I have a 1972 Chevelle with factory A/C. It keeps blowing the Thermal Limiter. What might the problem really be?

low r12 charge

I am in the process of trying to fill it with R-134a. I can't complete the process while the limiter keeps going.

by pass it

Sounds like system pressure is too high.
Retrofitting to r134a...you should use 80% of what r12 charge should be.
Expansion rate of r134a is greater than that of r12.
Let's say that your system is supposed to use 2 1/2 lbs.(40 oz.) of r12.
Multiply 40 x .8 = 32oz. Trying to add more r134 than this causes your high-side pressure to skyrocket...making the compressor work harder and a good chance it will trip the thermal limiter.(before other damage occurs!)
Also,did you start with a 4oz.container of "ester" oil charge.(used for"changeovers")
Mineral oil was used on r12 systems and is not compatible with r134,supposedly causes "sludge" to form in the system.
"Pag" oils are to be used in systems that had r134 from the factory.

I only put about 1/8 to 1/4 of a 14oz of Changeover 134A. It is 10oz 134, 2oz R-12 friendly oil, and 2 oz leak sealent. It only got to 30psi in the low side and I won't put any more in till it cycles through, but keeps tripping the circut.

So did you remove the old refrigerant oil first?

What is the high side pressure?

By the way IF you put your location in your profile someone in your area might offer to help.

I updated my profile. Hehe. This is my first day on the site. Anyway, Yes. I did get the R-12 suctioned out. I don't know the high side pressure. I have a gauge that only reaches 200psi. I don't know what the high side is supposed to be. The only gauge I have is to assist in the filling of the low side. I dont have a high side adapter/gauge.

Evacuating the system removes the old refrigerant and moisture but doesn't remove the old refrigerant oil.
Like BigFred said you can't leave the old oil when converting to 134 refrigerant.

Also, you really need to be monitoring the high side pressure.

I'd hate to know how many cars and trucks are out there with old oil still circulating through the AC systems after being converted over to the new refrigerant. When I first did changeovers years ago we were told 'NO WAY!' that it would work. So, we drained oil, changed o-rings, and generally had a not-so-fun time. After a while it became common practice to leave well enough alone, and even in my own vehicle I left ALL o-rings as they were, merely adding on the new port fittings along with ester oil and new refrigerant. And the AC system worked just fine. Many other ASE Master techs as well that I know did the very same thing.

oLDANI IS RIGHT ON. when it first came out it was taboo to mix the oil. Truth be known there is no way to eliminate all the old oil unless you replace The Compressor drier evaporator etc.. You will have residual oil on the insides of the lines and trapped in any low area. So what to do???? Well the reason the two oils arent compatible is they have different molocule anatomies and the main reason I have found is the weight. They seem to NOT mix good together and float on top of one another and we all know the oil/Refridgerent charge need to mix and travel the system together to help the compressor keep lubed. I would say first pull a vacuum down on the system (air is the number one killer to ac effeciency). Pull off the compressor and drain it turn it over by hand (keep track of the amount removed). Recharge it with the proper 134 lube and go for it. If you are talking about the low side thermal limiter it will keep triping until it reaches the proper pressure. If you are talking about the high side limiter tripping Then it is overcharged or there is another problem. (restriction, low fan volume, dirty condensor Etcc...) good luck.

Might be that it would work OK, I don't know because I can't recall ever converting one over without flushing out all the old oil.
I always heard that mixing the two types of oil would cause "wax" to formulate.

I know It sure would be a lot easier not to have to flush it all out.

All I ever changed was the line to compressor O-rings and that was only because I had to have them off to remove the compressor for getting it's oil changed anyway.

"(air is the number one killer to ac effeciency)"

The moisture in the air is the number one killer of air-conditioners.

Might be that it would work OK, I don't know because I can't recall ever converting one over without flushing out all the old oil.
I always heard that mixing the two types of oil would cause "wax" to formulate.

I know It sure would be a lot easier not to have to flush it all out.

