Got a 2003 Lincoln Navigator. The thing is not producing cold air at idle, basically anytime it is stopped. Checked the fan and it switches on high when the car is stopped. My friend hooked up gauges and seems to think the compressor is on the way out, but why would it stop cooling at idle if the compressor was going? To be clear it cools fine when it is moving.
maybe compressor cant compress enough at low engines speeds? when engine speed increases then the pressure is able to rise enough to become efficient again? just a shot in the dark, i hate working on AC
A weak compressor could do that. How's the charge level? Make sure the system is at full capacity before condemning any hardware.
Does it have a fan clutch? The condenser fan works in conjunction with the main fan, so if it's weak the secondary fan can't make up for it.
Check the charge with a full gauge set, use this rule-o'-thumb. Low side PSI should roughly 1/2 of ambient air temp F degrees number, high side (w/ compressor running) should be roughly 3X ambient air temp F degrees number.
Just to confirm. Does the A/C work find if you are stationary (park or neutral) and increasing the revs of the engine?
If it does work well with the above test then the system is low of coolant or the compressor is bad. In some compressor designs you can have a loss of x psi at idle but fluid dynamics allow for you to recover the pressure needed at higher speed. Imagine super slow stirring a drink in a cup. Fluid just flows around the stir stick. But go normal speed and you can build up a vortex of all the fluid spinning around.
If it still doesn't work then other issues. Yes the fan can be working but is the condenser super dirty or all the fins bent. Same for Radiator. MOst systems are designed for air to flow through both the condenser and the radiator so a old dirty radiator is like a solid wall up against the a/c condenser. I replaced the radiator in my 10 year old dodge neon and all of a sudden the A/C was making clouds out the car vents on that muggy summer day. Surprised me! I was scared it was coolant blowing into the car so I turned the car off and jumped out.
So...is it generally a thing that any modernish (2000+) car SHOULD be blowing full cold at idle? All of the ones I have owned needed some RPM to blow ice cold. And they were like that for years and never got worse. Wondering if they actually were/are low on charge.
AC being a little warmer at idle is normal, especially on cars with only a mech cooling fan and no electric fan. The compressor spins slower at idle, so there's some drop in cooling capacity. And if there's only a mech fan, there's less airflow over the condenser too. Mind you, it shouldn't get warm at idle, just a little less cold.
Some clarifications.
-- So we checked charge as the first thing, made sure it is fully topped off.
-- At idle the car just won't cool the inside very much if at all even with recirculate on. It is warm in the car with the AC on after sitting at idle. This is different than how it is has worked in the past where it would be cold.
-- Revving the car up doesn't make it cool better if you're stationary.
-- The condenser and radiator don't look bad from the outside. Not sure if there is some issue with blockage in there. Can't recall if I have done a coolant flush recently.
If revving it up while stationary doesn't cool things down but it cools better when moving, then it's guaranteed to be an airflow problem, not a bad compressor or other system problem.
I'll double check just to be sure, but we sat there revving it up and I wasn't getting colder air.
The condenser/radiator can look rather nice but still have lost a lot of efficiency. It can be clogged up inside too. If you haven't yet replaced the radiator and you don't have to tear the truck apart to get it out, then put in a nice quality new one to last another 10+ years. Think of it more as preventative maintenance.
While the radiator is out use a light pressure water spray from the engine bay out to clean out the A/C condenser. High pressure washers will actually be likely to bend the condenser fins. If you are super patient you can get a condenser fin comb and mechanically clear the gaps between fins while straightening them.
RedGT wrote: So...is it generally a thing that any modernish (2000+) car SHOULD be blowing full cold at idle? All of the ones I have owned needed some RPM to blow ice cold. And they were like that for years and never got worse. Wondering if they actually were/are low on charge.
Well I would say yes that many 2000+ cars blow very cold at idle and full cold above 2000rpm or so. Not all for sure but many. There is some magic to designing all the accessories driven by a serpentine/accessory drive belt and their pulleys so that they function from idle to redline. Better pump controls and alternator controls have allowed a reduction from multiple belts to one and soon probably to no belts just electric motor driven accessories.
Some cars have a cut off when the engine rpm is too low. If your compressor is kicking off and staying off at idle, I would be suspicious of this.
If the compressor stays engaged, but cools poorly at idle, I'd be inclined to suspect low refrigerant level, as others have speculated above. I have observed some cars are remarkably sensitive to the refrigerant content/level, others work with almost any amount, high or low.
All cars do lose some cooling ability at idle because of the slowness of the movement of refrigerant through the system. But loosely, I'd say it's no more than 10 degree temperature loss, if everything is in good working order.
In reply to Harvey :
I am a bit confused. The ambient temperature is 41degrees C. The low pressure gauge reads 60 PSI and the temperature reads 21 degrees C at idle. Then I gently spray water on the condenser and the pressure reduces to 45 PSI and the he temperature measured 15 C. I have a Daihatsu charade and one manual says 300 gms R134 is the quantity and another says 600 gms. Not sure which is correct as Daihatsu is not responding. Will filling 600 gms spoil the compressor? What is the correct process to charge refrigerant if the quantity is not known considering that the pressure changes as water is sprayed on the condenser.
Look in between the condenser and radiator for leaves, paper, rags or anything blocking the airflow. I think an 03 nav only has the mechanical fan with a fan clutch. Could be bad. Put a hi speed fan in front of the condenser to see if it cools better.
I dealt with this last week with my 2008 Ford E350. Same symtoms as original post.
Compressor was wearing out after 165,000 miles in Florida. Found metal particles in the filter. 
$2,400 dollars later its fixed. (cost a bit more because it has a rear unit also.)
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