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Username Post: A/C pressures too high (new system) 93 sub        (Topic#348541)
Posts: 26

Reg: 11-30-17
12-04-17 08:03 AM - Post#2716826    

I have a 93 suburban with front and rear air. When I bought it the air didn't work. The previous owner said that there was a leak somewhere but they couldn't find it. He had bought a new compressor, accumulator, expansion valve and orifice tube, but hadn't Installed them yet.

So, while I was tearing out the interior to restore it, I started pulling a/c components. The previous owner had injected system dye, but there were no leaks detected in the front evaporator with uv light when I pulled the dash to replace it, and no leaks on the rear Evaporator. So...

In order to replace the rear expansion valve I had to cut the evaporator out because the lines were rusted and you cannot order new front to rear lines (but you can order a new rear Evaporator). So I got that all put back in. The front condenser was in bad shape (lots of bent/broken fins, etc) so I put in a brand new Condenser. I also installed everything else as well.

According to this page
chevy truck a/c oil capacities

I needed 11 oz of pag 150 oil and 68 oz of r134. I used the specs for the 94 truck because mine (93) was listed as having r12 and I was doing a conversion. However the specs for r12 and r134 are the same.

The new compressor had a sticker that said I needed pag 46, so I used that in all components instead of the pag 150.

I removed and flushed all the components that I was reusing with
A/c flush.these components we're the front evaporator, the condenser y line, the condenser straight line, and the front to rear lines. I put the orifice tube in the y line as well (in the y line that goes to the front evaporator)

The compressor came with 1oz of oil in it. So to get to 11 oz, I added 1 oz of oil to the compressor, 2 to the front evaporator, 2 to the rear Evaporator,2 to the condenser and 3 to the accumulator.

I vaccuumed for 3 hours (pulled down to 29) and it held vaccuum overnight.

I had 12 oz cans of r134 (just r134, no oil in the can), and with the front and rear a/c on, started filling it up. Every time I switched cans I made sure to close the line to the low side and purge the air until freon came out of the Schrader valve on the manifold. I put in roughly 6 cans, but I figured that was ok because I probably lost 1oz from each can when I was making sure there was no air in the lines between each can change.

This is where the problems started. It took the r134 sooooo slow. It took me shaking the can every couple of minutes and took each can roughly 20 minutes to get into the system. Eventually the compressor kicked on and cycled on and off every 10 seconds, until I was about on the 4th can. Then the compressor stayed on until I was done.

The end result was this:

About 60 degrees blowing cold coming out of the front and rear a/c, (81 degrees outside 50 % humidity)

70 psi low side
300 psi high side. The high side would increase pressure over time until I eventually turned the truck off.

It was too cold out for these pressures to be so high, and the a/c is not near cold.

I had a house fan blowing on the condenser, but the condenser was really hot.

So what could be the issue here? I thought that I had followed what I was supposed to do, could I have missed something? Also, I noticed that AutoZone sells a electric condenser fan for the truck. My truck did not have one. Should it have one? I can't find the answer to that anywhere. Sorry for the super long post, I just wanted to give as much info as I could so that you guys would know what I did/didn't do. Any help would be greatly appreciated! It would cost more than 200 dollars to get the system professionally evacuated and refilled because of so much r134 this truck takes, I would like to avoid that if possible!

"12th Year" Silver Supporting Member
Posts: 2121
Loc: laurens sc
Reg: 04-13-06
12-05-17 02:50 PM - Post#2716972    
    In response to Mjricha

i would let some of the freon out. recently worked on a 71, converted to r134a and wound up running it low to get good pressures and cooling.
i have found through experience that especially on r134a i can not use the full amount on the sticker.
also when changing from r12 to r134a only about 80% of volume is required for r134a.

56 bel air ((since 2002)
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Posts: 26

Reg: 11-30-17
12-06-17 06:03 AM - Post#2717036    
    In response to 62chevy427

Ok thanks! Good to know. I wonder if that's applicable to the oil too. If so then I'd have to dump about 2oz and refilled the freon. Is there a good way to know how much 134 I'm letting out? Or should I just let it out until the pressure drops?

Posts: 26

Reg: 11-30-17
12-06-17 08:02 PM - Post#2717126    
    In response to Mjricha

Ok it's fixed!

I did let a very little bit out of the system.

Honestly I think the issue was cooling. I had the fan shroud off still while I was filling and I think the system just got really hot from idling for over an hour and a half while I was filling it up. Got everything put back together today and drove it up and down the block and my pressures we're 45 low and 190 high. Thanks!

"3rd Year" Silver Supporting Member
Posts: 458
Loc: Western NY
Reg: 06-15-02
12-27-17 09:58 AM - Post#2719099    
    In response to Mjricha

Glad it is working well now. Just FYI one visual way to tell if the original system was R12 or R134a is that the original condenser fittings for R134a had a groove through the middle of the hex nut (should be true of at least the front evap too). The groove was almost like picturing 2 "jam-nuts" with a gap between them. The R12 fittings were a solid hex. The sizes changed too but the groove served as a visual indication of metric fittings and R134a because the R12 English sizes were close. This actually applies to all GM vehicles around the conversion time with the exception of 1 and that was the old dust buster FWD "U" vans. GM pulled R134a ahead at the last moment and it was too late to change the connections.

Regarding the leak. In general the front to rear line in any rear system tends to be a high probability location for finding corrosion/ leaks.

Posts: 28691
Age: 48
Loc: Texas
Reg: 08-01-03
12-27-17 10:15 AM - Post#2719102    
    In response to toro455 only took about 2 seconds for me to realize what you meant by "dust buster" vans.

Good info; I've noticed the grooved fittings but never paid enough attention between the split years to notice which ones had either style. Having worked primarily with the GMT400 trucks I simply kept in mind that 1993 was R12, 1994 was R134a.


06 Silverado ISS / 06 Silverado SS / 06 300C SRT8

"3rd Year" Silver Supporting Member
Posts: 458
Loc: Western NY
Reg: 06-15-02
12-27-17 12:06 PM - Post#2719113    
    In response to someotherguy

Thanks. I couldn't recall exactly when each version changed (suburban/ Tahoe/ Crewcab and normal pick-ups) or if they all went at once.

When I read the OP's original post I was a little concerned the replacement condenser may have been of lower performance because there were lower performing condensers for the pick-ups which would physically interchange on the suburban. The overcharge plus extended idle especially without the shroud make sense.

Valued Contributor
Posts: 3963
Age: 35
Loc: wv
Reg: 08-29-12
12-27-17 04:16 PM - Post#2719149    
    In response to toro455

  • toro455 Said:
Just FYI one visual way to tell if the original system was R12 or R134a is that the original condenser fittings for R134a had a groove through the middle of the hex nut (should be true of at least the front evap too). The groove was almost like picturing 2 "jam-nuts" with a gap between them. The R12 fittings were a solid hex.

You learn something new everyday. Thanks for posting that.


'88 Chevy K3500, aluminum head roller cam 511inĀ³ stroker 10.5:1 compression, 96 NV 4500, 94-98 grille, 305/70-16 (33x12) BF Goodrich KM2s, 91 cluster swap

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