Debugging my AC problems

[Chris64]

I haven't found any other threads with this specific problem.

1974 K-20 Cheyenne - factory air

The problems so far:
* Fan speed is always on what I think is low.
I replaced the fan controller and it behaves the same. Changing the speed does nothing. I think I need to replace the resistor, but that's usually a problem when it's stuck on high I thought. I had to do this same thing to my Toyota PU decades ago.

* The AC doesn't "kick-on" at all. I expected a click or something.
There is a wire coming out of the little vacuum module. I'm guessing that's for the AC because something must tell it to turn on. What's a good tip for debugging?

The AC radiator in front appears to be leaking (unless it's something else) so I expect I'll be replacing several parts.

 

[75gmck25]

Fan speed will always be on low. That is the GM HVAC design and you can’t easily change it.

Not sure what you mean by the “vacuum module”.

A/C mode turns on power to the compressor, and then it is routed through a low pressure cutoff in the hard line near the evaporator. Switch has rubber plug that you can just pull off the switch, and then use a jumper to bypass. Just be careful - when you bypass the cutoff it may try to run the compressor even if the system is dry/empty. You can quickly ruin or lock up a compressor.

Do you know what refrigerant (R12, R134a, etc) and oil (Mineral, ester, PAG?) is in the system?

 

[Chris64]

Fan speed will always be on low. That is the GM HVAC design and you can’t easily change it.

Not sure what you mean by the “vacuum module”.

A/C mode turns on power to the compressor, and then it is routed through a low pressure cutoff in the hard line near the evaporator. Switch has rubber plug that you can just pull off the switch, and then use a jumper to bypass. Just be careful - when you bypass the cutoff it may try to run the compressor even if the system is dry/empty. You can quickly ruin or lock up a compressor.

Do you know what refrigerant (R12, R134a, etc) and oil (Mineral, ester, PAG?) is in the system?

I don't know how else to describe the little box with all the vacuum lines that sits behind the climate controller. I call that the vacuum module. It's the only thing that is connected to the lever when you switch the AC on.

I have no idea what refrigerant or oil. Whatever was stock in 1974. Is that radiator in the front holding oil? What I see leaking looks more like oil than anything, but it's not leaking so bad that it's dripping, it just looks wet.

I've never been successful at servicing AC so far. I'm not sure if I expect to be able to do it this time, but I'd at least like to have it in a state where it can be turned on before I take it somewhere. Is there a way I can check the levels before running it?

As for the fan speed, what I mean is that it's always on low, even when on high. It doesn't change speed on any setting, but I'm sure it isn't high because it's very quiet.

 

[75gmck25]

The vacuum module on the controls diverts vacuum to the right vacuum motors so you get air flow to defrost, heat, vent, A/C. The A/C compressor will work without vacuum to the controls, but the default air flow will be about 1/2 to floor and 1/2 to the defroster.

Original refrigerant was R12, and Ester oil (IIRC) was suspended in the system to lubricate everything. If you have it retrofitted they will probably flush the system, change the filter/drier and the orifice, replace all O-rings with new green ones, and then vacuum it and recharge with R134a and PAG oil. This assumes your compressor still works, and that you have no hose leaks. However, it’s not a common job anymore for many shops, since R12 has been obsolete for many years.

You can check the system pressure (probably around 25 psi with A/C off) with one of the cheap low pressure gauges, but you need one that works with R12 fittings. All the gauges on the shelf are probably for R134 fittings. Many stores also used to have the R12 to R134a conversion fittings, but I haven’t looked in a while.

 

[RaisedK5]

For the fan. Do the obvious first. Check all the fuses. Including the one inline on the firewall coming off the junction block just to the pass side of the brake booster. Then head over to the reference section and download the wiring diagram and start chasing wires. Not a super complex circuit. If you have four season air. Shoving the temp control all the way left should kick it to high speed fan regardless of switch position btw so that's another thing to check. And yes the fan runs constantly on low with the ignition on. Was a "feature" to keep fresh air moving through the cab.