I have a truck that is oscillating between 25lb and 99lb as the compressor cycles off & on, but it will not take the freon for some reason. I've left the freon gate open for 5 minutes at 1500 to 2000 RPM with no success. Any thoughts as to why it will not take the freon?

 

I charged my '09 (the blk one with an awesome tonneau cover) the other day from a totally empty system. The first can went fairly quickly, the second one took f-o-r-e-v-e-r. Certainly longer than any other system I have charged. It did the same psi oscillation you describe. I was just hoping it was normal for this system. Once I got it full, it works fine. Take care.

 

that's common and can be dangerous, the system is low on refrigerant and is cycling on the low pressure switch. If the pressure gets high enough (when the compressor turns off) in the refrigerant can it can burst. First do not leave the service manifold valve open when the compressor cycles off to prevent the can from over pressuring. Find the accumulator (near the fire wall) and pull the low pressure switch connector and add a jumper so that the compressor clutch stays engaged, this will prevent the pressure cycle and allow you to correctly charge the system.

Most likely it is taking the refrigerant while the compressor is running and then flowing back into the can when it turns off. Are you inverting the refrigerant can to charge with liquid?

Neal

 

Neal, many thanks for your input, and I'm glad you confirmed the danger. I was holding that can (inverted) and watched the gauge reach 99lbs, and thought, hmm, "this seems dangerous" but wasn't sure if the can was getting the full 99lbs or not. I'm assuming the low pressure switch is only a two lead plug that needs to be bridged? It seems strange to do this, is this what a garage would do as well?

 

Yes, it is the two lead plug located on the accumulator and yes the shop will also have to defeat the low pressure cut off switch to charge when using a 12 oz can. Once the system has enough refrigerant to keep the suction pressure above 18 to 20 psi the clutch will stop cycling. At those pressures the coil temps will be below 30* and the condensate will freeze on the coil causing reduced air flow.

Neal

 

Neal, many thanks! Your advice worked! I have it now taking freon with your workaround.

 

1.6lbs of r134a (freon) required, no more no less, over filling can put strain on the compressor. My local shop over-filled my system and I eneded-up having to service the whole A/C and replace the comp - its a lot of work!

Some PAG oil may be needed to compenste for any losses - exactly how much who knows, profesional systems capture the spent gasses + oil so the exact amount can be calculated.

You may have seen already, anyway its a good watch

 

I was just going to ask, how do you know when the system is full and can you overcharge it? I've "recharged" 3 or 4 different trucks and I usually just leave the can connected until the low-side gauge is between 40 and 55 lbs and I feel that it's not taking freon any more within a 3 to 4 minute timeframe. It seems to work for me, but I know it's probably not correct. How are others doing it?

 

Charging an automotive AC if done when completely empty and a vacuum has been pulled the best option is to use the exact amount on the label under the hood but I'll mention that rarely happens unless your a shop that does a lot of AC jobs.Usually the truck comes in with a no cooling issue and the system still has some refrigerant so then you have to resolve the leak and top off the system or repair what ever you find wrong. There a lot of things to consider when charging an AC, to much to go through while sitting behind a key board, the standard for AC world if you actually have been trained is while considering the ambiant conditions: engine speed between 1500 adn 2000 rpm's (close to cruise rpm), shop fan in front of condenser. AC controls set to MAX, Hi blower, open hood and front doors, open all AC vents. Set suction pressure based on ambient temps and refrigerant chart usually in the 28 to 30 psi on a CCOT system, head pressure is also critical to knowing the condition of the system and its cooling capacity so I would be looking for somewhere in the 120 to 300 psi heavily based on ambient conditions.

Keeping all this in mind the suction around 10" from the compressor or a little closer will tell the story, the compressor needs the cool refrigerant returning to help maintain it within a safe temperature operating range and to keep the refrigerant within a safe phase for the compressor, the refrigerant returning to the compressor should be cool to cold and more often then not the line will be sweating but not the compressor.

Taking all this into consideration if any of these parameters are out of limits then there is something else wrong,

Its something you learn over time while servicing automotive AC units.

Neal

 

As legerwn mentions, pulling a vacuum is the only real way to recharge, if the A/C cannot hold a vacuum for at least an hour, then it's pointless refilling.

I tend to feel over charging can put stress on the compressor, mine was shredded after my local shop to topped-up my A/C, it worked for a couple of months then siezed. It's not a nice job to have to do, so it's better to be careful with the re-charging!

As I couldn't find a Hummer HVAC specialist here in the UK, it was cheaper to buy all the test-kit (gauges, vacuum pump etc) and replacement parts and do it myself. After replacing the compressor, dryer, oriface valve, all O-rings and back-flusing, I followed ChrisFix's video and charged the system. After doing the work, at the lowest A/C setting the air-vent temperature was 32F(0C) with an outside ambient of 68F(20C)

Note its not actually "Freon" but R134a (1.6lbs)



Environmentally, the old refrigerant should be recovered, unless its been lost (as was my case)