Automotive Air Conditioning Information Forum

Vehicle: 1997 Chevrolet K1500 Silverado 305V8 R134

A shop recently recharged the A/C in my pickup. It was not empty but was sufficiently low on R134 the compressor wouldn't kick on.

I noticed the tech didn't put a floor fan in front of the condensor/grille while doing the work.

I assume this is gonna raise (at least) the high side pressure,

The tech was charging by pressure, not weight. I don't remember the high # but he filled it to where the low side was showing 36 psi and blowing cold air in the cabin.

What will the end result of not utilizing a floor fan blowing be? The system slightly undercharged because at the higher temps it indicates greater pressures?

Yes, I know what he SHOULD have done, evacuate, recover, vacuum, charge by weight, etc. I'm studying to get a 609 so I can do my OWN work, properly.

Go back and have the tech provide low and high side readings. We will be able to tell if it is within range.

wadenelson wrote: ↑Fri Aug 02, 2024 5:15 pm The tech was charging by pressure, not weight. I don't remember the high # but he filled it to where the low side was showing 36 psi and blowing cold air in the cabin.

You already know that charging must be the other way around—that is, charge by weight, not by pressure. And take the advice above as well. On a whim, maybe the tech hit the charge amount just right. Then again, maybe not.

wadenelson wrote: ↑Fri Aug 02, 2024 5:15 pm What will the end result of not utilizing a floor fan blowing be? The system slightly undercharged because at the higher temps it indicates greater pressures?

The only reasons one would have a fan blowing in front of the grille on a hot day during charging (airflow toward the condenser and radiator) are to keep the high pressure from rising too high and disengaging the compressor; and/or to help prevent the vehicle from overheating. I don't think it will make any difference now.

The specifications that I have for your truck vary with the model and compressor:
—Std Cab/Ext Cab with Denso compressor: 32 ounces net weight of R-134a, and 8.0 fluid ounces of PAG-46.
—Crew Cab with Denso compressor: 36 ounces net weight of R-134a, and 8.0 fluid ounces of PAG-46.
—Std Cab/Ext Cab with OE Harrison compressor: 32 ounces net weight of R-134a, and 8.0 fluid ounces of PAG-150.
—Crew Cab with OE Harrison compressor: 36 ounces net weight of R-134a, and 8.0 fluid ounces of PAG-150.

wadenelson wrote: ↑Fri Aug 02, 2024 5:15 pm Yes, I know what he SHOULD have done, evacuate, recover, vacuum, charge by weight, etc. I'm studying to get a 609 so I can do my OWN work, properly.

Congratulations in advance on your EPA Section 609 Certification

Unless they charged the policy, it's an open-book test.

Tim, it's been a while, but I believe it still is open book.

Tim wrote: ↑Fri Aug 02, 2024 6:04 pm Go back and have the tech provide low and high side readings. We will be able to tell if it is within range.

It was 85F, low humidity, and seems like the high side was either 250 or 275, i don't recall.

[/quote]The only reasons one would have a fan blowing in front of the grille on a hot day during charging (airflow toward the condenser and radiator) are to keep the high pressure from rising too high and disengaging the compressor; and/or to help prevent the vehicle from overheating. I don't think it will make any difference now.

The specifications that I have for your truck vary with the model and compressor:
—Std Cab/Ext Cab with Denso compressor: 32 ounces net weight of R-134a, and 8.0 fluid ounces of PAG-46.
—Crew Cab with Denso compressor: 36 ounces net weight of R-134a, and 8.0 fluid ounces of PAG-46.
—Std Cab/Ext Cab with OE Harrison compressor: 32 ounces net weight of R-134a, and 8.0 fluid ounces of PAG-150.
—Crew Cab with OE Harrison compressor: 36 ounces net weight of R-134a, and 8.0 fluid ounces of PAG-150.

Thanks for the specs.

wadenelson wrote: ↑Fri Aug 02, 2024 7:14 pm It was 85F, low humidity, and seems like the high side was either 250 or 275, i don't recall.

If you DO recall correctly, those pressures at that ambient temperature suggest an overcharge.

JohnHere wrote: ↑Fri Aug 02, 2024 7:26 pm

wadenelson wrote: ↑Fri Aug 02, 2024 7:14 pm It was 85F, low humidity, and seems like the high side was either 250 or 275, i don't recall.

If you DO recall correctly, those pressures at that ambient temperature suggest an overcharge.

Thank you.

My goal here is to improve my understanding more than to get answers about my truck. It's just...wait for it... a vehicle for learning! /s

Now, on to overcharge and undercharge and why they're both bad. Help correct my misunderstandings!

Obviously with undercharge, the system can't produce as many BTU's of cooling. Compressor isn't getting a nice Big Gulp of gas to compress. Too low and the compressor won't start. (Low limit switch)

Overcharge, the pressure rockets way up, and since it's harder for refrigerant under higher pressure to boil, change to a gas, same thing happens. The evaporator can't produce as many BTU's. AND you get greater wear and tear on the compressor, etc. Too high --- limit switch turns off the compressor. What else?

Obviously there's a sweet spot, in terms of charge, which everyone? almost everyone? today believes is easier to hit by charging by mfr suggested weight than by watching gauges.

How wide is that sweet spot, in your mental model? A couple of ounces? How often do you charge by weight and then decide using the gauges its still a little low or high? Ever? Never?

Same question but for refrigerant oil. How many of you guys are absolutely anal about measuring recovered oil and replacing the same amount.

TBH the guys I've hired over the years to do AC work for me - many seemed to be cutting corners, NOT doing things by the book, trying to "get done" (or perhaps limited by client budget) rather than trying to obtain the best possible AC performance.

Which is why I'm trying to get educated so I can do it right, at least on my own vehicles.

So THANK YOU for taking the time to help me learn, and I'm sorry if I come off as a jerk.

wadenelson wrote: ↑Fri Aug 02, 2024 5:15 pm The tech was charging by pressure, not weight.

That does surprise me, unsure why a shop would not have modern equipment.

wadenelson wrote: ↑Fri Aug 02, 2024 5:15 pm blowing cold air in the cabin

If the AC cools you and your passenger: fine. Cross your fingers that your truck only had a tiny leak due to attrition over the years, or you'll soon have more refrigeration issue of poor cooling.