A/C Cycling Questions

[Rerun]

My '78 Avanti has an R-12 system with a GM style compressor. The rest of the system is essentially the same as the Studebaker Avanti set-up. There is no binary switch. The control circuit consists only of the fan On/Off switch w/ rheostat, and the thermostatic switch, which is energized whenever the fan is on. It blows really cold air, and keeps the car comfortable on the hottest days.

My problem is that the system seems to cycle excessively. The compressor generally runs for 20-25 seconds, then shuts off. But, after only about 15-20 seconds, it comes back on. I can't believe that this frequent cycling is good for the clutch and/or drive belt. The sight glass is very clear of bubbles. Unfortunately, I don't have a set of R-12 gauges.

I recently replaced the thermostatic switch, but the cycling remained about the same. Is it possible that the system is actually overcharged? The end of the thermostat tube was tucked into the fins of the evaporator. Is this correct? Any other thoughts/suggestions would be appreciated.

[unimogjohn]

Jim, from my limited experience, frequent cycling is the result of low pressure, hence it is low on R12 and needs to be charged back to specs. R12 is getting harder to find, but it is available if you look and only need a can or two. But still expensive. John

[BobPalma]

What John said, Jim.

See if you can locate a simple, two-wire, pressure-sensing switch in the system, under the hood. On that GM setup, they are usually located on the top or side of the receiver-dryer "can." It looks kind of like an oil pressure sending switch for an oil pressure "idiot" light.

This switch exists to open the circuit to the compressor clutch, disconnecting the compressor clutch, when the pressure is too low due to low refrigerant. To test, you can pull off (unplug) the connector and jumper across the two terminals in the plug, effectively by-passing the switch.

Start the engine and operate the air conditioning system with the switch unplugged and a wire jumpering the terminals in the switch plug. If the compressor remains ON without the constant cycling you note, you've confirmed the system is low on charge. Since you said it is cold, it probably needs only one 12-ounce can of R-12 refrigerant to top off.

As John said, finding that will be a challenge since it's been illegal to traffic in it for over a decade. BP

[rockne10]

Yes, the thermostat tube should be tucked in to the evaporator fins.
It you can't find R-12 it may be possible to find it at a shop that has converted a system to 134. They usually evacuate the old R-12 and store it for recycling.

[Rerun]

Bob,

Thanks for your reply.

I understand what you are saying about the pressure (binary) switch. This system does NOT have one. There is power to the thermostat switch at any time that the fan switch is "ON". There is nothing upstream that would be turning the voltage on and off, as it comes directly from the ACC circuit. What is cycling the compressor is the thermostat switch. I have verified this with a meter and test lamps.

I thought that the sensitivity of the thermostatic switch might have bee too high, But I get exactly the same results with the new switch.

Still puzzled.

[Rerun]

Thanks, John

Yes, I agree that virtually all of the cases of cycling I have encountered are due to low Freon dropping out the pressure (binary) switch which is usually on or near the receiver/dryer. The strange part of the puzzle here is that this system does not have a pressure switch, at all. What is cycling the compressor is the thermostatic switch. There is constant 13.5V to the thermostat any time that the fan switch is "ON". I have test lamps on the system now, and it is definitely the thermostat that is cycling the compressor. I replaced the switch and still have the same results.

[Laemmle]

What John said, Jim.

See if you can locate a simple, two-wire, pressure-sensing switch in the system, under the hood. On that GM setup, they are usually located on the top or side of the receiver-dryer "can." It looks kind of like an oil pressure sending switch for an oil pressure "idiot" light.

This switch exists to open the circuit to the compressor clutch, disconnecting the compressor clutch, when the pressure is too low due to low refrigerant. To test, you can pull off (unplug) the connector and jumper across the two terminals in the plug, effectively by-passing the switch.

Start the engine and operate the air conditioning system with the switch unplugged and a wire jumpering the terminals in the switch plug. If the compressor remains ON without the constant cycling you note, you've confirmed the system is low on charge. Since you said it is cold, it probably needs only one 12-ounce can of R-12 refrigerant to top off.

