged.

fasteddie99 · 09-22-2009, 08:28 PM

Not exactly a moment of clarity by the Porsche engineers.

Canada Kev · 09-22-2009, 08:37 PM

If you can't dazzle them with brilliance, baffle them with bull****?

73ess · 09-22-2009, 10:26 PM

Thanks for the "The Mysteries of Porsche 911 Auto Heat" link Ian! That really helps. I knew someone would have collected everything on this somewhat complicated piece of Porsche engineering. Yes, definitely over engineered. The manual heating system is the best. I had an '88 911 for a while and always liked how my wife, who was always cold, could control the heat to her side separately from mine. Can't do that with the Auto Heat system.

Well, so, none of my blowers are turning on even when I set the control knob to the max "defrost" position. So I'll start with the Main blower and go from there.

73ess · 09-24-2009, 10:20 AM

Hmm....well, I just read in my "Original Porsche 911" book that the Turbos didn't come with a rear blower due to space limitations. So I guess that makes troubleshooting a bit easier. I just have to debug the footwell blowers.

73ess · 11:19 AM

I just looked under my 930 and discovered that some kind of aftermarket exhaust is installed. See photos here:

http://cmd.shutterfly.com/commands/pictures/slideshow?site=930photos&page=930photos&album=27

This may be the reason why my Auto Heat isn't functioning, eh? I'm guessing that the thermo sensor that kicks on the footwell blowers may be fried by this modified exhaust. But I'll check resistance on the sensor to be certain.

Any idea what brand exhaust this is?