All I ever changed was the line to compressor O-rings and that was only because I had to have them off to remove the compressor for getting it's oil changed anyway.

"(air is the number one killer to ac effeciency)"

The moisture in the air is the number one killer of air-conditioners.

"The moisture in the air is the number one killer of Air- conditioning" ?????????
Well the number ac killer would be "LOW or NO OIL" I was refering to AC efficiency. And wasnt looking for a reply but since you asked. Obviously "AIR" has moisture in it. I dont like to get too detailed but this time I will. Unless its pure oxygen and we dont have that where I`m from there is always humidity "Moisture" in the air. This is why we pull a vacuum down on it. Most guys think they remove the air by pulling vacuum down and pull out the moisture too, but unless you have a very strong vacuum pump all you are removing is the air itself. What you need to do is pull a hard long vacuum down on the system for more reasons then that. The water starts to turn to a gasous state under a 29 plus psi vacuum after 30 minutes or so. This happens at alot lower temp then under normal atmosheric pressure. Water boils at like 212 under normal conditions it boils at 80ish under a 30 pound vacuum (dont quote me on that) It kind of boils so to speak and the water vapor is then sucked out. The drier/accumulator holds alot of moisture This needs alot of long strong vacuum to get the moisture out of it if its not replaced.This I know forsure You will never flush all the old oil out of any system period. You may get most of it but flushing a compressor out is impossible and a bad idea to boot. The flush residue can be devistating to the compressor and thats the place where alot of the oil sets. You can (As I Do) Flush out the lines and evap & Condensor) But I would never suggest flushing the compressor out. So if you dont/wont flush the compressor out you will have old oil in it.
No response neccesary ____. I really get sick of people trying to punch holes in everyones comments. There is more then one way to skin a cat and my way may work???? and so may yours????? so lets all try and work together and be as informative as we can. Like I said before (I hope or I may have deleted it), Flush out the evaporatore & condensor, drain the compressor keeping track of how much came out. Fill it back up with the proper amount of oil and charge. And thats as good as you can do. If you need to know the proper charge a good way is to fill it 3/4 what r12 is with 134 and check your inlet and outlet temps on the evaporator (with a temp gun) . Add until they are equal then your full (you wont need to add much if any more). Your system holds what 60 ounces I`m guessing????? So about 40 ounces is a good starting point. This is TOOOOOOOO much info.

Thank you all very much. I now have ALOT of info to try to fix this thing. 100 degree weather just begs for an A/C system. ;)

Like I said moisture in ANY refrigeration system IS the number one killer.

Moisture and refrigerant mixed formulate Hydrofluoric acid.

Been removing moisture from air-condintioners with vacuum pumps for 40 years.

Like I said moisture in ANY refrigeration system IS the number one killer.

Moisture and refrigerant mixed formulate Hydrofluoric acid.

Been removing moisture from air-condintioners with vacuum pumps for 40 years.


Amen..totally agree.

Like I said moisture in ANY refrigeration system IS the number one killer.

Moisture and refrigerant mixed formulate Hydrofluoric acid.

Been removing moisture from air-condintioners with vacuum pumps for 40 years.

Moisture, old mineral oil, and air are what kill most AC conversions. Acid formulation is a real concern that doesn't show up for a few months/years. The old oil also takes up system capacity when you add the proper oil on top of it. The proper (ester or DEC PAG) will mix pretty freely with the mineral oil but then the R134 has a hard time carrying the slop through the system. Kind of like adding Mobil 1 without draining the old dino oil. It's overfull. Aside from acid and gel formulation think of it this way; would you put something in your engine that might starve it for oil?? Same scenario, smaller scale.
Most folks don't realize that the AC system is real similar to your engine as far as oil and contaminants. Would you purposly cut corners on your engine build that might starve it for oil? But it will run just the same for a while...? . If you cut corners on an engine build it might blow up and you waste money. It's the same with AC compressors. It's a close tolerance system just like your engine, it just won't leave you stranded.. If you want it to work right for a long time, you have to build/repair it right.