Automatic Heating System Help

91189 · 12-12-2004, 05:05 AM

I am trying to get my automatic heating system to work on my 1989-911. The servo motor seems to be unable to open the heat exchanger valves, and thus is constantly working when I dial on my heater control. I am afraid to burn the servo motor, so I leave it off.
I tried to adjust the cables at the valve openings, and I played around with the screw adjustment under the heater knob, but I am afraid I might have thrown everything out of sync!

Does anyone have a complete procedure for proper adjustments to the automatic heating systems?

I have a Bentley manual, but I need more details.

Also, does anyone have a copy of the article on heating systems which was published in PCA Panorama February 2001?

Can anyone help?

Thanks

Paul M.

Jadams1 · Jadams1's Garage

Does the heater lever move at all? If it doesn't, maybe the linkage rod has come off at one end?

91189 · 12-12-2004, 05:32 AM

The linkage rod is secure. The lever moves but only travels a small distance before it seems to be unable to continue. Meanwhile, the servo motor tries to move beyond the position where it stops. I left the side console access door open, so when I want heat, I help the servo motor by manually pushing the lever, but I am amazed at the amount of force needed to push this lever, so I think I might have disrupted something in the adjustment of the heat exchanger cables, or the adjustment screw under the heater position knob. Another observation is that the servo motor does not need help to close the heater from the position I pushed it to.

Thanks

Paul McCarthy

Jadams1 · Jadams1's Garage

The amount of force required is large, due to the gearing. When the lever reaches it max height, climb under the car and look at the flapper valves and see if the shutters are fully closed (all the heat directed to the cabin). If the shutters are all the way closed, the cable needs to be adjusted. Lever all the way down = shutter fully open, lever all the way up = shutter fully closed.

I didn't know about the the adjustment under the heater knob.

p.s. do you play the bass guitar by any chance?

91189 · 12-14-2004, 03:28 AM

Thanks for your help. I will adjust it according to your instructions. It makes sence. The screw, under the control knob, might adjust the voltage sent to the servo motor (but don't quote me on this)

Regretfully, I do not play bass, hence the reason for not owning the C. GT model!. I am however quite content with my vintage 89.

Paul

Mark McClure · 12-14-2004, 03:48 AM

Paul,

I had a similar situation.........I even bought weaker springs for the flapper boxes. The solution was the current to the servo motor. In my case it was a bad earth. In essenece the motor was earthing through a switch and could only get a limited current. Take the control box off and make sure that all of the wiring is correct.

On my car when we looked into the wiring a PO had wired it through a seat belt switch and a few other places. Sometimes it is easier just to run new wires.

John is right the amount of force is quite large. But the gearing seams to be up to it. I have noticed that on full heat my motor goes about every 30 seconds to re-adjust the lever. Like you I do not want to burn out the motor so full heat is very seldom used.

Hope that this helps........

Cheers

Mark.........

91189 · 12-14-2004, 02:41 PM

Mark. this sounds very much like what I have as a problem. I recently bought an ohms meter, and I will try to do the repair during xmas break beginning with flapper adjustments, then I will test the electrical resistance at the servo motor. I will also follow your recommendation to use the heater at a lower setting than full. The heat at position 6 or 7 is sufficient.

Thanks

Paul

fireant911 · 12-14-2004, 03:46 PM

My 1981 has automatic heat control and assuming that the two heater units are identical and just as troublesome... I went throught the procedures in the Bentley manual to adjust the potentimeter to 950 ohms while the dial was on the number 5 position and was never ever able to achieve this value. What I did end up doing was taking the screw out of the lever arm and pulling the emergency activator to the flapper boxes manually which has worked well thus far this season (which I lubed with PB Blaster which greatly decreased any resistance). If additional heat is needed I just move the dial off of the 0 number to activate the fan. If the automatic heater units are not the same please disregard this post.

91189 · 12-14-2004, 04:30 PM

Thank you Daryl,

I will certainly take this route if my attempt at repairing the unit is unsuccessful. It sounds like a simple and effective way to deal with this problem. It seems to me Porsche should have stuck with the manual levers, in lieu of this complex servo driven system.

Thanks Paul.

91189 · 04-02-2005, 04:39 AM

I finally resolved my problem with the automatic heater.
Ron Greene of Campbell Garage in Montreal (Porsche specialist) diagnosed the problem. It was traced to a loose power contact at the board connection.
Ron is a great guy, and honest mechanic. He is highly recommended by local PCA members.

Mark McClure · 04-04-2005, 05:10 AM

Good news..sometimes electrical problems are hard to find and turn out to be relatively simple fixes. Congrats.........Is it getting cold over there now?

91189 · 04-04-2005, 05:53 AM

Well Mark, I sure was relieved. The problem was so bizarre, that I could have ended up changing the sensors, the servomotor, or even the circuit board. Any of these issues would have been an expensive proposition.

Weather is getting better in the Northern hemisphere, took the car out of hibernation last week, and looking forward to 7 or 8 months of driving bliss!

Cheers.