Do I have a problem?

[DBBAir]

At the end of a 2600 mile trip with the A/C working fine for the trip, it seemed that it wasn't working well the last few miles. It could have just been that we got back into the cold weather and it wasn't needed. In any case, I figured it was the low pressure switch that had gone again since it was 3-4 years since I replaced the second one. So I replaced it. In doing so, I looked closer at the A/C operation and decided that the clutch wasn't turning off so that the compressor was always on. It was my impression that the clutch disengages roughly every 3-4 minutes and it wasn't (although the internet says it should cycle only every 20 min). So I got out my gauges and found that the static pressure was about 79psi for both the low and high side. That seemed normal. Then I measured the high and low side pressures and got 40-45 on the low side (spec is 25-36) and 250 on the high side (spec is 147-205). So now I'm jacked out of shape because the high is so high. The pressures were measured at idle speed. Although the temp at the ducts inside was 48 while in my driveway, I discovered it was a reasonable 39-40 when underway. So now I'm thinking I don't have a problem at all but that 250psi bothers me. Should it? I don't use the truck enough to get a good reading on it - only use it for trips so I'm concerned I'm going to have trouble on my next trip.

What do you think? Any help is appreciated.

[Cusser]

Was the air flow out the vents decreased? I ask because possibly the evaporator froze up.

[DBBAir]

Was the air flow out the vents decreased?

No it wasn't decreased.

[JohnHere]

What is the year, make, and model of your truck?

When you changed the LP switch, did it have a Schrader valve behind it or did you have to evacuate and recharge the system as well?

Pressures (except static pressures) should be measured at about 1,800 RPM, A/C on "recirc," windows and doors open, blower on medium speed.

What was the ambient temperature when you took the pressure readings?

[DBBAir]

What is the year, make, and model of your truck?

2002 Chevy Silverado 2500 Duramax

When you changed the LP switch, did it have a Schrader valve behind it or did you have to evacuate and recharge the system as well?

Yes, there was a Schrader valve so I did not have to recharge the system

Pressures (except static pressures) should be measured at about 1,800 RPM, A/C on "recirc," windows and doors open, blower on medium speed.

What was the ambient temperature when you took the pressure readings?

Yes, maybe I didn't have the RPMs high enough. The temperatures were about 78-80 with less than 20% humidity.

Thanks for the response.

[tbirdtbird]

How many miles on the truck?
Has anyone ever "topped off" the refrigerant?
If so, was any sealer added? Any history of leaks?
Avoid sealer at all costs.

Has the compressor or any other component ever been changed out on this 20 yr old truck? If so, why?
Get pressure readings the way that John asked and mist the condenser with a water hose and let us know both sets of pressures

[JohnHere]

In doing so, I looked closer at the A/C operation and decided that the clutch wasn't turning off so that the compressor was always on.

A fixed-displacement compressor (which is what I think your truck has) that runs continuously suggests the system is low on charge.

It was my impression that the clutch disengages roughly every 3-4 minutes and it wasn't (although the internet says it should cycle only every 20 min).

Several variables determine when and how often the compressor cycles. It's not as specific as that.

Then I measured the high and low side pressures and got 40-45 on the low side (spec is 25-36) and 250 on the high side (spec is 147-205). So now I'm jacked out of shape because the high is so high. The pressures were measured at idle speed.

Try increasing engine speed to about 1,800 RPM and re-check the pressures as described earlier.

Although the temp at the ducts inside was 48 while in my driveway, I discovered it was a reasonable 39-40 when underway. So now I'm thinking I don't have a problem at all but that 250psi bothers me. Should it?

Elevated pressures on the high side point to a condensing problem. It could be that when testing at idle speed, not enough air was passing through the condenser, raising the HS pressure. Try bumping it up to 1,800 RPM and re-test it. If that doesn't lower the HS pressure, mist the condenser with water, as suggested earlier. The pressure should drop noticeably.

One last check is to ensure the fan is running as it should. Does your truck have a viscous-drive fan or an electric fan(s)?

[DBBAir]

How many miles on the truck? 190,000
Has anyone ever "topped off" the refrigerant? Not in the last 9 years
If so, was any sealer added? Any history of leaks?
Avoid sealer at all costs.

Has the compressor or any other component ever been changed out on this 20 yr old truck? If so, why? No
Get pressure readings the way that John asked and mist the condenser with a water hose and let us know both sets of pressures Will do

[Tim]

Perfect pressures overall. 39 degrees while driving. Not sure why one would want to mess with it at all. As Cusser suggested, the issue could be freezing up. But it was stated the airflow never slowed down.

I'd leave it alone.

[DBBAir]

So, I did additional testing:

Air temp: 79 Humidity <20%
at idle: 36 on low side, 225 on high side, temps about 40 and greater
at 1800RPM: 30 on low side, 220 on high side, temp about 38 and greater
Misting at 1800 RPM: 30 on low side, 160 on high side, temps 38 and greater
There was considerable fluctuations in readings since now the clutch was on and off whereas with other tests, clutch remained on.

So what does the misting test mean where I get considerable lower pressures and clutch cycles? I don't really see any debris on the cooling stack

Thanks for all your help!