2002 S-Type 4.0

AC blows really cold from all vents when sitting idle. If you drive the car, the temperature rises to warm in all vents. Back to idle and its cold again.

Changed DCCV
Changed AC compressor, dryer
Freon pressures: hi side, 140 low side 42
Temperature sensors appear all equal in ohms 25-27 range

Comments Please...

 

Curious, as this is opposite of a common gripe.

Two quick thoughts:

Can you find a deserted road, preferably one with a slight downgrade? Get up to speed so the AC blows warm, and then carefully slip the transmission into neutral while coasting downhill. See if the cold air returns with the engine at idle but while still moving at freeway speed. If not, I'd suspect some sort of odd airflow problem through the condenser. Perhaps from a missing baffle or something similar that isn't an issue when the vehicle is stopped and the fan is the only source of cooling airflow.

If the cold air does return while coasting with the engine at idle, I'd suspect the refrigerant pressure sensor. This isn't an on/off pressure switch. It's a transducer that sends a variable signal to the PCM. The PCM interprets the pressure signal to determine when to engage the clutch. As the compressor runs and the low side pressure drops, the PCM uses this signal to decide when to shut off the compressor until the pressure rises above a certain value, and then the whole cycle repeats. If the transducer is reading a little low, the PCM may never let the compressor engage at higher RPM. The reason the AC works at idle is because the system is less efficient at low RPM and so the low side pressure doesn't reach the point where the PCM mistakenly shuts off the compressor.

 

It really could be anything.. It could be the expansion valve, low refrigerant, AC clutch, etc... It just might be best to visit a local/trusted AC guy unless you know how to troubleshoot the problem yourself.

 

Those ohms look wrong, what do JTIS etc say?

 

Had a brain fart earlier. Instead of coasting in neutral to see if the cool air returns, try going at it from the other direction. While parked and with cold air blowing, rev up the engine and see if the airflow turns warm. If it does, the fault is related to compressor RPM, not airflow through the condenser.

Also, I wanted to clarify my hunch about the pressure switch not reading correctly. Could be a dead spot in the transducer, and you only reach this with increased compressor speed.

You also mentioned 42 psi at idle. You should see lower than that, down around 25 I think. That could be as simple as a slipping drive belt, getting worse at higher RPM.

Did you try setting the temp control panel all the way down to LO? I think that bypasses the duct and cabin temp sensors. This could help with your troubleshooting.

 

I don't have access at the moment, but from my notes, the three sensors should read approximately 31k ohms at 70 degrees F. Since all three are reading the same (not sure what temp), any chance you left out the "k" ?

Earlier this evening, I got to thinking. (Don't worry, it's just social thinking and I can stop any time I want...) I tried to think of issues that could cause a loss of AC output as RPM increased. I had my lightbulb moment when I realized you may be fighting two problems.

The first could be a marginal AC system, as evidenced by 42 psi on the low side. You may have some cooling, but not enough.

The second could be a heater issue. On my '02 V6, I can tell when the coolant level is low because the heater blows cool air at idle, but is fine at higher RPM. Top off the coolant, and the heat output returns to normal at idle.

So maybe you've got something like that, which would actually be three problems. The climate control system has a problem and is commanding a warmer temp than you have requested. This could be caused by a bad temp sensor, failed control module, stuck DCCV, etc.

So for whatever reason, partial heat is being commanded on all the time, even though you didn't ask for it. If your engine coolant is low, the heated coolant doesn't reach the heater core at idle, so the only thing you get is cold air from the AC system. Rev up the engine, hot coolant reaches the heater core, and now you have an undesired warm air supply mixing with the cold.

For this latest rabbit trail I've suggested, check your coolant level. Since you have the V8, check the auxiliary coolant pump. Its only purpose is to provide coolant flow to the heater core at idle.

If that's okay, let the engine cool for several hours so the coolant reaches ambient air temp. Fire up the engine, and while the coolant is still cold, get up to speed and see what the AC now does. If it now blows cold at all speeds (until the coolant warms up), you had warm air being incorrectly mixed with the AC output.

 

I appreciate all of the help. I have sit in the driveway and at idle, the ac is cold. Rev up the engine and it gets warm. I will retest and try all of the suggestions in this thread and report back with my results. Again, thanks for giving suggestions to try. This has puzzled me for months.

 

Ok, after a month, still issues. Bought new gauges recently. The a/c still gets cold at idle. The a/c pressure is Low Side 33, High Side 250.

It seems the symptom of this car keeps changing. Now I can sit in the driveway, turn on the A/C and the air gets very cold in the car. Bring up the RPMS to around 2500, The low side gauge drops to 25 - 30 and the high side gauge climbs to 350 - 400 and the drivers side a/c vent gets warm. The high pressure switch on the a/c then starts clicking in and out. Let the car idle and all vents in the car become very cold and the pressure switch stops clicking. New A/C Compressor, New Dryer.

Any thoughts?

 

This is a jaguar s-type 4.0 Is this maybe an expansion valve issue? I apologize for the delay with responding to this post, but I have a heart issue that plagues me.