AC Not Cycling (1 Viewer)

[AppleTech]

So my truck (1996) was low on R134a - it has a slow leak. Just started getting hot from a charge up about 2 -3 years ago so I have put off fixing it.

I started refilling it today and was aiming for 28-35 PSI on the low side per the manual. I was on my way up to 35 around 32 PSI (I was stopping the fill to check intermittently) when all the sudden the AC compressor locked up and the belt spun around. During the fill process, it never clicked off once. I evacuated enough that the AC ran normally (feels good and no lockup) and at 1500RPM I'm seeing upper 20's for PSI on the low side. Not quite where it should be but its blowing cold so I'm not going to fuss with it.

The question I have is shouldn't the compressor be cycling on and off? And shouldn't the overpressure switch be shutting the clutch off when it gets too full? I kept adding because the compressor never cycled. It is old and probably original so I'm considering that I may just need to replace it, but is that overpressure switch internal to the compressor? Also am I wrong in my thinking and approach to troubleshooting?

 

[Brandon98]

How much refrigerant did you put in? Any chance you slugged the compressor with a hit of liquid 134 instead of gaseous? In all likelyhood, the compressor's seen better days. Get a Denso from Rock Auto, new receiver dryer, and get to work. In addition, your evaporator is probably nasty and the TXV is tired. The evaporator only takes about 2 hours to get in and out. Go with Denso. Avoid Four Seasons. 7 oz of PAG-46 oil, as I recall.

 

[kruisinkid]

pressure switch is all the way over by the battery and sight glass. without knowing weight of 134 you probably have to troubleshoot with the sightglass. adding an ounce of oil or 2 prob wouldnt hurt

 

[AppleTech]

How much refrigerant did you put in? Any chance you slugged the compressor with a hit of liquid 134 instead of gaseous? In all likelyhood, the compressor's seen better days. Get a Denso from Rock Auto, new receiver dryer, and get to work. In addition, your evaporator is probably nasty and the TXV is tired. The evaporator only takes about 2 hours to get in and out. Go with Denso. Avoid Four Seasons. 7 oz of PAG-46 oil, as I recall.

How would one give it liquid R134? I just have the canisters you tap from the local auto parts store going into my gauge set's center line.

 

[SUMMIT CRUISERS]

How would one give it liquid R134? I just have the canisters you tap from the local auto parts store going into my gauge set's center line.

If you turned the can upside down you would get liquid.

 

[AppleTech]

Ahah so that may be it then - someone had told me to shake the can around and try and get everything out of it but it sounds like that is a bad idea! So really my low PSI reading is correct even though I've got icy AC? Should I try and add some more in without tipping the cans until I get to the correct PSI and then the compressor would actually cycle?

 

[SUMMIT CRUISERS]

As far as shaking the cans, most cans I've seen have directions to twist between 12:00 and 3:00. If your AC is icy, sounds like you're good. How cold is it with a temp gauge in the vent? The vent temp needs to read somewhere between 34 and 40 before the compressor shuts off.

 

[flintknapper]

So my truck (1996) was low on R134a - it has a slow leak. Just started getting hot from a charge up about 2 -3 years ago so I have put off fixing it.

I started refilling it today and was aiming for 28-35 PSI on the low side per the manual. I was on my way up to 35 around 32 PSI (I was stopping the fill to check intermittently) when all the sudden the AC compressor locked up and the belt spun around.
Did you accidentally 'invert' the can? If so...you introduced 'liquid' refrigerant into the low side (a No-No) in an amount sufficient to slug your compressor. Try to avoid doing that.

During the fill process, it never clicked off once.

Normal, depending upon ambient temp and the refrigerant charge.

I evacuated enough that the AC ran normally (feels good and no lockup) and at 1500RPM I'm seeing upper 20's for PSI on the low side. Not quite where it should be but its blowing cold so I'm not going to fuss with it.

What was your high side reading? Were you using a manifold gauge or a low side only gauge?

The question I have is shouldn't the compressor be cycling on and off?

Under certain conditions the compressor might never cycle. It cycles based on two things (Pressure and Evaporator temperature). There is a thermistor/ ETS attached to the fins of the evaporator.

And shouldn't the overpressure switch be shutting the clutch off when it gets too full?
Yes, when it gets to 455 psi. Do you really want to go that high? (answer: no).

I kept adding because the compressor never cycled.

An A/C system is never to be charged based on 'cycling' . There is a prescribed amount of refrigerant the system is designed to work with. Since you were 'adding' refrigerant to a system with some amount of charge already in it, we can't 'weigh in' the correct amount. So...we must rely on gauge readings, the sight glass and vent temps. Not cycling.

It is old and probably original so I'm considering that I may just need to replace it, but is that overpressure switch internal to the compressor?

If your compressor is not leaking (significantly) and is still able to move refrigerant through the system (at pressures as high as 250 psi on the high side) then it is probably fine. But we can't know that unless a set of manifold gauges are used to read both the low and high side. You can also do a fair amount of troubleshooting with a set of gauges, if you are familiar with what to look for.

Also am I wrong in my thinking and approach to troubleshooting?

The one thing I would caution against...is that folks all too often have an A/C issue and immediately assume it is low on refrigerant. So...they go get a couple cans of 134a and purpose to 'pour in the cool'.

IF you haven't first put a set of gauges on the system to see what the pressures are (high and low side), you are really shooting in the dark.

Thankfully, most systems ARE just low on refrigerant...and adding some amount of 'freon' does in fact improve the situation for most folks. Hence the popularity of the recharging cans with the cheap low side gauge attached...that we see in auto parts stores.

The trouble with adding refrigerant without knowing (for sure) the condition of the system is that you can damage certain components or make matters worse.

I am not trying to discourage DIYers from doing A/C work. In fact...you will find no greater proponent for DIY. BUT....a person should first have a rudimentary understanding of what they are doing, why they are doing it and have the proper tools with which to do it.

Glad to hear that yours in now cooling well enough.

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