1991 F250 351 Manual trans

the air works and blows cold-for awhile. then it will seems to shut off. After awhile it will blow cold again.
the hotter it is outside, the more often this will happen.
don't know if this is a refrigerant problem or what???
anyone had similar?
thanks!

 

Put a pressure gauge on the low side service fitting (black or blue cap) and watch the pressure fluctuate when it blows cold and when it stops. The compressor should start when the pressure goes above 45psi and it should stop when it goes below 25 psi. If this is happening frequently you may be a bit low on freon. Or it could also be a faulty pressure cycling switch (located on the accumulator body with and electrical connector). It is normal for Fords to cycle about 3 to 6 times a minute.

Cheers.

kw

 

thanks for the info.

i notice that when i tap on the compressor clutch it will kick on.
also the lines and accumulator are frosting up??
it doesn't seem to cycle at all, it is either off or on. would i hear a click if it cycles?

 

There is normally a wire attached to the compressor near the clutch. Maybe it is not attached tightly enough (normally philips screw head). The compressor needs to cycle to keep the lines from freezing up. That would result in no or poor cooling due to blockage of refrigerant (made more likely if you have moisture in the system). Put a gauge on the low fitting and watch the pressures. If the pressure does not get low enough for the cycling switch to kick the compressor off you may have to evac the system to get the moisture out. The accumulator and associated lines near it normally get condensation but excessive frost may be your indicator that things are freezing up (moisture) and blocking the flow. By the way, the cycling switch is adjustable with a flat head screwdriver but that would be a last resort.

kw

 

Sound like a low charge to me but frosting could be a sign of blockage due to moisture (which shouldn't be in unless the system has been opened or poorly charged.

 

You say when you tap the compressor the clutch will kick on. It could be a weak clutch magnet not coming on when it's supposed to. Put a test light on the power side of the connector at the compressor. Run the a/c and watch the light. It should cycle on and off with the compressor, and they should be simultaneous. If the light comes on and the clutch doesn't engage, it's getting the right signals from the cycling switch but just isn't engaging like it should.