88-94 5.7 Suburban A/C Diagnosis and Tuning

1998_K1500_Sub

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PlayingWithTBI

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Just an observation; you're all saying pressures are too high and there's not enough differential or pressure loss between high and low sides? Can't a plugged evaporator - air flow and/or internal flow cause this too? At one time was there a combining of mineral and PAG oil during conversion from R12 to R134A?

IDK - just thinking out loud. Carry on :cool:
 

L31MaxExpress

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Just an observation; you're all saying pressures are too high and there's not enough differential or pressure loss between high and low sides? Can't a plugged evaporator - air flow and/or internal flow cause this too? At one time was there a combining of mineral and PAG oil during conversion from R12 to R134A?

IDK - just thinking out loud. Carry on :cool:
Lack of airflow would cause low, low side pressure. Lack of flow in the evaporator refrigerant side would show up as low pressure as well. AC system has to pull in heat to build high pressure. Literally only like 3 or 4 things cause high pressure on both sides. Most of the time here, the vehicle I am driving will turn on recirculate if not selected just to control the pressure.
 
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L31MaxExpress

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Like these I assume.


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Those look backwards.

 
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1998_K1500_Sub

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When I converted to R-134 I installed a pressure switch from a R-134 truck that I puled from the junk yard. The switch works...

A few years ago I was curious about the compressor cycling characteristics of my Suburban's Sanden at road speed... how long on, how long off, w/ F&R air in operation.

I hooked-up a light to the compressor clutch circuit at the clutch control relay (firewall location) and used a length of wire to snake from there into the cab so as to position the light atop the dash.

I drove from IL to northern WI (Tomahawk) for the weekend. Engine RPM at 70MPH is around 2200. Compressor RPM thus about 4000RPM (200mm crank pulley, 112mm compressor pulley).

What I found was interesting, at least to me. The compressor would run for what seemed like a long time... and I'm sorry I didn't measure it with a stopwatch, but I might guess 45s. And then it would shut off for a long time, roughly (roughly) about the same length of time.

These ON / OFF times were longer than I was used to seeing, e.g., on my smallish 1995 S10 Blazer w/ R4 compressor and R134 (a vehicle I no longer own).

What did I learn? The Suburban's large amount of refrigerant, and the large volume of plumbing in the system (long tubes to the back acting as accumulators on both high and low sides), meant the compressor had long ON times to move large amounts of refrigerant from one large space into another and, accordingly, long OFF times as those large spaces equalized.

I would be interested to see @Wildblue19's compressor's cycling behavior under conditions similar to mine or @L31MaxExpress.

My Suburban's in storage so I can't run out and make measurements right now.
 

Wildblue19

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I know you are probably fed up with the thing. Have you misted the condenser? Mist it a bit, then once it starts to dry out, mist again, and repeat for a few minutes. Curious where it equalizes out. See if you can drop the head pressure down under 250 and see where the low side ends up. If you can get down to the 250-275 psi range the low side reading should be down around 38-42 psi.
This pic shows as low as it would get stabilized with fairly frequent misting. High side under a heavy stream of water would drop much lower than pictured, below 150, but regardless low side never varied off of around 55psi.

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I did check front evaporator outlet temps constantly while adding the cans, mid way through the 4th can it started to plateau and remained +/-2 degrees of the min temp until that can was empty.

@1998_K1500_Sub My actual temperature probe broke somehow so I am stuck using the multimeter probe lol. It's a junky china one I got from the jungle website. I really need to pony up and get a decent Fluke.

Here's a real head scratcher. I measured the temps at the suction inlet of the compressor and it was far colder than the exits at both front and rear evaporators, like 58° at idle. I would expect it instead to be about the same if not slightly warmer...
 

1998_K1500_Sub

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Here's a real head scratcher. I measured the temps at the suction inlet of the compressor and it was far colder than the exits at both front and rear evaporators, like 58° at idle. I would expect it instead to be about the same if not slightly warmer...

If you measure directly across the accumulator, inlet / outlet, what you do see?

Your rear suction line dumps into the mounting block right at the compressor, does it not? I'm quite certain the suction plumbing on the earlier Subs is different then mine.
 

L31MaxExpress

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A few years ago I was curious about the compressor cycling characteristics of my Suburban's Sanden at road speed... how long on, how long off, w/ F&R air in operation.

I hooked-up a light to the compressor clutch circuit at the clutch control relay (firewall location) and used a length of wire to snake from there into the cab so as to position the light atop the dash.

I drove from IL to northern WI (Tomahawk) for the weekend. Engine RPM at 70MPH is around 2200. Compressor RPM thus about 4000RPM (200mm crank pulley, 112mm compressor pulley).

What I found was interesting, at least to me. The compressor would run for what seemed like a long time... and I'm sorry I didn't measure it with a stopwatch, but I might guess 45s. And then it would shut off for a long time, roughly (roughly) about the same length of time.

These ON / OFF times were longer than I was used to seeing, e.g., on my smallish 1995 S10 Blazer w/ R4 compressor and R134 (a vehicle I no longer own).

What did I learn? The Suburban's large amount of refrigerant, and the large volume of plumbing in the system (long tubes to the back acting as accumulators on both high and low sides), meant the compressor had long ON times to move large amounts of refrigerant from one large space into another and, accordingly, long OFF times as those large spaces equalized.

I would be interested to see @Wildblue19's compressor's cycling behavior under conditions similar to mine or @L31MaxExpress.

My Suburban's in storage so I can't run out and make measurements right now.
I have actually monitored that as well in datalogging the PCM by monitoring the ac request and ac relay status. The ac request is directly connected to the cycling switch. There is a 2-3 second delay between the ac request and the clutch relay engaging. Cycling depends on alot of factors, but my compressor seems to run more than it is off atleast when it is hot and the blowers are on high speed. Once the interior is cool and the fans run on mediun speed the cycling is more frequent and the compressor runs less.
 

L31MaxExpress

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This pic shows as low as it would get stabilized with fairly frequent misting. High side under a heavy stream of water would drop much lower than pictured, below 150, but regardless low side never varied off of around 55psi.

You must be registered for see images attach

I did check front evaporator outlet temps constantly while adding the cans, mid way through the 4th can it started to plateau and remained +/-2 degrees of the min temp until that can was empty.

@1998_K1500_Sub My actual temperature probe broke somehow so I am stuck using the multimeter probe lol. It's a junky china one I got from the jungle website. I really need to pony up and get a decent Fluke.

Here's a real head scratcher. I measured the temps at the suction inlet of the compressor and it was far colder than the exits at both front and rear evaporators, like 58° at idle. I would expect it instead to be about the same if not slightly warmer...
Feel around on the suction line hose from the Y to the compressor. You are looking for a spot where the line gets colder. It is possible although unlikely that the suction line itself has a restriction. Remind me where the low side service port on this truck is. Is it accumulator or compressor mounted?
 

L31MaxExpress

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If you measure directly across the accumulator, inlet / outlet, what you do see?

Your rear suction line dumps into the mounting block right at the compressor, does it not? I'm quite certain the suction plumbing on the earlier Subs is different then mine.
From memory the rear suction still has the same style Y as yours. Just a different manifold hose.

EDIT..Looking on Rockauto you are correct. The suction line has two separate paths all the way to the manifold block.
 
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