AC Woes

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Re 1999 Avalon AC performance is borderline...lots of bubbles in sight glass and 46-48 low temp coming out of center vent. Doors closed, windows down, recirc on, 1800 rpm, 86 degrees and humid. Put on the gages. High side looks ok, around 225. Low side is low, around 25 but bigger problem is low is not stable. It will hold 25 for 15-30 seconds then start pulling down. When fans kick into hi speed, drop accelerates and low side pressure switch kicks off compressor around 15. I would say I have a restriction in the orifice. Also, when I turn off car, it takes a long time for pressures to equalize. After standing there for several minutes, high side was still 150. Definitely not going to put serious money into this almost 22 year old car plus at 71, I really don't feel like doing a major ac overhaul even though I have all the tools and know how. Could I be missing something easy? Thanks.
 
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Ignore the bubbles in the sight glass. They do not mean anything with an r134a system. I have learned that diagnosing a r134a system with pressures is nearly impossible. A system can have 1/4 charge but show ok pressures. Have it properly recharged and go from there.
 
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I'd agree with Chris. The only way to be sure on the charge is have someone with a machine pull it all out and weigh the right amount back in. Anything else is wild guessing. Overall what you describe though sounds completely normal for a TXV system. I didn't think any Toyotas were CCOT like American cars. TXV systems don't cycle on pressure. The compressor will cut off when the evaporator temperature becomes low enough to risk freeze-over of the air side. At that point the air from the vents should be about 40 F. The suction line will be similarly cold. Try and get an accurate measurement on the line. The heater could be re-heating air. If the compressor is cycling on temperature, absolutely nothing you could do to the refrigeration system under the hood will lower vent temperature. The problem would be reheating or lack of air flow due to 20 years of crud in the evaporator.
 
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Originally Posted by artbuc
Re 1999 Avalon AC performance is borderline...lots of bubbles in sight glass and 46-48 low temp coming out of center vent. Doors closed, windows down, recirc on, 1800 rpm, 86 degrees and humid. Put on the gages. High side looks ok, around 225. Low side is low, around 25 but bigger problem is low is not stable. It will hold 25 for 15-30 seconds then start pulling down. When fans kick into hi speed, drop accelerates and low side pressure switch kicks off compressor around 15. I would say I have a restriction in the orifice. Also, when I turn off car, it takes a long time for pressures to equalize. After standing there for several minutes, high side was still 150. Definitely not going to put serious money into this almost 22 year old car plus at 71, I really don't feel like doing a major ac overhaul even though I have all the tools and know how. Could I be missing something easy? Thanks.
Most automotive A/C systems have a low pressure switch which turns the compressor clutch coil off , if the low side pressure gets too low . Chances are , the system is low enough on refrigerant that the low pressure switch is causing the compressor clutch to cycle . I personally try to run the low side pressure around 35 psi , depending on the inside & outside temperatures .
 

CT8

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Originally Posted by Chris142
Ignore the bubbles in the sight glass. They do not mean anything with an r134a system. I have learned that diagnosing a r134a system with pressures is nearly impossible. A system can have 1/4 charge but show ok pressures. Have it properly recharged and go from there.
This.
 
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Thanks for the responses. I never had a FSM for this car although I did have them for several other Toyota's with sight glasses. The FSM HDD illustrations showing what the refrigerant should like like when properly vs undercharged. When I replaced the condenser on this car many years ago, I carefully weighed in the charge and added the proper amount of ND-8 oil. The sight glass showed exactly what the FSM said it should. The key piece of evidence most did not mention was the low side holding steady until the cooling fans kicked into hi speed. Then it rapidly sucked down until the compressor kicked off at around 15 psi. Why would that happen?
 
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Originally Posted by Chris142
Ignore the bubbles in the sight glass. They do not mean anything with an r134a system. I have learned that diagnosing a r134a system with pressures is nearly impossible. A system can have 1/4 charge but show ok pressures. Have it properly recharged and go from there.
It only has apparently correct pressures when not in use...
 
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Originally Posted by artbuc
Thanks for the responses. I never had a FSM for this car although I did have them for several other Toyota's with sight glasses. The FSM HDD illustrations showing what the refrigerant should like like when properly vs undercharged. When I replaced the condenser on this car many years ago, I carefully weighed in the charge and added the proper amount of ND-8 oil. The sight glass showed exactly what the FSM said it should. The key piece of evidence most did not mention was the low side holding steady until the cooling fans kicked into hi speed. Then it rapidly sucked down until the compressor kicked off at around 15 psi. Why would that happen?
The fan and compressor may energise at the same time , in this case , when the pressure switch turns on .
 
