A/C issues

[cfitz]

Before I tear into the dashboard, I'm looking for some advice. On my '04 GMC 2500hd I just replaced the ac compressor and accumulator (ac clutch was wasted) and the ac wasn't working when I bought the truck, so Im not sure if this issue was there or not. While parked with ac on the idle will drop suddenly, blower motor speed slows and warm air comes out. Within 15 seconds or so, the process starts again and cold air comes out and you can hear the compressor cycle. System is fully charged with no leaks. On occasion it'll do this going down the road, but 90% of the time the air is ice cold. I'm thinking a blend door problem but I'm not 100%, as some of the conditions throw me off, and I have the factory fan clutch as of now......Thanks in advance!

[Jsdirt]

The blower motor slowing tells me you could have a short somewhere between the low pressure cutoff switch on the accumulator, to the clutch coil on the compressor. The intermittence of it, and your idle dropping has me thinking otherwise though ...

 

You said you can hear the compressor cycle, so the switch is working (if not it would either eventually starve the compressor of oil and ruin it, or, the compressor clutch wouldn't engage at all). Idle dropping suggests a big load on the engine. Did you add the correct and required amount of oil to the new compressor? Did you flush it with oil before installing? Did it come with oil, or was it dry? How much oil was left in the system, and if you don't know, how much did you add? Did you add some to the accumulator?

 

Usually if the vents blow warm while accelerating, you have a vacuum leak that's letting the blend door move when it shouldn't. When the vents blow warm, how do the inlet and outlet of the evaporator feel temperature wise? You'd have to pull over to check that, obviously.

[Doug_Scott]

With the drop in engine speed it also slows down the compressor, and reduces the charge going into battery. It also slows down the mechanical engine fan(mechanical as in belt driven). You need to find out why the PCM is not picking the idle up. Could be something in the IAC stepper motor, or even just the ports in the IAC full of crap and needs to be cleaned out with the proper solvent spray.

 

The only was it could be the blend door is if you have auto air. Otherwise the door stays put until you make changes on the controls. That is just the actual door I am talking about. You could have a vacuum leak in one of the controls that operates the blend door.

 

If it was mine, I would look into why the idle drops first. Note the idle dropping should not impact the engine when on the highway. But a vacuum leak in one of the vacuum circuits under the dash could work the blend door wrong.

[cfitz]

I did not flush the system prior to this, the clutch was seized on the compressor and it was cheaper to replace the whole thing. I drained the oil out of the old compressor and tried to pour the same amount back into the new one, using new pag oil. The compressor I installed did have a second switch on the back of the housing, with no pigtail on the truck side, but according to NAPA that was the only one they listed, and it was a denso new, not reman'd. I'm also wondering if there is an issue with my tune not bumping the throttle when the clutch is engaged, and I have a dbw throttle body. This issue is only present while parked and engine idling.. I'm going to try and get some diagnostics done, it was probably something I did or didn't do while installing it. Thanks for the input, I'll report back with results.

[Doug_Scott]

Take a good look at the throttle body. If it is the least bit dirty with black crap and you can see it by just looking in the air inlet of the throttle body you need to clean it. Take the IAC stepper motor off and clean the ports inside the IAC port, and clean the IAC motor as well. Try not to drown the stepper motor with solvent, spray it with the open end pointing down.

 

The computer does not raise the idle speed just because the a/c engaged. It tries constantly to keep the idle speed at a certain minimum value. When the compressor clutch engages, it slowly builds resistance to turn the compressor, which will start to bring idle speed down, at which point the computer will try to raise the idle speed back to where it is supposed to be. If the IAC is blocked or partially blocked, it may not be able let enough air in to raise the idle speed. This is not like the old days with a solenoid on the side of the carb that would be energized to raise idle speed when a/c came on. The computer does not even open the throttle body to raise idle speed. It introduces a controlled vacuum leak, and injects more fuel to maintain the o2 level in exhaust.

 

The spare switch on the compressor is likely for a different year or even model of the same vehicle. It is cheaper to just put a switch in and not use it, than it is to have to carry two complete compressors to cover the same vehicles. Happens quite often in replacement parts. Would be nice though if they would just drop a note in the box saying that.

[cfitz]

I haven't had a chance to look further into this, but the throttle body is brand new and the intake manifold was just off for knock sensors. The issue was there prior to me removing the intake. I'm going to look at the tune again, there may be an issue there.