A/C Thermal Limiter 72 Only
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Joined: Dec 2007
Posts: 1,124
From: Fort Pierce, FL
A/C Thermal Limiter 72 Only
After finally getting my 72 back on the road after leaking caliper issue, I drove to the alignment shop. The A/C seemed to work fine. When I left the shop, the A/C quit. I ran the troubleshooting sequence in the Service Manual and determined I had a blown Thermal Limiter. I have one on order and should have it next week.
Has anyone ever bypassed this and the Superheat switch and run power directly to the compressor? As far as I know the Superheat is okay and hopefully the Thermal Limiter will fix it.
Has anyone ever bypassed this and the Superheat switch and run power directly to the compressor? As far as I know the Superheat is okay and hopefully the Thermal Limiter will fix it.
Connoisseur d'Junque
Joined: Sep 2010
Posts: 21,183
From: The Hudson Valley
You can bypass it. Earlier models didn't even have one.
But, if your A/C starts to lose its cool (even when it's on at the Defroster setting), you have to switch it off so that the compressor doesn't run dry.
If you can keep track of whether your system is losing its charge, it's fine, but if the refrigerant leaks out and you keep the compressor on, you will burn it out.
- Eric
But, if your A/C starts to lose its cool (even when it's on at the Defroster setting), you have to switch it off so that the compressor doesn't run dry.
If you can keep track of whether your system is losing its charge, it's fine, but if the refrigerant leaks out and you keep the compressor on, you will burn it out.
- Eric
Connoisseur d'Junque
Joined: Sep 2010
Posts: 21,183
From: The Hudson Valley
Nothing kept them safe.
The official specification allowed for a pound of R12 leakage a year as acceptable (though many leaked much less), and getting your A/C recharged (or "serviced") in the Spring was routine.
If your A/C stopped being cold, most sensible people would turn it off (back in the days when you were expected to have some sense), but the compressor is also on in Defrost mode, and people weren't always aware of that.
If the compressor ran without refrigerant, it was also running without oil, as the oil is dissolved in the refrigerant, and it was a matter of time before the compressor would seize.
In the sixties, A/C was still a relative rarity, so this wasn't a major phenomenon among "normal" people, and the wealthy, who could afford A/C, could also usually afford the regular general service visits that would keep the the A/C topped up.
In the seventies, A/C began to become more common, and expectations began to trend toward less regular maintenance, but then with the fuel crisis fuel consumption became a major consideration, and people (generally the owners of used cars, who had not shelled out big bucks for A/C) would cut the belts off of the compressors, whether they worked or not (but especially if they didn't) in the belief that that would save gasoline (the same way they indiscriminately pulled off all vacuum lines to remove the "emission controls").
In the eighties, nothing worked, and new cars had the smaller compressors, and nobody really cared about the older cars and larger compressors anymore, so it was really more of the same as the seventies.
So, originally, the design of the A/C system included the expectation of regular service, which would include recharging as needed. As expectations of less regular maintenance increased in the seventies, GM included the safety fuse, which would not only turn off the compressor, but keep it off until you got to a service station where they could figure out why it was leaking. In the eighties, the pressure cycling switch system came into use, which automatically turns the system off when the refrigerant is low. This system is still used today.
My assumption is that GM got tired of replacing compressors under warranty on newish cars that developed leaks, so created a system that would force you to go back to the dealer to get it looked at. Blowing a 25¢ fuse to save a $250 compressor was a no brainer for them, but doesn't really mean much to you in 2014.
- Eric
The official specification allowed for a pound of R12 leakage a year as acceptable (though many leaked much less), and getting your A/C recharged (or "serviced") in the Spring was routine.
If your A/C stopped being cold, most sensible people would turn it off (back in the days when you were expected to have some sense), but the compressor is also on in Defrost mode, and people weren't always aware of that.
If the compressor ran without refrigerant, it was also running without oil, as the oil is dissolved in the refrigerant, and it was a matter of time before the compressor would seize.
In the sixties, A/C was still a relative rarity, so this wasn't a major phenomenon among "normal" people, and the wealthy, who could afford A/C, could also usually afford the regular general service visits that would keep the the A/C topped up.
In the seventies, A/C began to become more common, and expectations began to trend toward less regular maintenance, but then with the fuel crisis fuel consumption became a major consideration, and people (generally the owners of used cars, who had not shelled out big bucks for A/C) would cut the belts off of the compressors, whether they worked or not (but especially if they didn't) in the belief that that would save gasoline (the same way they indiscriminately pulled off all vacuum lines to remove the "emission controls").
In the eighties, nothing worked, and new cars had the smaller compressors, and nobody really cared about the older cars and larger compressors anymore, so it was really more of the same as the seventies.
So, originally, the design of the A/C system included the expectation of regular service, which would include recharging as needed. As expectations of less regular maintenance increased in the seventies, GM included the safety fuse, which would not only turn off the compressor, but keep it off until you got to a service station where they could figure out why it was leaking. In the eighties, the pressure cycling switch system came into use, which automatically turns the system off when the refrigerant is low. This system is still used today.
My assumption is that GM got tired of replacing compressors under warranty on newish cars that developed leaks, so created a system that would force you to go back to the dealer to get it looked at. Blowing a 25¢ fuse to save a $250 compressor was a no brainer for them, but doesn't really mean much to you in 2014.
- Eric
Registered User
Joined: Apr 2013
Posts: 979
From: Grass Valley Ca
The CCOT system was used in 1973 chevy pu with GM and did not use the thermal limiter fuse.Granted the POA system in my 72 98 worked the best ever in any car i driven EPA clamped down on r-12 leakage was normal.
Thread Starter
Registered User
Joined: Dec 2007
Posts: 1,124
From: Fort Pierce, FL
My A/C has worked very well the past couple years, providing I use it fairly regularly to keep the seals from leaking. I remember back in the 60s, 70s and 80s that it was advised that you run your A/C for a few minutes every week or 2 to keep the seals fresh and prevent leaking.
My car was laid up with a drum to disk swap and then leaking calipers, so it didn't run for almost 3 months. It probably lost some charge from the inactivity, which might have caused the limiter to blow. Hopefully the compressor is okay.
My car was laid up with a drum to disk swap and then leaking calipers, so it didn't run for almost 3 months. It probably lost some charge from the inactivity, which might have caused the limiter to blow. Hopefully the compressor is okay.
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