1970 C10 A/C not blowing cold after 60 MPH
 

My 70 C10 cools great until I'm going about 60 then the air goes to about room temperature.
It has been upgraded to modern Freon, compressor, condenser, and valves.
Everything else is still stock.
Any ideas?

Re: 1970 C10 A/C not blowing cold after 60 MPH
 

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I would start by determining what the pump is doing when the vent air is room temp @60MPH. Is the clutch turning the pump? Maybe rig a 12v wire off the compressor clutch positive and run that to a bulb inside the cab so you know when it's being demanded on.

I know that my A/C will sometimes freeze up at speed when outside humidity is high. That is, the evaporator will freeze externally and the air flow becomes less and less out the vent...still "cool" but barely flowing.

My guess is either you have a valve freezing stopping the flow of refrigerant into the evaporator OR the RPM at speed is causing too much pressure and your safety switch shuts it down (with perhaps a restriction). Overcharge can be a problem here too.

If your system goes room temp and then a few minutes later starts cooling down again, that would be pressure related methinks. An experiment would be turn off the compressor while vent=room temp @60MPH and keep driving for say 10-15 minutes then turn it back on...any cool air?

Hth,

-Kevin

Re: 1970 C10 A/C not blowing cold after 60 MPH
 

An old heater valve not closing all the way caused this to happen on my 69 C10 with a late 1990s Vintage Air system. The tech support guy said it worked OK at low rpm, but when the revs went up, the water pump output pressure and volume increased.

Also, you want to make sure the heater valve is connected to the output of the water pump, which is usually the intake manifold port.

Re: 1970 C10 A/C not blowing cold after 60 MPH
 

It's possible the drive belts for the compressor are slipping at rpm and not squealing enough to hear.

Re: 1970 C10 A/C not blowing cold after 60 MPH
 

Thanks for all the great ideas.
I have a manual valve on the heater hose that is off. It didn't take long for me to realize the heater valve was leaking in the Texas heat. So I doubt that's my problem.
The cool air comes back quickly when I drop the speed so I don't think freezing is my problem.
I was thinking overcharge when I posted. My gauge set is from the Freon 12 days and the hoses are shot, so I need to get one of those quick charge kits and see if it's in the red.
Will post back when I have an answer.

Thanks again...