Romanoaf

Hi all,
Quick rundown of my issue; temp **** on full cold, front blowers work great, as soon as i turn the temp up, the rear blower turns on high speed and the front blowers cut out but not entirely (read that could be a failsafe on the CCU)

Bridged 30+87 on the relay, high speed on rear blower works. Bridged 30c and 87c, nothing.

From Tore's site "Fan speed is controlled by a relay in the left electronic unit in the engine compartment, (see picture) and low speed is done by switching a series resistor/ballast with the motor, similar to the oil cooler/A/C condenser fans, and uses the same relay type. The low speed resistor is mounted on the air duct beneath the fan housing for cooling, and it contains a bimetallic switch for resistor overheat detection. This is done to protect the resistor if the fan seizes or short-circuits. This switch can fail over time and disconnect the resistor, thus no low speed on fan."

Is it possible to repair this switch?

Current codes on HVAC system:
41(condenser blower motor), 43 (Rear blower motr- Stage 1), and 45 (Inside sensor motor)

Roman

ToreB

Not possible with any reliable result. You can bridge the switch for a quick fix, but you'll risk overheating and possible fire if your rear fan motor should seize. A new resistor assembly is relatively cheap, about 30 Euro.
Inside sensor motor is the CCU fan, not cheap at all, 250-300 Euro.
Cheers,
Tore

Romanoaf

Thanks tore, what's the pn for the resistor assembly?

mojorizing

Roman,

See if you need to reset the thermo switch as mentioned in this link...

https://rennlist.com/forums/3501436-post22.html

LM964

My understanding for normal operation is that when set to cold (blue spot) the blowers will dial up the speed as per sped ****. However, when you move off the blue spot (any +temp) the blowers drop back to slow/low speed. Is this not normal operation? A/C units may be different but would have thought not. I found this too..
The rear blower fan will run at low speed for normal cabin warm air. Presumably then the front fans if at high speed add nothing more in value, hence the low single speed (defrost switch activation being the exception). The rear blower runs high speed to dump high temp air out the wheel arches if detected above 75oC.

I have a C4 with A/C and another C4 without A/C. The one without A/C is being a pain at the moment. Rear blower motor bearing stuffed (blowing 30A fuses) so fitted a new Bosch motor. Now troubleshooting why when the engine is running I get no front fan speed selection at all but then get speed selection when the engine is off but ignition on. CCU on the blink me thinks and will get that sent off for a recon. All soooo complicated...

Romanoaf

So this is normal operation? Would my issue then be a mixer flap problem? Considering a very low volume of air comes out at anything above minimum temperature. When I press the defog button, it blows strong for a second, then dies out again.

ToreB

Low air flow is due to either defective mixer/defrost servos, or a non-working rear fan. The rear fan shall run at low speed when at low fan speed settings on the CCU, then switch to high at higher fan speeds. The actual point of switching depend on CCU version, but also on the temperature setting and/or ambient/engine temperatures.

One or both front fans will shut down or reduce speed if the rear fan is not operating as it should.
My HVAC pages have a quick guide for checking both servos and fan, as well as a detailed explanation on the rear fan operation.
Cheers,
Tore

LM964

Great information and detail Tore - thanks. The whole 964 HVAC system was a huge leap for Porsche back then and a flippin headache for the rest of us!

Question; I fitted a new Bosch motor on the rear blower but haven't taken the car for a warm up run yet. Apart from directly applying a voltage to the motor is there any way I can test it's initial operation from the CCU in place without engine running (with ignition on only)? I understand I may need to run the engine and let full temp build up before the high speed kicks in though.

Marcus

ToreB

Thanks. I take it more as a challenge than a headache.
I always check the operation with ignition on and engine not running, that should be no problem at all, at least in room temperature or so. You may have to be a bit precise and slow-moving when turning up the temp dial to find the point when the rear fan kicks in. There also seem to be differences in CCU versions on the necessary fan setting to control the low or high rear fan speed.
Cheers,
Tore

LM964

Hmmm...Interesting. So, with the engine cold (or not at running temp) I should still be able to run the rear blower from the CCU heater ****?

I have a non-A/C unit and think this is a bit duff - all servos and flaps tested and checked with a friend but the CCU controls seem too have a mind of their own!

ToreB

As far as I know yes. The rear fan should kick in even with a cold engine. But, there are several versions of the CCU, both with respect to software and hardware, so I can't guarantee this.
I've never worked on a non-A/C CCU, but I expect this to be working in the same way as the one with A/C. The fan and temp potentiometers may wear over time, also the front panel switches corrode. That may cause problems, and one solution is to use some high-quality electronics contact cleaner, or maybe even change the parts.
Cheers
Tore

LM964

Thanks Tore, Seems I have another weekend of CCU removal and checks awaiting me!