adjusting low pressure cutoff switch after R134a conversion

[gmbellew]

my 1990 suburban was converted to R134a before i purchased it. i have never been happy with the AC performance at idle or in stop and go traffic. it cools well at constant speeds on backroads or on the highway. i plan to add an auxillary cooling fan to help put more air across the condenser at idle and stop and go traffic. but i also wonder if the low pressure cutoff switch needs to be lowered to account for the different refrigerant and if that will help cooling at idle? thoughts?

 

[bucket]

When the system was charged with r134a, was it less of a charge than what it called for with r12?

 

[gmbellew]

When the system was charged with r134a, was it less of a charge than what it called for with r12?

no idea how the conversion was done. i can see it is the OEM condeser and compressor, but that is all i know. i had the ac "checked" at firestone when i was getting an oil change a couple years ago, but don't have a lot of faith they know what they are doing.

 

[bucket]

It's been many years, but I think my '88 with rear AC called for 4.5lbs of r12? I put in less r134a, like 4 lbs maybe. It worked pretty well with all the stock parts, once I replaced the fan clutch that was a little worn out.

 

[Runnergeorge]

I hope this is the correct place for this. I have a 75 c10 with 350 original ac truck. I am replacing the parts with new and going to charge with r134. However the low pressure cutout switch I bought does not fit the fittings on the accumulator as pictures show it. Does anyone know where I can get the correct switch or fitting to make it work? Thanks!

 

[75gmck25]

Two replies.
1 - they usually estimate the R134 charge as about 75% of the R12 charge, and use pressures to adjust from there.
I don’t think a different low pressure switch will help performance. On older trucks some folks switch to a Ford blue orifice, which might work better than the GM white orifice. However, I’m not sure whether a 1990 uses a TXV or an orifice tube.

2 - I replaced the low pressure switch on my ‘75 GMC and did not have trouble finding one. I just looked it up online at rock auto and compared pictures to my switch.
Mine is screwed into the tube that comes out of the evaporator under the hood. On later models they are on a fitting on the accumulator, and I think you could still use that type of switch also.

Bruce

 

[gmbellew]

to update....

I had a proper charge on a sanden swap (recently checked) and I was still not happy with the AC performance, even with with an auxiliary fan on the OEM condenser.

So I turned the set screw on the low pressure cutoff switch almost 1 turn clockwise (reducing the cutout pressure by about 5psi per the service manual). that made a huge difference in vent temperatures.

at idle in a parking lot with max AC on, I measured about a 30 deg Fahrenheit drop. when driving down the road with additional air across the condenser, I measured about a 40 deg Fahrenheit drop. this was with ambient Temps of 100 degrees and humidity of about 40%.

Later, I was driving on a roadtrip with Temps in the mid 80s on the highway, and I had to take it off MAX, turn the fan to low in front and rear, and turn the temperature to about 1/3 toward hot to keep from freezing myself out (and I like it cold).

just thought I would update this post in case it helps anybody after a r134a conversion.

 

[bucket]

to update....

I had a proper charge on a sanden swap (recently checked) and I was still not happy with the AC performance, even with with an auxiliary fan on the OEM condenser.

So I turned the set screw on the low pressure cutoff switch almost 1 turn clockwise (reducing the cutout pressure by about 5psi per the service manual). that made a huge difference in vent temperatures.

at idle in a parking lot with max AC on, I measured about a 30 deg Fahrenheit drop. when driving down the road with additional air across the condenser, I measured about a 40 deg Fahrenheit drop. this was with ambient Temps of 100 degrees and humidity of about 40%.

Later, I was driving on a roadtrip with Temps in the mid 80s on the highway, and I had to take it off MAX, turn the fan to low in front and rear, and turn the temperature to about 1/3 toward hot to keep from freezing myself out (and I like it cold).

just thought I would update this post in case it helps anybody after a r134a conversion.

So to put it simply for the dumb people such as myself, you adjusted it so that the pressure has to be about 5psi lower for the switch to cut power?

 

[gmbellew]

So to put it simply for the dumb people such as myself, you adjusted it so that the pressure has to be about 5psi lower for the switch to cut power?

yes. the pressures r134A operates are lower than the pressures that r12 operates. so with the r12 pressure setting on the cutout switch, the r134 was never really getting cold enough before the compressor kicked off.

 

[bucket]

yes. the pressures r134A operates are lower than the pressures that r12 operates. so with the r12 pressure setting on the cutout switch, the r134 was never really getting cold enough before the compressor kicked off.

Got it. It's worth mentioning that not all systems can benefit from lowering the cutoff point. With many vehicles, in warm weather that fully taxes the system, the low side pressure will generally not drop low enough to cut off the compressor. You obviously have a system that operates very well.