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Originally Posted by WyrTwister
Originally Posted by artbuc
Thanks for the responses. I never had a FSM for this car although I did have them for several other Toyota's with sight glasses. The FSM HDD illustrations showing what the refrigerant should like like when properly vs undercharged. When I replaced the condenser on this car many years ago, I carefully weighed in the charge and added the proper amount of ND-8 oil. The sight glass showed exactly what the FSM said it should. The key piece of evidence most did not mention was the low side holding steady until the cooling fans kicked into hi speed. Then it rapidly sucked down until the compressor kicked off at around 15 psi. Why would that happen?
The fan and compressor may energise at the same time , in this case , when the pressure switch turns on .
That is not what I described. The condenser fans are running and the low side is relatively constant at around 25. When the condenser fans kick into high speed, the low side pressure starts dropping until it kicks the compressor off at 15 psi.
 
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All the refrigerant is being moved to the high pressure side which shouldn't happen without blockage. If it was a simple underfill, the low pressure shut off would happen a lot sooner, and you woud not see different pressures on low and high side seconds after shut off.
 
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Originally Posted by Jetronic
All the refrigerant is being moved to the high pressure side which shouldn't happen without blockage. If it was a simple underfill, the low pressure shut off would happen a lot sooner, and you woud not see different pressures on low and high side seconds after shut off.
I agree completely. Just not sure why it only happens when condenser fans kick into high speed.
 
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More condenser cooling, high side drops, making it easier for the compressor to pull down the low side. About half the people here have you convinced there's a switch on the low side doing something. There isn't one. Look at the lines yourself.
 
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Originally Posted by artbuc
Originally Posted by WyrTwister
Originally Posted by artbuc
Thanks for the responses. I never had a FSM for this car although I did have them for several other Toyota's with sight glasses. The FSM HDD illustrations showing what the refrigerant should like like when properly vs undercharged. When I replaced the condenser on this car many years ago, I carefully weighed in the charge and added the proper amount of ND-8 oil. The sight glass showed exactly what the FSM said it should. The key piece of evidence most did not mention was the low side holding steady until the cooling fans kicked into hi speed. Then it rapidly sucked down until the compressor kicked off at around 15 psi. Why would that happen?
The fan and compressor may energise at the same time , in this case , when the pressure switch turns on .
That is not what I described. The condenser fans are running and the low side is relatively constant at around 25. When the condenser fans kick into high speed, the low side pressure starts dropping until it kicks the compressor off at 15 psi.
Refrigeration and A/C works on temperatures and pressures . When the condenser / radiator fan moves more air , there is more heat rejection in the high pressure part of the system . The compressor is there by able to pull the low side pressure lower . If you have a low pressure switch , it may pull the pressure low enough to turn off the compressor clutch coil . In any case , after verbally beating it to death , my bet is it needs more refrigerant . Either try that or do not try that . But please try something and stop beating a dead horse ,
 
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Originally Posted by mk378
More condenser cooling, high side drops, making it easier for the compressor to pull down the low side. About half the people here have you convinced there's a switch on the low side doing something. There isn't one. Look at the lines yourself.
You are right. My memory is coming back...been so long since I have worked the AC. I have one pressure switch on the high side which is a combo low and high pressure safety interlock. I have an evaporator thermistor which is shutting done the compressor at around 15 psi. Funny thing is the high side stays relatively constant as the low side pressure drops when the condenser fan kicks into high.
 
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Originally Posted by artbuc
Originally Posted by mk378
More condenser cooling, high side drops, making it easier for the compressor to pull down the low side. About half the people here have you convinced there's a switch on the low side doing something. There isn't one. Look at the lines yourself.
You are right. My memory is coming back...been so long since I have worked the AC. I have one pressure switch on the high side which is a combo low and high pressure safety interlock. I have an evaporator thermistor which is shutting done the compressor at around 15 psi. Funny thing is the high side stays relatively constant as the low side pressure drops when the condenser fan kicks into high.
the high side remains constant because of compression ratio. it'll actually drop slightly to allow more gas to get pumped to the high pressure side, which is then cooled and condensed but doesn't seem to evaporate as it should. Is the return from the evaporator to the compressor icy cold as it should be with ample refrigerant flow?
 
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Jetronic, thanks for your helpful responses. Yes, the suction line is cold. Also, I will make a correction to an earlier post. The center vent temp drops down to 43-44 before compressor cycles off. Have been doing as much research as I can and everything points to a partially obstructed or malfunctioning expansion valve. If I can add enough refrigerant to stabilize low side without making high side excessively high I can get by. Car is comfortable even on a hot humid day and I only use her for a grocery getter. Alternative is to let compressor cycle until it craps out. Access requires evaporator removal. Five or ten years ago I would not have thought twice about fixing it. Not sure I want to do it now.
 
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