Help! Does anyone understand the footwell blower circuit?

[Mysterytrain]

Ian..back to your issue rather then theory. I would recheck that the rear fan is working at idle since it is controlled by the logic circuit. You will have power on terminals 5&6 of the controller. This power should also be directed [inside the controller box] to the secondary relay that controls the footwell blowers. The current passes thru that mystery component before it reaches the relay solenoid. You should have 12 volts from fuse 1 sitting on terminal 3 of the controller and if the controller is working correctly 12 volts coming out on terminal 4 that will feed the footwell blowers.
After rethinking this issue and the problems other carrera owners have faced when they remove the rear blower and still want the FW blowers to work I began to look more closely at that mystery component.
Ready for this?? That symbol looks to be a reed switch. On the drawing that Ian provided there is a large linear blob that could very well be a coil. If you go back and look at the controller picture you will see what appears to be two coils of aluminum wrapped around a glass tube...a reed switch?!?
If that is the case, when the rear blower is drawing current it creates a magnetic field and closes the reed switch, activating the secondary relay and allowing the FW blowers to operate. In Ian's case, I'm willing to bet that the new motor doesn't draw as much current at idle and isn't activating the switch. I don't have a clue as to why the switch would be there in the first place, perhaps to provide a delayed soft start for the footwell blowers so that all 3 motors don't come on at once?
Looks like the nicest work around is to jumper the reed switch inside the controller box.
Ian, let me know if that really is a reed switch in the controller box so we can prove or disprove my theory...if I'm out of my mind here I want to edit this posting...but if I'm right you carrera guys can backdate your heating systems without that big load resistor to replace the rear blower.

[ianc]

Hmmm... Nice theorizing Ron. I have no idea what a reed switch is, so I'll let someone more knowledgeable chime in.

ianc

[Mysterytrain]

A reed switch is a set of contacts located in a sealed glass tube. The contacts can be made to make or break when they are in a magnetic field. A reed switch works very much like a relay. I've been trying to blow that picture up that you posted to get a better look. I'm hoping someone chimes in with a larger picture of the guts or a schematic. I would be interested in looking at a failed unit if someone has one laying around I might be able to draw a schematic from the actual unit.

[BGCarrera32]

Originally posted by Wavey
What the Hell were these highly educated German engineers thinking?

"Let's wait and work on the hvac setup during Oktoberfest."

[Wavey]

HA! That would certainly explain things... Maybe the solution is to be drunk when you try to use it.

[ianc]

I've been trying to blow that picture up that you posted to get a better look.

Hi Ron, and thanks for the explanation of a reed switch. It wasn't me who posted the shot of the controller, but the other Ian (McArthur). I don't know if that's a stolen shot, or one he took himself. Guess we'll wait for him to chime in,

ianc

[dfink]

Tho I have not taken it out to be sure it appears to be a reed switch. Now the one on my broken unit is open. There is a large aluminum coil that run around the tube about 4 or 5 times. I did jumper this device to see it my footwell blowers would work and they did not. However I may have a faulty relay inside the controller also. My replacement controller works fine. Also interesting note that if you manually close the smaller of the two relays inside the controller the footwell blowers come on. It will do this even with the ignition switch turned off. The germans do things a little different than american cars. On most american cars the switching device whatever it is, is between the power and whatever the device is. The device is then permanently grounded. The porsche appears to send power to the devices then completes the ground with whatever switch is used. Either will work but The american version is safer as with the switch off the majority of the circuit is dead. In the porsche even with the switch off the majority of the circuit is still conntected to the battery and any short would soon cause trouble.

example;
American;
battery------->switch<------device-------->ground

Porsche ;
battery----------device-------->switch<----->ground

[aj88cab]

That device is indeed a reed switch and it does control the relay that sends power to the footwell blower rotary switch (via termial 4 on the controller). The coils you mentioned do wrap around the reed switch and operate it. The coil is the circuit to the engine (edit) blower. I have suspected this is the source of the problem since I began having the same problem with my footwell blowers and opened up the controller but have not been able to find a diagram like the one Ian posted. Great find Ian. By-passing the reed relay sounds like a reasonable work-around. But I would really like to figure out why it is happening.

