2005 PT Cruiser - Cooling Fan Operation w/AC Engaged

[MarionB2BAuto]

I have look up description and operation in Mitchell, but it still seems a bit vague. I am assuming that like all other cars I have worked on, the cooling fan is supposed to come on whenever the AC compressor is engaged. This one does not, and the AC system is very weak when at idle and not moving. Also I have noticed that the engine stumbles badly when the compressor engages. Originally it came in with a complete empty system, and a line that had rubbed thru. We have replaced 3 lines, and have the proper charge, and correct oil level (as best we can tell). My biggest concern is with the fans. Aren't they supposed to come on when the compressor engages? they will come on when the engine temp gets to proper temp for them to engage, but won't come on when the AC system engages.

Thoughts? I am still researching.

[MarionB2BAuto]

UPDATE

I have verified that I can command both the high and low speeds using a scan tool. So fan operation and that part of the wiring is fine. Still unsure why it wont come on by itself with the AC system engaged.

[ricmorin]

I believe you are correct. From what I can see, the PCM controls the fans. The PCM also gets the high pressure AC signal, so it's safe to assume that the PCM would control the low speed and high speed fans based on AC engagement and high pressure signal. Take a look at this value in the scan tool. My first thought is the pressure transducer is bad. Very common for a Dodge.

The fact that the compressor engages tells me the low pressure switch is working and there is 'something' getting through the high pressure transducer. Probably stuck and some lower value and not moving.

Worth a look.

[MarionB2BAuto]

When you say "high pressure transducer" are you referring to the high pressure switch? or Something else?

EDIT:

OK, I found it, it is also known as the high pressure switch...now that I know that, I have a question on it. If this pressure switch were bad, wouldn't the compressor not come on at all?

[steven kiser]

Compressor will engage if the high side cut off switch isn't functioning. It cuts out only when the pressure exceeds a certain limit. When the compressor engaged the fan should trigger as well as the iac or idle up to stop it from stumbling. I believe there is a relay that controls the fan and the pcm relies on the relay triggering so it will raise the idle. Check for bcm codes.

[ricmorin]

Interesting. My diagram calls it a transducer, yet wiring-wise it looks like a switch. Do you have a PID in the PCM for high side pressure?

[MarionB2BAuto]

OK, this is what I have found so far. According to my wiring diagram the high pressure switch is closed until pressure gets to the preset point, then the switch will open, telling the PCM to disengage the compressor. The low pressure switch and high pressure switch are in series, so this make sense (LPS is normally closed unless pressure drops BELOW preset, HPS is closed until pressure on high side gets too high. As long as pressure is within the LPS and HPS range, compressor can be engaged)

After doing some other testing, the low speed fan finally failed to engage when commanded on (evidently after engine and fan motor get hot, the low speed will not engage, but the high speed will)

Circuit testing shows that the fan connector is getting power for both high and low speeds, but only high speed operates. When getting close enough, and limiting all outside noise (i.e. shouting "Everyone turn off your radios and fans for a minute! I'm trying to hear something!!") I was able to hear the distinctive electronic hum of a motor that is getting voltage, but not turning. It seems that after it gets hot, the fan fails. We have a replacement fan on order.

The interesting thing is that when the car first came it, it had a low charge due to a leak in the line from the compressor to the condensor. It is my belief that when at idle, the high side pressure was high enough that it caused the line to fail at the compression fitting, leading us in a tangent diagnostic direction. We ended up repairing the resultant damage, not realizing the CAUSE was somewhere else. (thank God I always test drive every car, no matter what the repair!!)

Thanks for all of your help. Sometimes it just takes a thought from another tech to get you in the right direction.