Model: 1500 silverado
Engine Size: 5.7
Refrigerant Type: 134a
Ambient Temp: 95
Pressure Low: 65
Pressure High: 110
Country of Origin: United States
I had a problem with my compressor not cycling on. At first it would cycle on if I touched the switch and then finally it would not cycle at all.
I replaced the low pressure switch from Advance auto and the compressor cycled on.
Got 2 hours down the road with the temps in the mid 90's here in central Texas and it quit blowing cold air. I pulled over and found the line from the compressor going to the evaporator had iced over. I turned the ac off and drove until the ice melted and turned it back on and froze again.
This is when I realized the compressor was on all the time the dash ac button was pushed. It would go off with the button in the cab and would come on when the button was off with the defrost on.
I left Austin and when I got into Corpus Christi, I exchanged the switch at the Advance auto there, still would not cycle off with ac button on or defrost on.
It would cycle off if the plug at the low pressure switch was pulled off at the accumulator.
I took it and had the 134a removed at a local shop, he was on his way out of town when I brought the truck in and will be back next week so I decided to wait for him.
When he removed the 134a, it weighed a little over 2 lbs.
For the heck of it, I hit the ac button in the cab and the compressor came on, and when I hit the button off, the compressor cycled off???
He said that is not supposed to happen.
Before the system was evacuated, the low pressure was around 65psi and the high was around 110psi.
He is thinking either the orifice tube or the evaporator has a restriction.
The compressor is under warranty from Advance, it was new and not reconditioned. He wants me to remove the compressor and replace it with a new one and is going to flush the system .
Any thoughts, it hot down here in Texas.
95 red hot silverado
I cannot seem to rectify the statements "evaporator had iced over" with "before the system was evacuated, the low pressure was around 65psi".
At 65 psi, there's no reason for the compressor to cycle off, and assuredly it will run continuously with the a/c or defrost on. However, you will not be icing up the evaporator in those circumstances either.
If the evaporator is icing up, I'd expect low side pressures down in the 25 psi or below type of range. Did you take measurements of the low side pressure WHILE the evaporator was iced up? If you were measuring 65 psi at that time, there's likely something wrong with your gauge set.
On the other hand, a high side pressure of only 110 (assuming it wasn't a problem with the gauge set) suggests a bad compressor as well, since you had over 2 lbs of refrigerant to work with. Sounds like you've had a compressor replaced in the past, but somebody failed to properly flush the system. Metal debris will destroy a brand new compressor just as easily as it will an old one.
Edited: Sat June 07, 2014 at 11:57 AM by webbch
Do not replace compressor. This is purely a control problem. The switch should be open at zero pressure. If system is still empty, measure ohms of switch. If it's closed circuit, remove it from the accumulator and measure again. The switch may not be opening the shrader valve on the fitting, making it unable to see the system pressure. Or you've just gotten into a run of lousy switches. There is another type of switch (used on variable compressor systems, it would be the wrong one for your truck) that opens at 10 psi. That one will not prevent freezing.
To test with system charged, while measuring the pressures, put the interior fan on low and rev the engine, this should drop the low side into the 20's and the switch should turn off.
Edited: Sat June 07, 2014 at 1:03 PM by mk378
As of this morning;
As I mentioned, the system is empty.
The static pressure measures 10-11 psi on the low side and 0-1 psi on the high side after letting the truck sit all night.
Started the truck with gauges on this morning and the high pressure stayed at 0-1 psi and the low pressure dipped down to 20 or so in HG vac (20 inches of HG)
I haven't had a chance to measure the ohms across the low pressure switch, but I will when I get my meter back.
Also, the clutch still cycles on no matter the pressure.
So the system was empty but the compressor came on and low side went into vacuum? How long was it run this way? Hopefully not too long as nothing was lubricating the compressor while it was running without a charge in the system.
I agree with mk378, there is obviously a problem with the control system.
I bought a can of 134a at w**-mart that had a stop leak, oil, and dye in it. It also had a hose and a gauge, so now I'm an AC pro!
On a full charge, low side pressure of 65 and high side of 110 with the compressor engaged as far as I could tell from the problem description. Assuming these are valid gauge measurements, that suggests to me that either the compressor isn't being given anything to pump, or it's incapable of pumping. Given that 2 lbs of refrig was pulled out during evac lead to the latter conclusion. mk378, are you suggesting the former due to control system probs? As mentioned, the "icing up" of the evap in the description is inconsistent with a low side pressure of 65 psi, so I'm missing something here.
It was running nice and cold until it froze up in Texas -- too cold. To go digging into the refrigeration loop is just not the right conclusion.
I apologize for the empty posts! I dunno, I'm a floor mechanic and find this interesting.
Jag987, yes," the system was empty but the compressor came on and low side went into vacuum".
It was on for 5-10 seconds on 3 occasions, first to check with the ac tech when the system was evacuated , 2nd and 3rd time to verify the switch cycled on under 10psi. Mk378 brought up the point that perhaps I had a lousy switch or maybe a switch that cycled on at 10psi.
OK, I'm thinking the switch is lousy unless my pcm is shot, plug wiring harness issue or I'm missing something else.
I replaced the compressor, condenser, orifice tube and accumulator last August and the system was very cold up until the compressor wasn't cycling on. I assumed it was a lousy switch when I replaced it, now the compressor runs continually. The ac runs all year round with the exception of a few weeks at a time down here in central Texas.
The ac gauges (oem brand) are new from Auto Zone. I close them up, attach them to the high and low pressure ports then open them up until the needle on the gauge moves to make the readings.
So, I am leaning towards a restriction somewhere and hoping it is not in the labor intensive evaporator. I could not verify the evaporator was icing over since it is behind the firewall, but the lines from the compressor to the firewall was iced over including the accumulator.
When you unplug the switch, it stops like it should-- this shows that the rest of the electrical system is OK. The problem is that the switch is not opening at low pressure. This could be because the switch is bad, or the valve between the switch and the accumulator is not letting the pressure reach the switch. You need to find your ohm meter or buy a new one and test the switch. Quit running the compressor with no refrigerant or you will need a new one.
I checked the old switch and it measures 0 ohms
The new switch measures .2 and one of the prongs is loose.
I have the same truck except 98, if you have a 65 psi on the suction it's either 100ÃÂ° in the cab or it's way overcharged. When it's cooled down in my cab, my suction is around 35, IMO if the system is charged correctly, it should not freeze up. The low pressure cut out "I was told at one time by a GM mechanic" is for just that, to cut out the system if the charge is low to keep from damaging the compressor, especially with 134A it depends on the correct charge to move the oil. I have never thought of the low pressure cut out, to be used to "temperature" cycle switch for the compressor. I like mine to run all the time I just use the control on the dash to adjust it if it gets to cold, but in this Texas heat, and me being in the Residential and Commercial a/c business mine stay on full blast and as cold as it will go all day long. Also I would think cycling all the time would not be good on the clutch, going down the road at 70 mph.
Don't take life seriously... Its not permanent.
On my 98 GM truck, I have a high pressure switch on the back of my compressor, and a low pressure switch on my accumulator, so I have two.
Don't take life seriously... Its not permanent.
The truck has an R4 compressor, right?
The low side switch that never turns off in normal operation is used on cars with variable compressors such as the V5. The R4 is not variable, it needs to cycle off or it is going to over-cool and ice up.
Any LPCO switch should be open circuit when it is at zero pressure, such as when you take a new one out of the box.
Like most things GM, first cost, not reliabilty, is the main concern.
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