ac pressure suddenly shoots up to 400 psi

[Jimmy D]

Recently while under hood of my 1991 there was a fast spray of freon from the relief valve on the back of the compressor. I shut it off and looked around and saw pag oil on the valve cover. I started it back up and it seemed fine, cooled fine. Later it did it a couple more times. Then while idling the compressor locked up and began burning the belt. I had the gauges hooked up and I saw the high side reading was at 400. I quickly shut it down and the pressures came down as well. I grabbed the compressor clutch to see if the compressor was seized up and it was not. It turned freely. The compressor doesn't have but a few thousand miles on it. I removed the hoses from the compressor to check for debris and it was pretty clean inside. I removed the orifice tube and it was also pretty clean so I do not suspect blockage or compressor failure. I have to wonder if the cycling switch on side of the accumulator is sticking allowing the high pressure to run up there? I know the cycling switch will stop the compressor from running if the system pressure is too low but will it let the system pressure get too high if that switch fails?
Also, there is no high pressure switch on the compressor itself, only a bleed off valve on the back.

 

[PlayingWithTBI]

I have to wonder if the cycling switch on side of the accumulator is sticking allowing the high pressure to run up there? I know the cycling switch will stop the compressor from running if the system pressure is too low but will it let the system pressure get too high if that switch fails?
Also, there is no high pressure switch on the compressor itself, only a bleed off valve on the back.

What year truck?

Yes, the cycling switch is only for low pressure shut off. You need to install and wire the high pressure switch in the back of the compressor. That switch usually is designed to shut off at 430 PSI and turn back on at 200 PSI.

www.oreillyauto.com/detail/c/murray-climate-control/murray-climate-control-2-terminal-multi-purpose-switch/mry0/35961/v/a/3009/automotive-truck-1988-chevrolet-c1500-pickup-rwd?q=AC+high+pressure+switch&pos=1

https://www.oreillyauto.com/detail/c/murray-climate-control/murray-climate-control-2-terminal-multi-purpose-switch/mry0/35961/v/a/3009/automotive-truck-1988-chevrolet-c1500-pickup-rwd?q=AC+high+pressure+switch&pos=1

 

[Supercharged111]

If I understand AC operational principles well enough, if it gets that high something is wrong. Either there is no airflow across the condenser and/or the AC is just blowing too hot for whatever reason.

 

[1998_K1500_Sub]

If I understand AC operational principles well enough, if it gets that high something is wrong. Either there is no airflow across the condenser and/or the AC is just blowing too hot for whatever reason.

^^^ this

Fan clutch problem?

Not enough "fan" (old style 5-blade mebbe)?

Fan shroud missing / incomplete?

OE 1991 tube-n-fin R12 condenser is inadequate for R134a (imagine that)?

Non-optimal orifice size in the tube?

Yes, the cycling switch is only for low pressure shut off. You need to install and wire the high pressure switch in the back of the compressor. That switch usually is designed to shut off at 430 PSI and turn back on at 200 PSI.

^^^ this, with emphasis

The R134a conversions practically require a high-side pressure cut-off switch.

 

[Jimmy D]

What year truck?

Yes, the cycling switch is only for low pressure shut off. You need to install and wire the high pressure switch in the back of the compressor. That switch usually is designed to shut off at 430 PSI and turn back on at 200 PSI.

www.oreillyauto.com/detail/c/murray-climate-control/murray-climate-control-2-terminal-multi-purpose-switch/mry0/35961/v/a/3009/automotive-truck-1988-chevrolet-c1500-pickup-rwd?q=AC+high+pressure+switch&pos=1

https://www.oreillyauto.com/detail/c/murray-climate-control/murray-climate-control-2-terminal-multi-purpose-switch/mry0/35961/v/a/3009/automotive-truck-1988-chevrolet-c1500-pickup-rwd?q=AC+high+pressure+switch&pos=1

As stated it is a '91, and does not have an electric high pressure switch on the compressor. This compressor is aftermarket some shop put on while my dad owned it. I'll look tomorrow to see if there are some wires unplugged behind the compressor. The ac has been working flawlessly for 8 to 10 years.

 

[1998_K1500_Sub]

As stated it is a '91, and does not have an electric high pressure switch on the compressor. This compressor is aftermarket some shop put on while my dad owned it. I'll look tomorrow to see if there are some wires unplugged behind the compressor. The ac has been working flawlessly for 8 to 10 years.

It may have never had an OE high-pressure cut out, but it should have one / be retrofit with one regardless of past history. Such is the nature of R134a vs. R12.

 

[Jimmy D]

I just looked at a Delco and Murray compressors for a 1991 at O'Reillys and neither of them come with that pressure switch so that leads me to believe the '91 did not originally have them. In the upper rear of my compressor and on the Murray and Delco units same place is a plug with a circlip that looks about the right size for that switch above so that's probably the place to put a switch. And splice it into the positive or negative wires to the compressor clutch?

 

[1998_K1500_Sub]

I just looked at a Delco and Murray compressors for a 1991 at O'Reillys and neither of them come with that pressure switch so that leads me to believe the '91 did not originally have them. In the upper rear of my compressor and on the Murray and Delco units same place is a plug with a circlip that looks about the right size for that switch above so that's probably the place to put a switch. And splice it into the positive or negative wires to the compressor clutch?

The 1995 R4 did, probably all R134a R4 systems did; the switch goes in a hole in the back (which you've found).

The switch goes in series w/ the compressor wiring, typically the "+" circuit.

Consult the 1995 service manual if you want to see how the factory actually wired it

 

[PlayingWithTBI]

Such is the nature of R134a vs. R12.

Yes, R134a runs at a higher pressure than R12. On R12, pressures that high were pretty much unheard of except, when something's wrong in the system. When an R134a system gets over heated, like when you're running all day in the 100° + temps, then shut off the engine. The engine compartment continues to cook so, if you start it back up soon after, you will see those high pressures. Back then, GM put in that high pressure relief valve which will bleed R12 to the atmosphere! There was a wire connected to the no longer available one wire HP Switch which, when it saw high pressures it would close off the Recirculation Door to warm up the evaporator and reduce pressure. Maybe releasing R12 to the atmosphere was a contributing factor in outlawing it.

There are threads here on installing the HP Shutoff Switch in an older system that doesn't have the "MAX" button on the Control Panel. You wire the HP Switch in series with the LP Cycling Switch so, if either goes out of designed limits, it'll shut off the Compressor.

In the upper rear of my compressor and on the Murray and Delco units same place is a plug with a circlip that looks about the right size for that switch above so that's probably the place to put a switch. And splice it into the positive or negative wires to the compressor clutch?

I was typing when you posted this - yes, you have to install your own HP Switch.

 

[Jimmy D]

This truck was "converted" to r134 when this compressor was put on with it's stock condensor. And as said, it has been working flawlessly and plenty cold for 8 to 10 years now. The truck was not used much so only a few thousand miles were put on it but since I've had it I have driven it several days a week in the last couple of years. It just now crossed 88,000 actual miles. I still believe the compressor is fine but will look into adding that switch for insurance.