As John said, finding that will be a challenge since it's been illegal to traffic in it for over a decade. BP

R12 is sold everyday on Ebay..........no problem

[maxpower1954]

Yes, one of the modern myths - the only thing about illegal about R-12 is manufacturing it. Nothing illegal about selling the 12 ounce cans and Ebay is full of them. Not even that expensive - I got five cans for 80 bucks a few months ago. Russ Farris

http://shop.ebay.com/i.html?_nkw=r-12+12+ounce+can&_sacat=0&_odkw=r-12&_osacat=0&bkBtn=&_trksid=p3286.m270.l1313

[BobPalma]

Yes, one of the modern myths - the only thing about illegal about R-12 is manufacturing it. Nothing illegal about selling the 12 ounce cans and Ebay is full of them. Not even that expensive - I got five cans for 80 bucks a few months ago. Russ Farris

http://shop.ebay.com/i.html?_nkw=r-1...286.m270.l1313

Interesting, guys. Thanks for the clarification. BP

[railway]

If no other switches in the clutch circuit, I think the thermostat is doing its job. You stated that it is cold on the hottest days, Great. Sounds like a good system. If you live in a dry climate, the thermostat can be adjusted for the clutch to remand on for a little longer amount of time. The cycling of the clutch is to keep the coil from freezing up like a block of ice. The ware on the clutch can be adjusted, with proper tools, a common service on GM compressors, and other "round type" compressors. My best advice if it's cooling the cabin on the hottest days----- Leave it alone and enjoy the drive. Another trick is to move the thermostat bulb nearer the top of the coil. But it needs to be in the coil.

SERVICE TIP: On our Studebaker A/Cs and aftermarket units, one of the things that should and needs to done is cleaning the expansion valve bulb. Corrosion will after a period of time cause the bulb not to read and insignificance cooling will occur, and most tec, thinks it needs more refrigerant. Cleaning bulb, remove the installation tape around the bulb, remove the clamp, sand the suction pipe and bulb replace clamp(make sure it is tight), replace installation tape. Installation tape has two jobs, Keep the heat out of the contact spot, and to keep water from dripping on the car floor.

[Rerun]

Thanks, Ebon

My concern is that with the short "OFF" time of the compressor it is coming back on under too much load. That is to say that there may still be too much liquid in the line to the expansion vale and cause the compressor to tend to "hydro-lock".

Maybe I'm concerned about nothing, but it does seem to be taking its toll on the belt. I've never had an A/C that cycled this often, so I've been a bit concerned. Maybe it's just that the capacity of the GM compressor is so much greater than the old York types that it is a mismatch for the expansion valve and/or evaporator.

I'm trying to locate a local A/C tech with gray hair that might remember R-12 systems and still have a set of service gauges. This would give me a better picture of what, if anything, is wrong with the system. Meanwhile, I'm enjoying the cold air.

Thanks for the tip on the expansion valve bulb!

[Laemmle]

Another old time 'trick' to see if you have enough Freon is to look at the sight glass on the expansion valve......if the system is full you will not see Freon passing by the window......it will appear empty....only if you see liq passing by the sight glass do you know the system needs topping off.
Now this is far from correct..........as the proper way is to hook up a set of service valves with proper reading guages......but the sight glass method is just a fast down and dirty method of checking if you feel the unit is not putting out enough cold air.

[Laemmle]

It's a shame that Studebaker did not have the time or methodology to have the Avanti have A/C with the blower.....yes I know modern day R2s do have A/C retrofitted.........but back then I always thought the "Vette was great......tons of HP and comfort of A/C.....

[DEEPNHOCK]

The low pressure cutoff switch was a legal mandate added later on.
Today, a legal a/c shop is supposed to add the switch before servicing (or repairing) an a/c system,
or they could lose their license.
My swag on your issue is the system is overloading your a/c clutch circuit breaker.
Try hooking up a jumper lead and running the clutch direct (for a while) to isolate the the clutch coil from your regular harness.
That's a free test, anyways
HTIH
Jeff

I understand what you are saying about the pressure (binary) switch. This system does NOT have one. There is power to the thermostat switch at any time that the fan switch is "ON". There is nothing upstream that would be turning the voltage on and off, as it comes directly from the ACC circuit. What is cycling the compressor is the thermostat switch. I have verified this with a meter and test lamps.
I thought that the sensitivity of the thermostatic switch might have bee too high, But I get exactly the same results with the new switch.
Still puzzled.

[maxpower1954]

The low pressure cutoff switch was a legal mandate added later on.
Today, a legal a/c shop is supposed to add the switch before servicing (or repairing) an a/c system,
or they could lose their license.
My swag on your issue is the system is overloading your a/c clutch circuit breaker.
Try hooking up a jumper lead and running the clutch direct (for a while) to isolate the the clutch coil from your regular harness.
That's a free test, anyways
HTIH
Jeff

I thought the low-pressure cut-off switch was only a requirement on R-134A conversions. Russ Farris