Air conditioning 1972 Buick Electra

[Guest 49 plymouth]

Hi I am trying to sort some problems with my AC. The car was stood for many years and has thd older coolant. Firstly when ever you switch the ignition on the heater fan runs on its lowest setting regardless of where I set the switches. Secondly the AC pump wont cut in. If I bridge any two off the three wires coming from the pump the clutch cuts in and runs smoothly. I notice the pipe coming from the bottom of the heater box turns white with frost almost straght away. Whille it is running like this the AC on off switch works correctly and I checked the pressure and its spot on. The pipe I mentioned that runs out of the bottom of the heater box that gets ice cold does so up to a valve that has a thin copper wire coming from it that returns to the heater box- somthing like the wire or tube that you would see on a old temperature sender. The system beyond this fails to get very cold and the air from the vent it not particularly cold.

[Pete Phillips]

I'm no A/C expert, but I can tell you that the fan always runs on low speed in GM cars of those years--it's part of the ventilation system and is perfectly normal.

Pete Phillips, BCA #7338

[NTX5467]

Chryslers did that too, even ran the a/c compressor. Supposed to keep the humidity out of the air outlets, or something of that nature. Air output should be via the defroster vents?

I suspect the reason you have to jump the wires in order to get the compressor to run could be the "low pressure" switch, which ensures that sufficient R-12 Freon is in the system BEFORE it'll let the compressor run. Protects the compressor, too! IF you see no "bubbles" in the sight glass on the receiver/drier (the cylinder-shaped item in the lines near the condenser), then that's "the check" that Freon level is correct . . . "bubbles" (other than when the POA valve cycles, means there's not enough Freon in the system.

There can ALSO be a "thermal limiter fuse" in the compressor's wiring. This snaps inline with the compressor wiring and is usually in plain sight adjacent to the compressor. If it's not working, then the compressor doesn't run. It's there to sense "compressor load" and turn off the compressor if the Freon is too low or the load on the compressor is too great (i.e., excessive "head pressure" in hot weather). It was a quite frequent "change item" in initial troubleshooting of the system, back then.

The larger, in-line valve near the a/c box is the POA valve. It cycles to keep the evaporator core at optimum (i.e., non-freeze-up) operating temps (approx. 40 degrees F a/c vent outlet temps). You might try tapping on it with the compressor running as it might have stuck. It also varies the amount of Freon flow in the system for varying heat loads and such (rather than the later method of cycling the compressor on and off).

The low pressure switch on the receiver/drier should be "two wires", but the thermal limiter fuse usually had three terminals on it. That might be the one you're jumping across, bypassing it? Get a new one to see if that helps. BE SURE the system has enough Freon in it, using the sight glass observation point.

When ever you add Freon, make sure there is PLENTY of air going across the condenser, not relying solely upon the engine fan for that air circulation . . . especially in hot weather. Without doing that, the head pressure will remain "high" and the system might not accept any additional Freon, even if it's low.

ALSO ensure that no heater/engine coolant flows through the heater core when the heat is "off". Make sure the heater flow control valve works as it should. ANY seepage of engine coolant through the heater core during a/c operation (in the "full cool mode") can heat the air enough to make it seem that the a/c is not cooling well.

The "ice on the lines" is supposed to indicate which part of the system is "over-capacity" as for "cooling capacity". Which section of the line indicates which side of the POA valve the over-capacity" is supposed to be. POAs ARE adjustable, with an internal screw that's used to calibrate it when it was first built.

When those GM a/c systems are working correctly, they work GREAT. In some cases, as also with similar model years of Chryslers, they might not have fully upgraded the system air flow capacity to match the larger interior volume (and sun load) of the '72 era cars, compared to the ones which came before them. Might need to run it on "inside air High" a little longer than in prior years, as a result.

Keep us posted on your progress.

NTX5467

[Guest 49 plymouth]

I'm no A/C expert, but I can tell you that the fan always runs on low speed in GM cars of those years--it's part of the ventilation system and is perfectly normal.

Pete Phillips, BCA #7338

Thats intresting I wonder why they did that but thanks for the info.

[Guest 49 plymouth]

Thank you for taking the time to write all of that. You have given me some pointers a great help I will see if I can find a thermal limter fuse as a start. Is it possable to test that it has three wires comming from it.

[60FlatTop]

The low pressure switch will prevent the compressor from pulling the low side of the refrigeration system into a partial vacuum and sucking air into what is supposed to be a hermetic system.Automotive systems have more tolerance than precise systems, but added air will still screw up the pressure/temperature relationship. Jumping the compressor ain't good.

I think a conversion from R12 to R134A is $300 to $500 in a shop. It's 86 right now on the tundra of New York State. Better get her done.

Bernie

[avgwarhawk]

The system is low on Freon.

[old-tank]

The system is low on Freon.

That is correct. And when you do charge it, be advised that the sight glass is useless. Best to get a service manual where you will find that the pressures are much higher than on systems with a cycling compressor.

[Guest 49 plymouth]

Its was the Thermal Limiting Fuse. Thanks for help.