2008 Trailblazer 4.2L 4WD LT 158k Original A/C components
I noticed A/C acting intermittently at times. Gets much worse at higher ambient temps. Primarily cuts out on Expressway for 10 minutes or more and then seems to recover for another 5-10 minutes before repeating. When it runs, it is cooling decently. Figured evaporator was icing up or perhaps some type of heat soak issue with clutch or switches. Yesterday, I took these measurements in the driveway:
You can read pressure charts all day and get different normal pressures. The high side seems low at idle. The low side seems OK at idle but drops to the threshold for the switch at engine RPMs you’d expect on the highway… Thoughts?
First thing I would do is take some scan tool readings and see if the pressure sensors in the A/C system for both the PCM and HVAC system appear to be correct (match the actual values).
Then, with the system stabilized at 1500 RPM I would add refrigerant an ounce at a time to see if I could get the low side up a few psi without significantly raising the high side. I think the system is a bit undercharged.
You need to recover the old freon to know how much it has in it…
I would do a recover, pull a vacuum and charge with known, correct amount of freon and see how it does…
Almost looks like the desiccant bag maybe slowing the flow (moister in the system), causing a slight blockage… that could be the reason when high side goes to high and low side to low at 1500 rpms over the idle numbers…
EDIT: Live data probably a good idea too… lol (didn’t think about that) thanks asemaster
My pin-head diy’er thinking is to first make sure the condenser’s cooling fan is spinning when it should, and that the evaporator’s drain hole is open and draining freely.
First, let me thank you for taking the time to respond with your expertise and insights. Given the age of the system, I am concerned if it turns out to be a partially plugged orifice tube. That would require a lot more time and cost- perhaps too much cost to make it economically feasible for this older truck. Although the point of recovering and measuring the actual amount of refrigerant in the system now is probably the best course of action, I am going to try and see if I can get the low number up above the switch threshold at normal cruising RPMs. Of course, being careful to not overfill it. If that works, great. Otherwise, I will evacuate the system and pull the orifice tube to see if the compressor is failing internally and plugging up the orifice screens…
That’s the hard way to do it. I know some pro’s who would just hook up their gauges. They could tell from the pressures if it was low on refrigerant.
That is my fear.
I’ve been delaying the inevitable and limping along with this truck but if this needs a complete system and cleaning, time to go truck shopping…
If you are a shop with a machine, it is easy and you can do it while working on something else… But if not or DIY, I agree you can just use gauges…
But with the info shown on the killer spread sheet (props T.T.) the head pressure should be about 204psi, with the head pressure jumping to 245psi and low side down to 20 psi, that is not showing low, that is showing a restriction… now the idle pressures at 40 psi and 150 shows low and or a week compressor… imo anyway…
I also normally see AC off pressures about 110psi each… so the 90/90 looks low to me in my area…
Isn’t that just temp dependent? 79/79, if refrig levels are good, suggests it was somewhere around 75F ambient air temp to start. The 90/90 likely b/c everything was just heated up from running the truck. Or am I off base there?
Good point, I am used to high humidity and temps 85+ when doing AC work… Don’t do much when the temps are in the 70’sF… So if that is the case, then it should be (close to) full…
Again, that is why knowing how much is in it helps…
