AC Trial and Error

yevgenievich said:

For serpentine condensers I found OSC 4544 to be closest in thickness to original. It is slightly larger than 4721.

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Microtube parallel flow is all I have used in the past ~5-10 years. They are much thinner and are much more efficient. A microtube parallel flow that is 1/3 the thickness of the OE style serpentine condenser will easily out perform it. The thinner parallel flow will also more airflow through the whole cooling stack as it is much less restrictive to airflow through it. The condensers on both of my vans and my Tahoe are only 5/8" thick and work exceptionally well even on a 115F day.

I have used the OSC 4721 on several SUVs and trucks. It is a mictrotube parallel flow and my aunts Escalade I put the last one on dropped 50 psi on the high side compared to what it ran in the same heat with the stock condenser. I also replaced the dying condenser fan on it.

OSC 4721 shows up as a serpentine unit which is why referenced a slightly larger serpentine OSC 4544. GM put serpentine on dual unit systems and parallel flow on single air systems.

I am running NC4544A parallel flow currently.

yevgenievich said:

OSC 4721 shows up as a serpentine unit which is why referenced a slightly larger serpentine OSC 4544. GM put serpentine on dual unit systems and parallel flow on single air systems.

I am running NC4544A parallel flow currently.

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GM put parallel flow on the dual systems as well. The 01+ Express vans have them. The OSC 4721 units I have had are without question a microtube parallel flow. The stock image also clearly shows that.

L31MaxExpress said:

GM put parallel flow on the dual systems as well. The 01+ Express vans have them. The OSC 4721 units I have had are without question a microtube parallel flow. The stock image also clearly shows that.

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I have just recently bought one osc 4721 and serpentine was recieved. Discription also states serpentine. I went through probably five condenser purchases recently and everyone is using same stock picture regardless of what they are actually selling.

OSC 4544 was largest serpentine that I found and NC4544A was largest of the two parallel flow that I had

What is your vent temps at idle vs driving? I am currently using the red tube and realized I should have used blue or possibly the white. Ac works ok at highway speeds but the compressor cycles to often to keep the vent temps down as low as I would like. They hover around 48F while driving. I pull up to a red light and they actually drop a couple of degrees until I take off again.

95 Tahoe said:

Ac works ok at highway speeds but the compressor cycles to often to keep the vent temps down as low as I would like. They hover around 48F while driving. I pull up to a red light and they actually drop a couple of degrees until I take off again.

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Sounds like (excessive) hysteresis in the cycling switch could lead to this.

The separation b/t high and low pressure On / Off states is greater than desirable.

Or simply the high-pressure "On" pressure is higher than desirable.

You might try another switch and/or one of the adjustable switches @L31MaxExpress has mentioned:

FOUR SEASONS 36496 adjustable switch
FOUR SEASONS 37219 mating connector

95 Tahoe said:

What is your vent temps at idle vs driving? I am currently using the red tube and realized I should have used blue or possibly the white. Ac works ok at highway speeds but the compressor cycles to often to keep the vent temps down as low as I would like. They hover around 48F while driving. I pull up to a red light and they actually drop a couple of degrees until I take off again.

Click to expand...

A properly functioning stock switch does not cut the compressor until ~22 psi. Should be seeing center vent temps of about 35-40F at that point. I have the Red tube in both of my vans. They will get mid-low 30s at the center vent.

1998_K1500_Sub said:

Sounds like (excessive) hysteresis in the cycling switch could lead to this.

The separation b/t high and low pressure On / Off states is greater than desirable.

Or simply the high-pressure "On" pressure is higher than desirable.

You might try another switch and/or one of the adjustable switches @L31MaxExpress has mentioned:

FOUR SEASONS 36496 adjustable switch
FOUR SEASONS 37219 mating connector

Click to expand...

It is cycling on low pressure. I've watched the gauges while revving the engine to confirm. System seems properly charged per the pressure temperature chart. I'm reluctant to add more refrigerant as I am in the top of the range on the high pressure side. At 90-95 degrees using fresh air open at idle I see 45 PSI/275PSI. At idle on recirculation I see 32 PSI/170PSI

I am currently using a stock low pressure switch that cycles off at 26 and on at 45. I used a similar adjustable switch but it didn't work adjusted below 25-26 PSI it would cycle on and off with in a second.

L31MaxExpress said:

A properly functioning stock switch does not cut the compressor until ~22 psi. Should be seeing center vent temps of about 35-40F at that point. I have the Red tube in both of my vans. They will get mid-low 30s at the center vent.

Click to expand...

The stock switch I'm using cycles off at 26 PSI. How do you keep your evaporator from freezing at 22 PSI, that is like 25 degrees?

95 Tahoe said:

It is cycling on low pressure. I've watched the gauges while revving the engine to confirm. System seems properly charged per the pressure temperature chart. I'm reluctant to add more refrigerant. At 90-95 degrees using fresh air open at idle I see 45 PSI/275PSI. At idle on recirculation I see 32 PSI/170PSI

I am currently using a stock low pressure switch that cycles off at 26 and on at 45.

Click to expand...

Cannot charge off pressures alone, need superheat and subcooling readings. I try to run 6-9F of superheat at the accumulator outlet. I recently added 8-10 oz of refrigerant to the 1987s system and saw less than 10 psi change in pressure but the superheat went from ~15F down to 5-6F and it cools noticeably better. If your accumulator outlet is not ice cold, the system is low on refrigerant. My accumulator actually starts to pickup frost on the exposed aluminum before the cycling switch cuts the compressor.

Tested in 103F and 35% RH, mine was holding 35/235 with the pusher condenser fan off. With the condenser fan on it dropped to 30/215. At 1,500 rpm it was 25/230. At 2,000 rpm the low side dropped a bit more. Cutout is happening at about 18-19 psi.