Peter_964C2

Hi,

I have a problem with my 1991 C2.
The problem is that both front blowers and the rear blower (engine compartment) do not work.
I was able to let the rear blower run by providing direct power.
Same for the front blowers.
I had the possibility to replace the "Heater Contol Unit" and the "Blower Final Stage" but that didn't solved the problem.
All the fuses and relays are good. I've checked most of the wires and connectors, and they are looking good. I can move the servo's via the Heater Control Unit'

My question is..
I read that the front blowers will not work if the rear blower doesn't work.
There are 2 sensors near the rear blower, the "NTC Heater Fan" and the "Resistor Blower".
If one of those sensors are broken, will my front blowers still work ?
I don't have spareparts for those 2 sensors. How can I test the sensors ?
Can I bypass the sensors to test the blowers ?

Regards
Peter

Computamedic

Peter,

The heating system in the 964 is pretty complicated and the CCU adds a degree of mystery since it uses it's internal logic to control the servos and blowers.

It is true that the two internal blowers will not operate for more than a few seconds if the rear blower is not running (or, at least, operational). The rear blower produces a positive air pressure which is sensed within the ducting and the CCU will not enable the internal blowers if that air pressure is not detected.

The resistor you mention (mounted in the ducting from the rear blower) is the thing which enables the blower to run at slow speed. It's in the ducting to cool it down. They often fail and there is an updated version which incorporates a safety bi-metal strip. The sensor (mounted a litle above the resistor) senses the air temperature and enables the CCU to keep the blower running if the temperature is above a set limit. If this fails it usually keeps the rear blower running until the battery goes flat!!

If you set the blower switch on the CCU to OFF and the temperature dial to the RED dot the rear blower should start up (with ignition on). Move temp to BLUE dot and it should (evenutually) stop. If this doesn't happen there is likely a problem with the CCU.

If you don't yet have Adrian's book (Porsche 911 Enthusiast's Companion) you should make a point of getting it. It contains a considerable amount of detail on how to debug the heating system (as well as much much more).

It's a little time consuming to go through the system in depth but most problems turn out to be blown fuses or bad wiring. CCU faults are rare - although it does happen. There are a few companies who will repair the CCU rather than buy a new one.

Regards


Dave

Peter_964C2

I have the "Porsche 911 Enthusiast's Companion" book.
and I used it for debugging the problem.
As I mentioned before, I was able to replace the CCU with another one. Same for the "Blower Final Stage".
It doesn't matter how the switches are set on the CCU, no one of the 3 blowers are working, not even for a second.
What is the purpose of the second sensor mounted in the ducting from the rear blower, the "NTC Heater Fan" ? Is this the "air presure" detector ?

Computamedic

Peter,

I'm not sure what you mean by "Blower Final Stage". There is a relay in the engine compartment electrical panel (which is the same as the a/c and oil cooler fan relays). The NTC sensor (there is only one) senses the temperature of the blower air (from the engine fan) and forces cooling air across the heat exchangers if the temperature is above 40 deg C, high speed if above 75 deg C. The differential pressure valve is mounted after the heat exchangers, before the split to the left and right air ducts.

I wonder if your ballast resistor is open circuit. If it were no current would pass through the blower if slow speed were selected (as it would be initially). You could check it with a meter.

Regards


Dave

Peter_964C2

The "Blower Final Stage" is located in the front of the car, near by the servo's.
It is mounted on a piece of aluminium. I guess this is a kind of amplifier that amplifies the signals from the CCU to front blowers.
The relays and the fuse in the motor department is OK. If I bypass the relay with a wire, the rear fan blows.

Computamedic

Peter,

I think the only way forward is to see if you can measure voltages on the relay (i.e. is the relay being activated) and, if not, work backwards from there. If you're convinced that the CCU is OK - unlikely to get 2 with the same problem - it has to be a wiring issue between the CCU and the blower relay. If there was a problem with the ballast resistor it wouldn't stop the relay activating. You could short the two terminals of the blower resistor just to check - it just means the blower would run at full speed instead of slow speed.

I can't really be any more help than that.

Good luck

Regards


Dave

Peter_964C2

Hi Dave,

I appreciate what you have done..
Now I know more about the way it should work, and it will be usefull during troubleshooting.
I will try to check everyting on sunday, and I let you know if I made progression or not..
Have a very nice weekend.. ;-)

SNOWBIRD

Peter, after reading all the threads on this subject, I decided the most likely culprit was the rear blower relay, so I just got one and put it in. All the erratic and non-operating nonsense has stopped (see https://rennlist.com/forums/964-forum/190837-help-rear-fuse-panel-question.html ). The relay is a VW PN #811 951 253 $43.00