[DougA]

Hoping there's a Ford AC expert here.99 F550 is blowing the freon out of what looks like a high pressure relief valve on the hose hooked to the compressor.Thought it was a bad valve,replaced the hose,recharged,worked for a while now doing it again.Issue with the expansion valve,or something like that?Thanks

[jpete]

I'm not an expert but if the metering device was blocked, then I suppose that could lead to high pressure but it seems like there should be a high pressure switch to shut the compressor off.

Also, that should mean the compressor would suck the low side down to the point that the low pressure switch would cut out.

Take a look at the compressor clutch and see if it is locked up so that it can never disengage.

I have a 97 F150 and a coworker had a 99 and we both had to replace our failed compressor clutches.

[Dean]

Got gauges?

A restriction on the low side will cause the low pressure control to cut off the compressor and the refrigerant to back up in the condenser (pump down) BUT it won't cause the head pressure to be that high unless the system is way overcharged.

[Andy's Auto Repair]

Is it a Shrader or a black nylon ball? What is your discharge pressure at operating temp idle? Why are you driving a F**d?

The big hose between the evaporator and compressor is return back to the compressor as a low pressure gas. The thinner hose from the compressor carries the high pressure gas which condenses into a high pressure liquid in the condensor. The skinny line between the condensor and the evaporator is the liquid line, which in 1999 could have the orifice tube built in or be removable depending on which plant built the truck. Which line has the fitting that leaks?

[DougA]

Originally Posted by Dean

Got gauges?

A restriction on the low side will cause the low pressure control to cut off the compressor and the refrigerant to back up in the condenser (pump down) BUT it won't cause the head pressure to be that high unless the system is way overcharged.

I've got gauges,but It's lost its pressure now,didn't want to waste anymore freon till I knew what was causing this.When I charged it last time,I put in the amount called for.

Originally Posted by Andy's Auto Repair

Is it a Shrader or a black nylon ball? What is your discharge pressure at operating temp idle? Why are you driving a F**d?

Believe it's a Shrader,not my truck it's my sons service truck.

The big hose between the evaporator and compressor is return back to the compressor as a low pressure gas. The thinner hose from the compressor carries the high pressure gas which condenses into a high pressure liquid in the condensor. The skinny line between the condensor and the evaporator is the liquid line, which in 1999 could have the orifice tube built in or be removable depending on which plant built the truck. Which line has the fitting that leaks?

[71350SS]

Doug,
Does your truck have an engine driven fan ? or an electric fan? or an auxillary electric fan to cool the condensor.if it is an electric fan and doesn,t come on at idle it will cause the pressure relief valve to open.As the high side PSI starts to rise, without airflow past the condensor it will just continue to climb up till the pressure relief opens.This can also happen if the high side cutout switch is faulty too,but something has to cause the pressure to rise first.
Did you here any noises that sounded like a pack of firecrackers going off all at once?Thats the sound that the FWD cars would make when the pressure build up would release all at once,but some just make a loud hissing sound.
Also make sure your A/C condensor isn't plugged up with leaves or mud.Also as mentioned before check for any kinks or dents in the smaller of the 2 lines between the compressor and evaporator and either of the 2 lines leading to the condensor in front of radiator.

[DougA]

Thanks,It's my sons service truck,he's a self employed heavy equipment mechanic.I don't have the truck here,but he'll bring it over sunday and we'll check those things out.Thanks

[lsrx101]

Originally Posted by 71350SS

Doug,
Does your truck have an engine driven fan ? or an electric fan? or an auxillary electric fan to cool the condensor.if it is an electric fan and doesn,t come on at idle it will cause the pressure relief valve to open.As the high side PSI starts to rise, without airflow past the condensor it will just continue to climb up till the pressure relief opens.This can also happen if the high side cutout switch is faulty too,but something has to cause the pressure to rise first.
Did you here any noises that sounded like a pack of firecrackers going off all at once?Thats the sound that the FWD cars would make when the pressure build up would release all at once,but some just make a loud hissing sound.
Also make sure your A/C condensor isn't plugged up with leaves or mud.Also as mentioned before check for any kinks or dents in the smaller of the 2 lines between the compressor and evaporator and either of the 2 lines leading to the condensor in front of radiator.

This ^^ x2.

Something is causing the High Side pressure to skyrocket and blow off the pressure relief. The most common cause is poor/no airflow over the condenser. This is usually caused by a faulty fan clutch or the heat exchanger stack blocked with dirt or debris (or both).

Soak the radiator and condenser with a strong detergent like Simple Green or Purple Power. Let it sit for a few, then blast it thoroughly with your garden hose You'll be amazed at how much crud washes out.
If you can, separate the radiator and condenser, then clean the area in between them. Again, it's amazing how much crap gets caught between the 2 heat exchangers.

The engine is very tolerant of airflow restriction and won't overheat until a fan clutch is nearly dead or the heat exchanger stack is almost completely blocked. The AC system is VERY sensitive to condenser airflow issues, it doesn't take much at all.
Usually, the first indication of a weak fan clutch is poor AC performance at idle.

Being an F550 it "might" have an air or hydraulic fan clutch, or so I recall. I never spent much time working on trucks bigger than F350. If so, make sure the clutch is engaging.

[DougA]

Thanks Tom,that could very well be the problem!This truck spends 12-14 hours a day,working in this grey river rock dust.He works at a Hydroelectric plant on the Susquehanna River,right down on the riverbank,servicing heavy equipment.Stuff is like cement,packed in every where.Truck looks like a grey ghost at the end of the day.Will check that carefully.