Remove electronic heat control?

[Coultl]

I’m restoring my ‘84 911 which was equipped with the electronic controller for the heater valves. I’d like to remove it (pretty sure it’s not working) and go back to the manual levers.

Is there a writeup on how this is done? Any watch outs? I won’t have the booster fan in the engine bay so I don’t have to worry about controlling that.

Honestly, I still don’t fully understand the system. Any good write ups for diagrams I can read?

Thanks in advance!

[Bill Douglas]

[Bill Douglas]

It makes quite a good cup holder.

Many years ago, but I think it just unscrewed, and the levers were already connected to the flapper boxes.

[Alan L]

There are mechanical connections in the housing - ball joint things that connect the electronic unit to the control wires to the flaps. You will need to disconnect those. Which means removing the housing between the seats. Easiest to remove the seats to do this. One at least, but two is probably preferable. Gives you some room to work around. There are small (8mm hex head?) bolts holding the housing.
Yes they can be problematic - but I have repaired mine. The main issue was stiff cables and the ball joints detaching under load.
Alan

[wazzz]

If you plan to repair it instead of ditching it, I can provide you with a diagram of the electric connections and electronics inside the unit that I created a while ago. I did this as a kind of general schematic to help understand how the complete automatic heater works.

[Coultl]

Wazzz, that would be amazing. Please send!

Removing the unit isn't an issue, I'm most concerned with what it's doing other than just moving the heater valves. What other functionality will I lose.

[Alan L]

That is all it does. No other purpose.
Bear in mind the thermo sensor hanging by your rear view mirror is part of the system too. If that isn't functioning then the unit won't.
Alan

[wazzz]

Here is two versions of the schematic. I don't know which one applies to your car. One is for the earlier model of PCB, which I traced only with pictures found on the web, and the other one is for later model of PCB, which I traced from a broken unit I got from a MY87.

Wire colors are not shown as I was mainly focused on working principle and didn't want to clutter the diagrams. Hope they can help anyway.

Earlier PCB



Later PCB

[Coultl]

Very helpful. Thank you!

[steven c]

I just got mine working and it does control the fan motor on top of the engine that blows air thru the heat exchangers to the cockpit. Mine didn't work because the 2 temp sensors in the system were not working so I just unplugged them from the circuit board in the black control box. So now rather than automatic, I have to move the round switch on top of the unit to the amount of heat I want. Works fine for me, but I may go to the next step and repair the temp sensors.

[wazzz]

Originally Posted by steven c

I just got mine working and it does control the fan motor on top of the engine that blows air thru the heat exchangers to the cockpit. Mine didn't work because the 2 temp sensors in the system were not working so I just unplugged them from the circuit board in the black control box. So now rather than automatic, I have to move the round switch on top of the unit to the amount of heat I want. Works fine for me, but I may go to the next step and repair the temp sensors.

Since you disconnected the temp sensors, moving the round switch can only turn the heater on or off, with no more control of the heat. In the schematic above, you can see that disconnecting the sensors makes the pot RV1 useless as they are all wired in series. Obviously the system in your SC must be a bit different, since there are no footwell blowers and no relays, but the principle must be the same. The PCBs appear to be the same between SC and earlier 3.2. The system in your SC would be closer to the "earlier" version of the diagram. Just omit the relay, the footwells and the part with switches SW-E and SW-G.

[steven c]

Gilles,

It actually opens the flappers in small increments for each click up and the same back down without the temp sensors. What I didn't know until the motor went back in was that it controls the fan on top of the motor too. Comes right on with the first click. Now what happens with it all the way to defrost I have not explored yet. Will see on that one.... Thanks

[Alan L]

The rear fan is activated as soon as you ask for heat.
Alan

[mysocal911]

There're various servo motor controller boards, e.g. 911 3.2, 911SC, 911 Turbo, which are not interchangeable, even though they plug-in.
Typically, the motor I/O pins are different. Good luck in finding the correct board, if the heater unit is missing one.

[mysocal911]

Heater control test setup;



It's as simple as that!

[wazzz]

Originally Posted by Alan L

The rear fan is activated as soon as you ask for heat.
Alan

Yes, that's the result of switch SW-C (see diagrams above) closing at position 1 and above. It sends a ground to the heater relay (electronic unit) which then powers up the blowers. That's the way it works on 3.2. For earlier models (2.7 and SC), a standard relay is used for heater relay.

[Coultl]

Just got my parking brake with manual levers. There’s a switch that triggers when either heat lever is raised. Is that for the rear fan (the one in the engine bay)? Does it also activate the footwell fans? Thank you!

[GeorgeK]

https://forums.pelicanparts.com/porsche-911-technical-forum/811080-1984-cabrio-long-term-project-3.html

Post #49

[wazzz]

Originally Posted by Coultl

Just got my parking brake with manual levers. There’s a switch that triggers when either heat lever is raised. Is that for the rear fan (the one in the engine bay)? Does it also activate the footwell fans? Thank you!

Yes, but this can only work if you still have the heater relay unit in the engine compartment, that's one of the two metal boxes on the left. The switch commands the relay which in turn drives the main blower and the footwell blowers, but the footwell blowers can only work if the main blower is in working order.

If you want the schematic for the heater relay unit, I also have traced it and can send it.

[Michael.]

Question... I have SSIs and no aux fan in the engine bay. The electronic unit only controls heat on/off. Is there a way to remove the box and replace it with levers?