I once thought the circuit to the speedometer was suspect. It also goes to the power top control unit and the cruise control control unit, most probably for disabling those devices above or below a certain speed. But I guessed that it controls the blower temperature switch to only allow it to operate when the the car is not moving and should not affect the footwell blower operation.

The following link is my favorate for explaning the basics of this control circuit.

heater control circuit

Andrew

[Mysterytrain]

Great news...I didn't want my theory on how the rear blower controlled the footwell blowers to be total fantasy. So, it appears that backdating the heat on a carrera is now a simple proceedure that can be accomplished one of two ways. You can open the Control module and jumper around the reed switch or you can modify the module socket connections by removing the green/red wires from terminals 5 and 6. Then remove and relocate the black wire from terminal 4 to the 5 or 6 terminal. The first relay will now switch on the footwell blowers and will be controlled by all the sensors in the system. Be sure to tape the unused wires. The down side to changing the socket wiring is that the next owner will be totally confused by the mod if they reinstall the rear blower. Secondly, the engine overtemp sensor might want to turn the footwell blowers since the system still thinks the rear blower is still there. You would need to defeat that function too, although it appears that the stock system operated in the same half assed way. I guess the factory figured that in the summer the footwell blowers would be switched to the off position, if and when the rear blower came on.
I think the slickest solution is to jumper the reed switch in the controller and maybe mark the unit as modified or for the real crafty install a switch in the module.

[Rot 911]

What else does that control box operate? I might open mine up this weekend and see if I can find the reed switch and bypass it.

[Mysterytrain]

Kurt,
The control module has 12 connections. From what I gather the module appears on Carrera cars with manual heat and AutoHeat. The AutoHeat systems have the additional circuits in the console between the seats for the interior and exterior duct sensors. The control module alone has the oddball speedometer sensor on terminal 1, Terminals 9 & 11 to the starter that defeat the system during cranking and terminal 10 that goes to the blower temperature switch, which I have been told uses the rear blower to flow air thru the heat exchangers when the engine gets too hot. The other connections bring power in or out to control the blowers.

[Rot 911]

Ron, thanks for the info. So in your opinion, using a wire to jump the connections on the reed switch should allow the footwell blowers to operate as if the engine blower was still in place?

[ianc]

Hey Ron,

Sounds like you're on to something. I haven't been able to devote any time to this in the evenings since cylinder #2 stopped firing.

As soon as I can get that sorted out (badly fouled plug but good compression; injector or guide?), I'll have a whirl at this and see if I can get it going.

In the meantime, any who tinker with it please post their results!

ianc

[Mysterytrain]

Kurt, Ian..yes, it looks like a jumper internally is all you need. If you take the module apart please take some pictures so that we can see the front and back of the module. Based on the picture that Ian provided I'm thinking the backside of the board has solder pads for each of the components. All you would need to do is solder a wire from one end of the reed switch to the other. I'm attaching a modified drawing that shows the jumper wire in red.

[Mysterytrain]

Here is another drawing that has been relabeled based on the drawings found early in this thread. Let me know if I need to fix the wire color coding.

[Rot 911]

Ron this weekend I will pull the controller and open it up. I will probably take a picture to get confirmation of the reed switch.

[RSREE]

Great thread guys! I'm following this one carefully because, I too, would like some heat in my 84 Carrera with autoheat(= noheat) and for which I am about to install the Grainger motor in the engine room.

Unfortunately, I have nothing constructive to contribute on this thread but I might make the suggestion that if we're not successful in turning the heat on, we could always retrofit our freezing p-cars with the Eberspacher Stationary Gas Heater that did a great job in my 56 VW Bug. Someone just sold one on eBay for $350, so they can still be had today!

[Mysterytrain]

Sounds good! It will be great to sort out this stupid little problem.

[Rot 911]

Ok guys here are the pictures:

[efhughes3]

So, the idea here is to jumper the connections of the reed switch on the back side of the board? Is that the reed switch there in the center, the part with the silver coil wrapped around it like a spring?