1996 Cadillac Seville new AC question

[da3248]

Hi,
About 5 weeks ago I replaced an ac system in my 1996 Cadillac Seville. The only parts that aren't new are the evaporator, high side refrigerant sensor and the high side pressure switch. I pulled a proper vacuum and let it sit for an hour before recharging and all was normal. It has been running great until yesterday when it just shut off while I was using it. I checked the fuses and relay and both are good. I took the relay out and jumped it to see if the compressor would kick on and it did. I tried a new relay since I had one and it didn't change anything. The digital readout says very low refrigerant, but when I put my gauges on the pressures are perfect. To double check, I lightly pushed in the Schroeder Valve on the accumulator and there's a lot of pressure. I used oil with the UV dye and do not see any leaks, but again, there seems to be plenty of pressure. Any ideas would be appreciated. Thanks!

[JohnHere]

How did you charge it, by precisely weighing-in the R-134a or by the pressure readings? It sounds to me like it might be low on refrigerant.

[JohnHere]

Here are the specs: 32 ounces R-134a and 8 ounces PAG-150 for vehicles with a Harrison compressor; 32 ounces R-134a and 8 ounces PAG-46 for vehicles with a Denso compressor.

[da3248]

How did you charge it, by precisely weighing-in the R-134a or by the pressure readings? It sounds to me like it might be low on refrigerant.

A mechanic, who's lift I was using, put the refrigerant back in. He didn't put the full amount in as he said the pressures were good and didn't want to overcharge it. I don't understand much about pressure and that relationship, but why would my readout say very low when it might only be about 6 ounces shy of the recommended amount? If that's the case, why did it work so good for 5 weeks? It was much warmer when the procedure was done, and has gotten much cooler since. Maybe that's why? Thanks for the helping me understand, mush appreciated.

[JohnHere]

A few observations. You can't charge by pressures, only by weight. Pressures will help diagnose a system, but they won't tell you the exact amount of the charge. And currently, none of us knows just how much refrigerant is in your vehicle's system.

So what you should do is have a professional mobile A/C shop recover whatever refrigerant is in there now, evacuate, and recharge it to spec, which is 32 ounces of R-134a as said earlier. Then you'll know for sure that the system has the correct amount. Hopefully, you have the correct amount and type of oil in it, which is also very important.

Mobile A/C systems are sensitive to the amount of refrigerant, and just a couple of ounces low can often make a difference in how a system operates. If your A/C is six or more ounces low, as you surmised, that's about 20 percent deficient, more than enough to shut-down the compressor to prevent internal damage due to lack of lubricating oil, which is carried by the refrigerant.

When ambient temperatures were higher, so were the pressures, probably just enough to allow the compressor to run with the charge the system had. Now that the ambient temperatures are lower, so are the pressures. And if the low pressure is now below the threshold of the low-pressure cutoff (LPCO), the compressor will shut down. I think that's what's going on in this instance.

[da3248]

A few observations. You can't charge by pressures, only by weight. Pressures will help diagnose a system, but they won't tell you the exact amount of the charge. And currently, none of us knows just how much refrigerant is in your vehicle's system.

So what you should do is have a professional mobile A/C shop recover whatever refrigerant is in there now, evacuate, and recharge it to spec, which is 32 ounces of R-134a as said earlier. Then you'll know for sure that the system has the correct amount. Hopefully, you have the correct amount and type of oil in it, which is also very important.

Mobile A/C systems are sensitive to the amount of refrigerant, and just a couple of ounces low can often make a difference in how a system operates. If your A/C is six or more ounces low, as you surmised, that's about 20 percent deficient, more than enough to shut-down the compressor to prevent internal damage due to lack of lubricating oil, which is carried by the refrigerant.

When ambient temperatures were higher, so were the pressures, probably just enough to allow the compressor to run with the charge the system had. Now that the ambient temperatures are lower, so are the pressures. And if the low pressure is now below the threshold of the low-pressure cutoff (LPCO), the compressor will shut down. I think that's what's going on in this instance.

Thanks for the detailed explanation. Your assessment sure sounds plausible to me. I do have the proper and right amount of oil. I'll take it to the shop and have them recover and recharge it. I'll try to remember to let you know how it turns out, but crazy busy lately and I'm getting older and forget things from time to time. If not, thanks again for all the help!

[GM Tech]

Caddy's are a different animal-- Their algorithm detects loss of refrigerant by monitoring pressures AND inline temperatures before and after the orifice tube. So they know when you are losing charge and hence the very low refrigerant message-- Believe that message- it is doing its job-- Face it, you have a leak- put dye in the system and look a day later with a black-light- to see where it is leaking.

Again- only charge by weight- if someone tells you the pressures are good- they don't know automotive a/c at all- run from that shop....