pcweber

97 Camaro. It blows constant 42 degrees at idle, never cycles. I drive on highway the comressor kicks off after 2 miles for about 10 seconds, then back on. Acts like high side cut off. I always assumed with more airflow (highway speeds) the high side presure goes down so I cant see how the high side could be kicking it out. But how can you tell as I cant have gauged hooked up on highway....

bobdec

If you take any A/C DTCs out of the picture any of the following will disable the clutch.
-RPM over 4100
-AC refrigerant pressure over 414 psi (fixed setting in PCM)
-AC refrigerant pressure under 38 psi (fixed setting in PCM)
-Evaporator temperature less than 36* F (can be changed in tune, but 36 prevents freezing coil up)
-Throttle position sensor greater than 99.9% (can be changed in tune, mine cuts AC at 40%)
My guess is you never hit 36* when idling so compressor stays on and when running you probably hit 36. It's completely normal for the compressor to cycle on/off constantly when driving. Really depends on the amount of, and temp of, the air passing the evaporator coil. Mine cycles on/off at idle and when running. My Datamaster log shows it's because of the 36* threshold. But I do keep a slightly higher charge than I should, really not recommended, but I like cold air in stop/go traffic.

jloshotz

Agreed. Higher rpm of the compressor produces lower back pressure of the system...when the back pressure falls below the specified threshold, the compressor will cycle on the low pressure switch, preventing evaporator icing. You may very well be experiencing a normal running condition.

pcweber

OK, a bit more info. When running down highway the vent temp never gets below 45 adn when it starts cycling goes to 55, then starts dropping and at around 50 it cycles off, etc. So in relity with more airflow at hwy speeds the overall temp out vents is higher. Not cool enough for comfort even on an 80 degree day in the car. So I wondered about heat soak from the heater core and the mix door not closing all the way. I put a $12 coolant valve inline with one of the heater core lines today to rule that out. Did not change things. Is there a way I can eliminate or confirm if it is the evap. sensor triggering the cycling by unplugging it or disabling it for testing? THanks for the replies thus far. My daughter thinks she will die in about a week without cold AC in her car....

bobdec

Suggest you find a set of AC gauges or take it in for one of those 'Free" AC checks if you can find one. Normal AC will work better and have higher pressure at higher engine RPM's. If your sucking the refrigerant out of the low side it's possible your charge is to low. Running blower at high even at idle puts lots of air past the coil, road wind does not really effect the HVAC air flow inside the car. However at this point w/o the gauge readings we are just guessing at a low charge. As far as the evap sensor, the PCM it cuts off compressor at 36* and turns on back on at 39* if working correctly AC clutch cycling is not detectable inside the car as the 3* difference is to low to feel. There is no way to bypass the sensor with jumpers or opening the circuit, opening or shorting it will cause a DTC P1532 or P1533 w/o SES light.

pcweber

Yep, I have gauges. 2 days ago at 80 degrees ambient temps low side was 40 and high side was 250-275 depending on high or low cooling fans cycling. As an FYI it was a bit cooler yesterday at around 68 degrees and it would still cycle though temps out the vents were around 40 before it started to cycle. My comment around HWY sppeds and air flow are directed around airflow over the condensor. I cant ever recall a vehicle not cooling better at high speed. I ahve 6 vehicles licensed and insured an every one of them the AC works far better at HWY speeds than idle. This Camaro has me stumped. When it gets back to 80's again in next few days I am going to try driving it on HWY with guages on somehow.

jloshotz

That head pressure is way too high. You either have a restricted txv, air in the system, or are grossly overcharged and going off on high head. It's a starting point, I would lose the entire charge, evacuate the system through a vac pump and weigh the new charge in to tag specs, typically found on the receiver/drier, or fan shroud cover. You may just have non-condensibles (e.g. Air), in the system. You don't know what someone could have done before you, and way too many are dangerous with a set of gauges in their hands. Start by doing this, and it may just be all that needs to be done. If it still acts up, the txv may be the culprit. I just changed mine out, and my a/c has never worked better. Good luck and post back with results

pcweber

I did vacuum the system at -28 for 45 mins before starting so should have no air in it. I was told that until 96 they had no orifice tube and in 97 (my year) with a change to a diff. compressor they went back to an OT. Does that sound right?

jloshotz

Interesting, not sure. I know the 95, which is what I have, does not have an OT, and was very straightforward to change. Hopefully someone with a 97 can chime in on this. Your head pressure really shouldn't see over 175, and with the back pressure where it is, it really sounds like a restriction, perhaps with an overcharge....did you scale the charge in as well? What method did you use to break the vac?

pcweber

The below verbiage is what someone on a specific car AC only forum replied. His user ID was GM Tech if that means anything. He indicated in an earlier post a 97 has a V5 compressor... I dont have a way to wiegh, jsut a guy in his shop trying to keep daughter's car going. Thanks for any and all help.


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Should be an OT, switched back to OT when they went to variable compressors-- the fixed displacement H-6s of 93-96 vintage used an H block TXV at evap entrance underhod...

bobdec

Not sure if this helps.. The '98 has an orifice tube where the '94-94 have a mixer valve called a "Thermostatic Expansion Valve Assembly" . But I do not have a '97 service manual. Also the Hi side pressure at 80* F should be between 150 and 235 so you seem in the ballpark. As you stated, when moving more air through the condenser coils, it will drop the compressed liquid form 134 temp and should provide cooler air at the evaporator. Buy the way the F-Body books all say the temp from the air ducts s/b 20* F cooler than ambient temp. At 80*F you should be blowing 60* air..

pcweber

Got some more info today as it was finally warm again. SO I taped the gauges to the windshield so I could see pressure while driving. Again, I can let it idle for 20 mins and the temp out vents are cold 45ish and pressure steady at 40ish & 225ish. So, I drive. It climbs to around 250 on high side and stays steady for about 1.5 miles. Low side stays between 35-40 the whole time. (as long as comp. In engaged). All of the sudden after mile 2 the high side starts climbing fast. Swings all the way over to 500, comp kicks out, drops pressure, comp kicks on, then a vicious cycle of high pressure, cut off, etc. back and forth. I stop, let it idle and it all settles out and cools. Is the roll of the TXV to prevent this high side rise? Also, I went home and made a jumper one time that stuck the fans on high. That made my idling high side lower at around 200psi but when I drove it the same spike issues happened. I am stumped. Unless movement is creating some weird lack of new air situation under the front of the car as these Camaros rely 100% on sucking in air from below.

jloshotz

Still strikes me as a restriction or non-condensibles...Have you determined if it is an OT system?

pcweber

Here is a pic of the lines going into the evap. Nore the high presure side where the sensor goes into looks to be of a larger 4-5" section where maybe an OT could be? There are no dimples in that line there but it does reduce to the right. Also, I dont see anything lookign like a TXV where the lines head into the EVAP. As an FYI I did vacuum the system at -28 for around 40 mins prior to charge and this is about the 10th system I have charged after vacuuming over the past 4-5 years so no way am I an expert but not first rodeo.

https://picasaweb.google.com/lh/phot...Fm0?feat=email

jloshotz

What method did you use to break the vacuum?