Heater Control Circuit

[Early_S_Man]

Hey, guys!

I may have found a replacement for the small relay ... at a price you won't believe. It is a 24 Volt relay, but that may be a good thing in Carreras with Paris-Rhone alternators! It should work OK down to 9 Volts or so, and is rated at about 15 Amps.

http://www.allelectronics.com/cgi-bin/item/RLY-275/500/24_VDC_SPDT_PC_MOUNT_POWER_RELAY_.html

[Jascha]

You are "Top Dog" in my family!


Should we do a 'Group Buy'


Special Quantity Discounts!
10 to 99 $ 0.45 each
100 to 999 $ 0.40 each
1000 or more $ 0.30 each

[aj88cab]

Originally posted by TroyGT

The only small oddity that I noticed is that the footwell blowers seem to stay on all the time... albeit on very low setting. My rear engine compartment blower stills cuts on and off with the levers. The footwell blowers operate independently of the rear blower... meaning I can turn either on without the the other being on first. Is this by design or do I still have a problem here?

Thanks.

-Troy

Back when we were investigating this footwell blower circuit, I believe I discovered an error in the diagram that we were using as indicated in these two images.

Original Diagram (with jumper shown)


Corrected Diagram (W/O jumper shown)


The reed switch (footwell control relay coil power) is not feed from pin 7 & 8 as indicated in the first diagram. It is feed from pin #12. The #12 pin is hot with the ignition ON. If you added the jumper the corrected diagram would result in the footwell relay contact being closed when the ignition is ON and would result in the "always running condition with the heater levers OFF" that Troy is experiencing.

It has been a while since I had my head in this circuit, but I am pretty sure this is the correct circuit, by visual inspection and testing. But I have been wrong before

Also remember that position "0" is not OFF , and that "0" and "1" on the FW Blower switch is the same speed. But that really doesn't have anything to do with the "always running" condition we see here.

Andrew

[Early_S_Man]

Boy, Stuttgart hired some engineers-in-training and gave them some unsupervised work drawing circuit module diagrams that we are just now finding out -- 20 to 25 years later, are WRONG!!!

The two-stage rear window defrost relay is another example!

Things like this didn't happen under Dr. Piech! Or, at least the mistakes didn't wind up in print in service manuals all over the world ...

[dfink]

There have been several that have installed the jumper and I don't think it caused their footwell blowers to stay on. I think MysteryTrain is correct that he probably has a faulty relay. They are pricey little buggers. I would see if you could get one from a salvage yard. If you really, really, really want to mess with the one you have let me know and I will send you the one I have torn apart. You can use it for parts. Just don't be mad when something burns up.

[dfink]

The never ending story. It has questioned why the elaborate reed switch.
In normal operation the footwell blowers will not run if the car is not running. If you turn on the switch then turn on the heater controls the footwells will start momentarily then stop. This function has been oberved by others.
Could it be that with the car not running the current draw is small enough that the footwells don't run because the reed switch is not activated. When the car is started the current flow is higher and the reed switch activates. This would help with not draining the battery when the car is not running.
Or is this function controlled by the speed sensing circuit. If controlled by the speed input the fans should not come on momentarily and the fans will run with the car running but not moving.
A current surge when the heaters are turned on could momentarily activate the reed switch causing the footwells to come on then go back off. The never ending story of the heater module.
So for the guys that have performed the jumper. Do your footwell blowers run with car turned off but the heaters turned on. If so perhaps we have found a useful function for the reed switch.

[aj88cab]

Like I said I have been wrong before.

But consider this, if the first diagram is correct and the oil breather temp sensor (engine blower temp switch on the diagram) sees a high temp condition it sends a signal (via pin #10) that turns on the engine blower. If the first diagram is correct the footwell blowers must also come on. That doesn't make sense (and I do know I'm talking about the 911 heater control circuit here). It makes more sense that the two control circuits are isolated.

Please, if you have an open control unit, trace the circuits and I think you will find the second diagram is correct.

On the other hand, I can't explain why the jumper seems to work for some folks without the FW blowers always on. I installed the jumper and my FW blowers ran whenever the ignition was ON regardless of the heater levers position.

If I am wrong I need to know and I'll throw away my "corrected" diagram, and you'll never see or hear of it again

[dfink]

There are a few differences in different versions of the relay. I noticed this while tracing the circuit a couple months ago. The circuit board that was originally posted for the jumper discussion. Is slightly different than the one I posted for dissection of the circuit. It appears however that the reed switch is supplied by a very large trace section that originates from 7&8 and connects several components in the center of the board. Pin 12 appears to power several components for the control circuit. I am going to take a wild stab that 11 goes through some sort of power conditioning circuit before it hits the control circuit.

My board



Original board

[dfink]

Also keep in mind that nothing happens until the controller tells that main relay to close. The drawings are a simplified and do not include the complex control circuit that determines when the rear fan comes on. All the auxilliary inputs go to the control gates to determine if the engine compartment fan will be turned on. The current flowing through the wire around the reed switch then closes the reed switch which then closed the relay for the footwells.

[TroyGT]

For safety sake... should I just buy a new heater control unit and place inline fuses on my footwell blowers? I don't want to increase my chances of a fire in the engine compartment or in my footwells.

I love this thread though... it is fascinating trying to figure out the German mind with regard to engineering our cars.

-Troy

[Jascha]

Here is the Porsche Service Technical Bulletin (8/12/1987) about the in-line heater fuse

[Mysterytrain]

I have also wondered about the logic circuit..ie: the engine is in overtemp, the speedo isn't sending a signal..so the engine compartment blower should come on...but not the footwells! The diagram that we worked from is not a schematic. The only way to really understand what is happening is to draw the schematic, which I guess could be done from the pictures of the boards.
I always thought the reed switch was odd and I've questioned the ground connection in the diagram for the small relay. The large relay shows the ground going back into the logic module. What I'm suggesting is that there might be a different ground arrange for the small relay in other versions of the circuit. The factory might pull the voltage up on the ground side of the small relay to control when it functions.
I would be curious to know if the modules have rev numbers on the outside so we can see which ones work with the reed switch jumper and which ones do not.
Troy..if you replace the controller I would be interested in borrowing yours to draw out the schematic.

[Mysterytrain]

Other things to ponder...pins 9 and 11 on the controller are connected to the microswitch in the heat lever or cam in the autoheat consoles. They are also connected to the starter solenoid circuit. So when you crank the engine 12 volts is applied the pins 11 and/or 9 depending on position of the switch. After the engine starts the 12 volts is removed from those pins and I'm betting one of them starts to look like a ground at least to the logic circuit. Is this the ground for the engine blower? WHich is kind of like the old SC single relay circuit?
Then there is the mysterious pot in the corner.

[TroyGT]

Ron... if I replace mine, then you can have my old one.

Jascha... thanks for the inline fuse specs. I gotta those installed soon.

-Troy