Hello All..i have a 99 f150 4.6. Yesterday I was driving about 80 mph or so and switched on my AC..no cold air and it seemed as if the compressor did not engage..seemed odd as it worked perfectly on Sunday. About 20 miles down the road i stopped to gas up and cycled the AC and it worked as it should..ice cold no issues and it is operating as it should today...my question is does the system have some type of feature that keeps the compressor from engaging at high speed or do i have some AC problems heading my way?? Thanks in advance for any feedback

 

i dont know of any high speed features for a/c. maybe the selector switch is starting to fail. or the high pressure switch is failing. my pressure switch failed and the a/c compressor clutch kept locking and unlocking, and it sounded like a plug wire was arcing. i'd put a set of a/c gauges on it and see what the pressures are.


other than that you may get more and better responces over in the hvac section.

 

MattsFord has already supplied an answer, but there is something to add. Did I read right that you were going at 80M/hr. on a Georgia Highway?. And you didn't get stopped?. That couldn't have been Rt. 20; could it?.

Look: About the compressor at that possible Rev/min. you could have temporarily glazed the solenoid clutch upon engagement. This would have allowed the clutch to spin with out engagement for a period, and caused it to overheat. This would have left a burned coating between the clutch plates.
You may be able to spray the clutch plates with some "Electrical Contact Cleaner". Spray a little on the clutch plates, and then operate the AC system. This should remove the burned coating; if that is what had occurred.

Let's see at around 80 M/hr. you should have been somewhere above 3K R/m. somewhere when the plates hit.
"Spin-Out" is probably what happened, but it is not normally permanent. I wouldn't continually engage or disengage the AC over 70M/hr. myself; but that's me.

I had made a simple Interface Box that controls my AC so that if I accelerate above a given velocity my AC disengages the clutch so that it doesn't load my engine during that time. It is timed so that it resumes when I reach a given speed or a consistent speed for a given amount of seconds.

 

Well, the system will be cycling the compressor on and off anyway any time you have the AC running, so I don't see how this would be any harder on the clutch.

 

I was saying that at 80 M/hr. speed with probably a engine R/m over 2-3KR/m.;engagement could be a bit extreme. IMO

What I stated in my post doesn't mean that there would always be a fault, but in case there is; that this would probably be the cause/symptom of that problem.

Considering there's significant back pressure within the system, and that the initial charge state is more fluid than gas in the compressor's input. The Hydrodynamics of the situation would be that the charge plasma would be "softer" against the torque being applied by the clutch shaft. The plasma when "cold" in less compliant than when it has been charged or compressed (warmed) for at least a minute of operation.

Now look at engaging the AC down around 1-2 KR/mi. the system has a bit more time to stabilize with regards to the liquid, and gas mixture. That first charge cycle has a different dynamic than cycling on and off at "reasonable" speed. I am not sure if he had a Nippon-Denso compressor, but sometimes they do require a little more time to spin up from stasis to an operable charge.

Therefore when he engaged his AC later on or the next day it ran fine. He was building up his charge from essentially idle speed; which gave his system more time to stabilize.
This shouldn't have caused a permanent problem, but I suggested cleaning the clutch as a Prev. Maint. option. The clutch material can "glaze" if over heated. If there is any glazed clutch material present the compressor may not engage properly later on;making it appear inoperative.
I usually clean my clutch pulley at the start of the AC season or if I replace my serpentine belt.
Take it Easy