AC diagnostic on a neo-classic Camaro

[Atlas]

tldr; Shade tree mechanic stumped by variable displacement AC compressor.

 

Not Silverado related, but for my 2000 Camaro SS. I've never really used the AC since acquiring the car a good handful of years ago. I plan to take it on a trip this summer so I wanted to make sure the AC was in good shape. On an 73 degree day in the shop with the car idling I set the AC on "Max", temp set to cold, and fan on hi. The compressor turned on and air turned nice and cold.

 

• I let it run for several minutes but noticed the compressor never cycled off.
• The engine was up to temperature enough that it began cycling the cooling fans on high.
• I had a set of manifold gauges hooked up and it was consistently at about 25 psi on the low side and between 200 and 225 psi (warm engine) on the high side. Per temperature charts, the low side is low (should be 30-35) and the high side is a little high (up to 170 per chart).
• I read the low pressure as potentially being under-charged underscored by a continually running compressor
• This was also underscored by temp readings of mid-20 degrees at the vents. Not just cold, freezing cold.
• Switching off the AC but leaving the HVAC fan on high produced a deluge of condensation underneath the vehicle. I was getting ice buildup on the evap core most likely.

 

The low reading (25psi) concerned me that the compressor wasn't switching off so I swapped out the pressure switch. No change in behavior, still ran constantly.

 

The AC clutch works fine as it engages/disengages with the HVAC switch on command. The compressor relay is good as I swapped it with two different known good relays just to be sure. Having eliminated that, and the pressure switch, I added refrigerant, thinking the constant run and low "low" pressure were signs of a slight undercharge. Makes sense, the car is 26 years old and it doesn't appear the AC system has ever been touched.

 

Adding some R134a didn't meaningfully change the low side pressure. And that's when a lightbulb flashed upstairs. While I consider the car "old", it's possibly "new" enough to have a variable displacement compressor. Did some reading and sure enough. Dangit. I don't work on these for this reason.

 

Adding refrigerant means the compressor will just compensate and won't really change pressures until it's severely overcharged or undercharged. But at least I wasn't getting ice/frost anymore, but instead high 30 degree temps out of the vents. That's more normal, but with variable displacement now I have no idea where my charge level is at. It's probably overcharged now. The high side even with the engine radiating serious heat was never really over about 225.

 

The static pressure at room temperature is dead on, before and after the charge. Both high/low equalize after some rest.

 

I'm thinking I'll need to take it to a shop. I want the proper charge level so I'm not working the compressor too hard. The only way to get an accurate charge is to evacuate and then re-charge with the exact amount specified for the system -- at least that's what I'm reading.

 

Anyone here with modern automotive AC knowledge?

 

Edited Sunday at 02:16 PM by Atlas

[carkhz316]

Were you repairing a previously working AC system? When filling with refrigerant, you need a way to weigh it properly or calculate the amount needed to add to an empty system. Pressures are only relative to temperature and a tool to use for diagnosing. Static pressure means almost nothing in regards to refrigerant charge and will also change with temperature. That said, they're not THAT fickle about exact fill amounts. You have to be considerably over or undercharged for it to impact A/C operation so dramatically.

[Atlas]

4 hours ago, carkhz316 said:

Were you repairing a previously working AC system? When filling with refrigerant, you need a way to weigh it properly or calculate the amount needed to add to an empty system. Pressures are only relative to temperature and a tool to use for diagnosing. Static pressure means almost nothing in regards to refrigerant charge and will also change with temperature. That said, they're not THAT fickle about exact fill amounts. You have to be considerably over or undercharged for it to impact A/C operation so dramatically.

 

It was previously working, but the continually running compressor threw me for a loop. It wouldn't be out of the realm of possibility that a 26 year old AC system had lost some of its charge and could be exhibiting low charge indicators, so I thought I had it licked. Add some refrigerant to get the low and high pressures in range like on the old 134 systems.

 

I learned the continuously running compressor is normal, and that I need to know how much is in the system, as well as needing to meter what goes in. I'm only used to working on older R134a and R12 systems that aren't variable displacement. This is how we learn...LOL.

 

Long story short, I had a 20% off coupon for a new Jiffy Lube that opened nearby. I took it up there yesterday and they did an evac/recharge using their brand new machine from Mahle. Had a good conversation with the guy, they loved the car and I was out of there for $159 plus tax. Digitally measured charge, exactly 1.5lb of refrigerant went in.

[richard wysong]

How much did they evacuate?

[Atlas]

1 hour ago, richard wysong said:

How much did they evacuate?

 

That would be the piece of data that would tie all of this together, wouldn't it?!

 

They didn't tell me and I completely forgot to ask--I would guess it's measured somehow because they have to estimate recovery tank capacity/fill somehow. But at least I'm starting from a known baseline now even if I don't have that figure..

[richard wysong]

On the machines I have used it is displayed during and after evacuation