Hello,

I have a 1978 f250 that I recently bought.

I was wondering what the reason would be that my a/c blows cold for a few seconds then starts blowing hot?

Thanks for your help!

 

Does the compressor clutch stay engaged? Radiator fan pulling decent air? Fan shroud still in place?

 

clutch is staying engaged, shroud is still there.

 

Update: It stays cold for about 30-45 seconds then gets hot.

I also noticed the dash tubing for the far left vent is non-existant. Where can I get more of the tubing?

 

Well, it could be a number of things. Freon low, expansion valve bad, plugged dryer, restricted condenser, I say run it and feel all the lines. The ones from the compressor to condenser and condenser to firewall (expansion valve) should be hot, maybe just uncomfortably warm at idle. Now, the one from the firewall to the suction of compressor should be cold. If the proper lines are warm/cold, then I would look into the cab not properly circulating the cold air.

Check to see if your heater valve (vacuum actuated) is working properly. It should shut the coolant flow off. Check the proper operation of the hot/cold setting. All the way on cold AC on should be recirc. Just toward warm is a detent, and that is fresh air. See if your little door under the passenger side dash is opening and closing.

This is all for factory installed AC, not dealer option. Good luck. I am leaning towards Freon low and/or system plugged.

 

It is a factory system.

Checked the lines and it didnt feel like they got very warm.
Checked heater valve and it works fine.
The door under passenger side dash is opening when heat is on and closes when a/c turns on.

I know the tubing behind the dash needs to be replaced. Where can I get the correct stuff?

If the system needs a recharge does any one know of a a/c shop in the okc metro that will do r-12?

 

Well, if the thing isn't making much heat on the lines, That means that the Freon is not circulating properly. This could be many reasons, like the Freon might be somewhere in the atmosphere on furlough. It might need more Freon, but without hooking gauges it is hard to determine. As for a 1978 system, I would evacuate, leak test, change dryer, expansion valve, and compressor oil and leak test again, then recharge. If you find any stop leak looking stuff, or dye, or anything but Freon and oil, try to get that stuff out. Good luck. If I was less than like 15 hrs away, I would be happy to do that for you.

 

Very well could be moisture the system freezing up and restricting the flow of freon.

 

Run engine at 1500 rpms and the compressor clutch is turning. If the line from the outlet of evaporator to the compressor is ice cold, then the a/c air out of the vents should be cold . If the line is cold and the air is not, then you have a control issue. Ex. hot air blend door not closing, water valve not shutting off, poor air flow through the evaporator. (Clamp off one of the heater hoses and see if the air gets cold.)

If that line is warm, hooked up gauges to it to determine what the cause is.
Most likely is has leaked, look for oily areas around the compressor and hose fittings. Freon contains compressor oil.