geldgrube

Hi all, I've searched but I can't find anything that addresses my specific problem on Rennlist.

My rear blower doesn't come on when I shut the car down to cool off a hot engine. So far, I've confirmed that the rear blower works properly through the inside temperature and fan speed controls (i.e. low speed as well as high speed works as expected) so that rules out the blower relay and the low speed resistor as well as the CCU itself.

The only thing left (I think) is the temperature sensor in the blower. When I measure its resistance, it goes from 8K ohms cold to 2M ohms hot so it appears as if its reacting to heat. The interesting thing is that I disconnected the temperature sensor with the engine OFF and the fan came on so it looks like the CCU sensed "high" temperature and turned the fan on. Bad temperature sensor??

Does anyone know what resistance range it takes to "trip" the CCU to engage the blower? Any other ideas?
Thanks,
Ron

ToreB

The Climate Control Module supervises the air duct temperature with a NTC thermistor. Normal value at 20degC is 12,5kOhm, and about 4.4kOhm at 45DegC. The measured value can be inspected wioth a OBD diagnostic tool connected to the CCU.
The CCU shall run the fan at low speed (40-45degC) and high(62-57degC) when ignition is ON and mixer valves are closed. (minimum heat setting) At igntion OFF, the fan run in low speed if the air duct temperature is above 75degC, and shall stop when below 70degC.
See my HVAC details page for further on this topic, Section Rear (AUX) Fan.
Cheers,
Tore

ble2011

Mines comes on now and then. At least for me, I've noticed the engine has to be "very" hot for it to turn on.

TMc993

+1 Mine came on yesterday (Ambient temp 96 degrees F) for the first time this summer.

JB 911

While running or after shut down?

KNS

I thought I was also having problems with my rear blower fan not coming on. The temps do have to be pretty hot for the fan to come on. After a lot of research and sending my CCU in to get checked out it turns out that everything was okay.

And, yes, pulling the sensor out will cause the fan to switch on - normal.

geldgrube

Tore: did you mean 4.4M Ohms hot? I measured about 2M Ohms on mine and Kurt confirmed that pulling the sensor out (i.e. creating an "infinity" resistance reading) turns the fan on.....so that would imply that resistance increases as the sensor heats up.
Thanks to all that replied.

bobbyp

Mine came on last night about 5 minutes after parking it and turning off the ignition and then stayed on for 10 minutes or so.

Normal operation?

ToreB

The rear air duct temp sensor is a 10k NTC thermistor, the resistance will decrease with increasing temperature. Measurements in the megaohm range indicates a defective resistor.
I guess the CCU have a failsafe mechanism that will start the rear fan if the sensor fails or is disconnected. It will also trip a fault code in the CCU. (15=Rear fan temperature sensor)
Cheers,
Tore

TMc993

After shutdown...

JB 911

The not coming on for 5 minutes can be normal but running for 10mins continuously is too long. Edit see my post on page 2 - 10mins run time in hot weather is probably fairly normal. When mine comes on, it cycles. I'd say it runs for around a minute each time. As Tore pointed out, It's just trying to drop the duct temperature a few degrees, which probably happens in a matter of seconds. I'm guessing the time it runs is how long it takes the thermistor to sense a change in temperature and/or perhaps a minimum run time. If yours runs for 10 mins straight, it might be a sticky relay. They are inexpensive enough to replace, or to try to catch it in action, remove the electrical panel cover in the engine bay and try tapping the relay while the fan is in a 10 minute run session and see if it turns off.

I don't have my engine tray on. I imagine if you did, that fan would come on a lot more. One thing I notice is if I leave the engine lid open it doesn't come on at all. Airflow is good!

gmorat

I thought it could run for up to 20mins?

ToreB

The rear fan can run up to 20 minutes after ignition off. This is the same time as the CCU stays on.

Cheers, Tore

JB 911

Correct. when I wrote 10 minutes is too long I meant, continuously. Edited my post. Sorry for confusion...

techman1

Hot in Florida, spirited drive. Parked the car in a hot garage, and 5 minutes later the fan came on and stayed on for almost 10 minutes. Too long?