71 GMC Factory Air Problem

[dman535]

The air on our 71 stopped working, the low side pipes were not even getting cold. I added some freon and its getting cold now - but still no cold in in the cab. I have good airflow, cable seems to be free - but still no cold air inside. Everything worked fine about 6 months ago.

You will notice in the pic - that the entire evap core is getting cold..Any ideas?

[motorcritter]

Check your pressures- it looks like you converted it to R134a, a proper flushing would get rid of the old compressor oil, which isn't compatible with the oil used for 134a. That can cause a blockage, and the evaporator is a likely spot. If you added refrigerant, there's a leak somewhere, and that can cause internal problems (water can cause quite a few problems in the system, none of them good). Air and moisture gets introduced when there's a leak, and can cause dead spots in the system. The new 'stop leak' stuff works on contact with water, and if there's contamination in the system, that stuff can create a whole lotta problems that are not easily fixed. Good luck, but I don't think this ones gonna be a real simple fix.

[D-rock1981]

Do you have a ac gauge set? If you do hook it up and tell us what your readings are.

[Andy4639]

Looks like it's freezing up from the looks of it. Is there any water dripping off of it under the truck? If it' freezing up it's not cycling and is just getting to cold are it's plugged up from something.

Took this off of Google.

If it freezes up it has a restriction usually, expansion Valve ,the thing that is doing it ,but need to put some gages on it and see if the low side is going under vacuum since the low side in the suction side and it its vacuum ,but when the Gage's are hooked up on the high side and the low side ( suction & discharge ports) with the A/C on Max Fan on high the low side will stableize at about 29.9 inches of mercery or 30 to32 inchecs of vacume, if it starts going lower and the more rpm's added and the hight side climes drangersly higher that 250 275 300 the high side line can and will explode, so if this is happening you defenlety have a restriction, depending on what system r-12 or r-134-and this don't matter its the type of vehicle does it have a orifice tube or expansantion valve they both do the same thing ,and usaly this is what is wrong providing its full of freon and fan and all else is ok, this is most likely the place to look ok, good luck take care

[Eddie H.]

without gauges, you're just guessing. This type of system isn't designed to ever cycle the compressor on and off. It uses a combination of a POA valve, and a TXV valve to regulate the proper amount of refrigerant flow through the evaporater core. The frost on the POA valve would indicate that your low side pressure is too low. This can be caused by several things including not enough freon in the system. Is the sight glass on top of the drier clear, or milky? If you just added freon to an empty system without evacuating it first, you could have air trapped in the system. The best thing to do would be completely evacuate the system, and check for leaks. If leaks are found, fix them. (If you have to open up the system, replace the receiver drier), then evacuate again and add the proper amount of freon.

[Wrenchbender Ret]

There is a box under the dash that has a diverter flap in it. These are nortorious for going bad. The valve comes loose & won't let the air go through the core.

[Eddie H.]

Originally Posted by Wrenchbender Ret

There is a box under the dash that has a diverter flap in it. These are nortorious for going bad. The valve comes loose & won't let the air go through the core.

Yep, that's another common problem with these systems. The blend air door, directs air through the heater or evaporater cores depending on the temperature control position.