1996 Cadillac Seville new AC question

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da3248
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1996 Cadillac Seville new AC question

Post by 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!
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JohnHere
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Re: 1996 Cadillac Seville new AC question

Post by 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.
Al9
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Re: 1996 Cadillac Seville new AC question

Post by Al9 »

Careful with the jumping a comp back to life stuff - this is what can happen if the problem is not enough refrigerant (static pressures can be really deceptive) and the compressor is not getting enough oil flowing back as a result.

Whenever a low charge is suspected, recover, weigh and assess. No other way.
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JohnHere
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Re: 1996 Cadillac Seville new AC question

Post by 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
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Re: 1996 Cadillac Seville new AC question

Post by da3248 »

JohnHere wrote: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.
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JohnHere
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Re: 1996 Cadillac Seville new AC question

Post by 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.
Al9
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Re: 1996 Cadillac Seville new AC question

Post by Al9 »

Beware, a system with a not so efficient condenser/condenser fan (or severely overcharged on oil) can behave like it's overcharged even when the correct amount of refrigerant is inside. My car's AC had the former issue (a severely worn electric fan motor that didn't allow the fan to move enough air through the condenser), also had a slow leak (leaky TXV o-ring) at some point so i noticed my issues somewhat improved months before the system started showing low charge symptoms, has been to 3 different shops even in sweltering hot summer days and no one ever came up to me and told me "Boy, your cooling fan is faulty, lets change it", so i think mechanics just don't care about how effectively the system is rejecting heat. They give a cursory look for correct pressures and that's it, without inquiring further.

Where i live, attaching gauges and evaluating a system in a public place outside of a shop is likely to result in someone calling the cops and a huge fine. Driving with gauges attached like someone suggested me to do? Likely to result in severe windshield/hood/AC line damage due to POOR road conditions, in addition to the cops and fine part. Shops, the only ones authorized to do all of this, just don't care too much, and get severely annoyed whenever a client inquires too much about how the repair gets carried. Noisy compressors? Paranoid driver most likely, and that's it. So i couldn't afford to read (or have someone read) pressures.

I had to fit a new cooling fan assembly out of pure "parts changer" attitude (after getting the comp and TXV replaced, making sure the system was flushed and then loaded with the correct amount and type of oil, double end capped PAG 46 and no fancy performance improvers, by the shop, even had to buy a second hand Denso TXV and hollow it out so that they could do the flush since they lacked the necessary adapter, and then getting another noisy compressor and also one HP cutoff event in return), and that fixed my system issues for good (noisy compressor and poor pull-down, that is, cold air took its sweet time to come out, in hot sunny days, especially with the correct refrigerant charge amount inside).
da3248
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Re: 1996 Cadillac Seville new AC question

Post by da3248 »

JohnHere wrote: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
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Re: 1996 Cadillac Seville new AC question

Post by 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....
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