jgray

I have been having this issue since I bought my jeep. After i'm driving for a while and I'm sitting in traffic I notice my fan constantly cycles on and off or from hi to low (if its a 2 stage fan). I also noticed that when I turn on my A/C (which I don't use often) it starts to blow cold then gets hot. Then it has a burst of cold air then gets hot again. I had a mechanic friend of mine take a quick look at it and the pressures are good in the A/C, he seems to think it could be a high pressure switch for the A/C. Am I dealing with 2 different problems or are the 2 related? The compressor kicks on but the system still blows hot air. My jeep doesn't over heat it stays at normal temp the whole time.

Mr.T

For the 3.8L engine, it's normal for the electric radiator fan to switch speeds based in coolant temperature, but if it the speed change is erratic (or erotic) that would be abnormal. But with the A/C is running; the radiator fan runs at high speed regardless of coolant temperature to have air flow through the condenser, and coolant temperature is regulated only by the thermostat. Sounds like you should try a different mechanic, it should be fairly easy to troubleshoot.

Stay in touch

jgray

Right about the fan, however the fan does kick on to bring the head pressure down on the a/c system so it doesn’t blow a seal or hose correct? That has been my understanding of it. The fan comes on to load the system so the pressure drops to a safe level to allow the a/c compressor to continue to pump refrigerant. I could be wrong but I though that’s how the system worked. Like I said I’m new to the way this Jeep operates so I could be way wrong. I’ll have someone else look at it and keep you posted. Thank you for your reply.

Mr.T

The A/C hi-side has a pressure transducer that can be read from the PCM via the OBD2 port. Service manual isn't at hand, but I'd guess that there is logic in the PCM for high hi-side pressure to de-energize the compressor clutch. Just observed that the radiator fan turns on a few seconds after turning on the A/C with a cold engine -- That could be from logic based on hi-side pressure > some setpoint, or maybe just a time delay on compressor clutch energized.

resharp001

I don't think I've ever heard that before. So you're saying that if the a/c is on, that fan is on high regardless of coolant temps? Is that something they would have changed when moving to the 3.6L, cuz my '13 definitely doesn't operate like that. My fan kicks to high when the coolant gets to ~224* if I recall, and then kicks back down to low when the coolant temp drops down to 210* again. The a/c is pretty much always on here in TX. Maybe yall's 3.8L fan doesn't sound like a dang airplane taking off when on high like mine does.

jgray

Oh mine is loud. It’s just that it happens all the time a/c on or not. Sometimes runs for 5 seconds other for minutes but it’s always cycling high then low or on off. Still haven’t figured if it’s a 2 stage.

resharp001

I'm in Texas, so gets dang hot here too in the summer. I got frustrated last summer with some really warm coolant temps. One night I went out and started that sucker in the drive and observed over 20 minutes or so of it running exactly when the fan was kicking to high and back to low. I do find it helpful in really hot climates, like mine and yours, to monitor coolant temps closely when things really heat up. Does the jeep computer, or an aftermarket programmer allow you to monitor coolant temp? It's just one of those things where every bit of information is helpful to piece the puzzle together.

That is really interesting that the 3.8L rad fan apparently runs on high anytime the a/c is on, just like Mr. T reports. I'm not sure why they would have changed that in '12 going to the 3.6L.

Mr.T

I'm not positive, haven't observed enough it under enough various conditions. There's only 2 fan speeds on the 3.8L and it might choose the speed based on the A/C high side pressure transducer signal, maybe even in conjunction with the coolant temperature -- The logic isn't fully disclosed in the manual, plus I've set the fan speeds to start at lower temperature with less deadband in the PCM (with HPtuner software).

You're probably right about the fan on the 3.8L being quieter on highest speed, and moving less air as well. Makes sense that with more ponies the 3.6L needs more airflow.

Mr.T

Looks like I was wrong about always running the fan on hi-speed with A/C -- Just did another test and the fan was on lo-speed with a cold engine and cool nighttime temperature. Maybe it switches to high speed if the hi-side pressure is high enough, but less than the clutch cut-out pressure.

For the OP, there's also a temperature sensor on the evap (inside coils), the compressor clutch is de-energized when below a certain temperature. I'd start by observing the coolant temperature (via OBD2) when the radiator fan changes speeds (see temperature setpoints below). Next might be trending the hi-side A/C pressure and watching the compressor clutch.

For troubleshooting reference, at a standstill the fan starts lo-speed at 216F (reset 207F) and hi-speed begins at 223F (reset 216F). There's a small difference above ~28MPH, but these are the stock low-MPH values from my 2008 PCM's stock tune file.

jgray

So after checking everything out I decided to go ahead and replace the hi pressure switch for the a/c it was only $50. After I did that everything went back to normal. I don’t think I have even heard the fan kick on hi since I replaced it. Even after getting off the freeway, and the a/c is working great! Thanks for all your help!