bjornebo

When I got the car the AC was not working. After studying Tore's excellent HVAC website and Brian's awesome 2007 post "Complete A/C diagnosis" I felt confident to go DIY. I got a Neiko aluminum AC manifold gauge set from Amazon and checked A/C pressure last weekend. 28 psi on both high and low pressure sides with the engine not running. This would likely explain why the A/C was not working (compressor will not come on if high-side pressure is less than 37 psi). I added a 12 oz can of R-134a. The A/C compressor is now working, and with compressor running high-side pressure is 130-140 psi; low-side pressure is 11 psi. Per the table in Brian's post both are within the acceptable range for the ambient temperature which was ~55). A/C now puts out cold air.

So this weekend I decided to go through the "Quick guide to test most HVAC components" on Tore's web site to make sure everything else is working properly. Everything was fine except for the operation of the rear fan in the engine compartment. The fan only works on high which would indicate that the ballast resistor has failed and I will replace it.

Finally to my question - the other strange thing about the operation of the rear fan is that for cabin fan settings of 1, 2 or 3, the rear fan comes on at a temperature setting of 28 or above and goes off if the temperature setting is reduced to 25 or below. Per Tore's web site the fan should come on if the temperature is set to 20 or higher. Any ideas on what might be causing this?

TIA

Partisan

It depends on the cabin temperature AND the temp setting, if the compartment blower is switched on or not. When no hot air is required, it will not be turned on.

Maybe the little fan on your CCU is stucked? This results in false cabin temp reading, because the sensor gets heated by the CCUs own power dissipation.

Regards,

Michael

BesideTheBox

+1 on this above.

20 would be correct if the cabin temp was sensed by the CCU to be 20.
Then the aux fan would be on above 20 and off below 20 (roughly).

bjornebo

Thanks. I will take out the CCU and check the motor and temperature sensor.

Livefreeandrive

I get hot air all the time even after servicing AC? Where is this Tore website?

ToreB

Thanks for the feedback on my HVAC DIY site. The rear fan will start running (and the mixer servos opens) if there is need for heated air from the engine. This depends on the ambient and cabin temperature, as well as the temp **** setting.

I would recommend to clean the cabin temp sensor in the CCU, as well as check the CCU fan. A sensor full of fluff or a malfunctioning CCU fan will both make the sensor to sense the temperature inside the CCU enclosure. This gets very hot over time, and this will fool the CCU to believe that the cabin temp is very high, thus closing the mixer flaps. (and stop the rear fan)
Cheers,
Tore

bjornebo

I verified that the CCU fan was working but there was a big dust bunny in the temperature sensor. I blew that out and also cleaned the tip of the sensor with some mass air flow sensor cleaner. I put the CCU back in the car and the rear fan now behaves correctly.

Thanks to RL and Tore B's website this was a reasonably simple DIY.

Livefreeandrive

THANK YOU TOREB!!!