My daughter has been driving the XC70 and she told me last night that the cooling fan was louder than normal. After looking at it it sounds fine but its just on high speed as if it was sitting traffic at 105° with the AC on. Even engine cold it ramps right up to high after start (AC off) when when the engine is off it turns off like normal. Key on engine off it's not running. No OBDII codes and no pending codes. Intake air temp and coolant temp read normal with my scanner. The AC failed a few years ago (internal compressor failure) and have been running without AC since then. Could the AC pressure switch affect this? is there a way to test or bypass since the AC doesn't work anyway? What else can I check? 2005 XC70 150,000 miles.

 

I'd start with the fan relay. if you can pop off the cover, take a look at the contacts to see if they are fused in one place otherwise I'd just throw a new relay in for a baseline. After that I'd suspect the fan itself.

The AC pressure switches are only in play when the AC is running so I wouldn't suspect them, plus you can simply unsnap the connectors and jumper the pins to emulate the sensor going on/off (there are two pressure sensors - one for high pressure, one for low - the low pressure sensor is on the AC line coming from the firewall back to the compressor).

 

I don't believe this model has a relay (I could be wrong, if I am wrong, where is it located). 2 heavy gage wires with constant battery voltage and fan on/off/speed control is triggered but the purple wire and the fan is internally modulated. Something is commanding the fan to run at high speed but only after the car is started. Where are the AC pressure switches can? I think I saw one underneath near the bottom of the compressor but it's 3 or 4 wire. Where is the other? I'd like to take those out of the equation if possible.If I interrupt the purple wires, the fan goes off. I think that means I can rule out the fan assembly/modulator being bad.

 

upon further reading, you are correct the after 02 there is no relay so the controls are in the fan assembly so the problem has to be in the temp sensing or in the fan assembly itself.

 

Today disconnected the ECT and jumpered the terminals at the connector and disconnected the AC pressure switch and there was no change. I think at this point it's down to the fan or the climate control module? I don't have the technology to test either of these though. Something is commanding the fan to come on. It shuts off when the key is off an only runs when the engine is running (key on, engine off the fan's not running). Anything else I am missing?

 

A guess: Could the coolant temperature sensor have gone bad and "safetied" to issue a false HOT condition such that the fan would be told to run on high?

Failing at 150K and 15 years is normal.

Lots of words but really simple. Thermostats sometimes fail in such a safe (aka "open") configuration.