Ok, you oracles or all weirdness that happens in a Dodge truck, I am stumped. Last week I fixed my leaking A/C system - I replaced a leaking Schrader valve, evacuated the old coolant right down to 29 inches of mercury of vacuum and then refilled with 2 cans of spiffy new refrigerant. Let me tell you, I expect to see snowflakes in the cab of the truck now, the air coming out of the vents is super chilly. Now for my problem. I can't get the A/C to turn off. The compressor runs (it cycles properly) in every setting of the HVAC system - (defrost - yeah I know it is supposed to there), floor, dash, combined, A/C and Max A/C).

I don't think this is related to my repairs of last week. I am pretty sure I heard the compressor clicking on and off before I fixed everything up but the refrigerant was so low that the A/C wasn't actually doing any chilling.

Any suggestions about a condition that would make the compressor always think that I want A/C?

 

Probably the hi/lo switch IIRC....

 

The A/C compressor will be enabled in all but two settings on the position ****. It will be on with the first two positions from off with the snowflake, split level, and then the defroster/floor and defrost settings. The compressor is disabled on the dash vent only setting and floor vent only setting.

A low charge can cause the compressor to run continuously. How much refrigerant did each can hold? And how much does the little blue sticker call for under the hood?

Also what are your high and low side readings while it's running, and what is the ambient temperature?

 

When you evacuated the system with you spiffy new pump did you let it sit with the pump off and gauges closed to see if it would hold a vacuum? This is a true test of if the system still has a leak. I suspect it does as the compressor running continuously usually means it's low on freon.

 

Those two above know more about this than I do.

Try what they suggest.

 

That is a very important step, is making sure it holds vacuum.

Another good thing to do is if the system is a few years old, is to replace the reciever/dryer (that can near the firewall on the passenger side) if the system gets opened up. A good thing to do during a major component replacement (like a compressor) This puts in fresh dessicant which pulls any moisture out of the sytem as the refrigerant passes through. Moisture + refrigerant = something acidic which is bad for the entire system.

I do not think your low pressure switch is bad. If it was, your compressor would not even engage. You have at least enough in there to not trip this switch. Even if the switch failed on, the compressor would cycle normal based on the thermostat setting, and give you no protection if you did have a low charge. Again, I do not think your switch is bad

I suspect you either have another leak, or you didn't put in enough refrigerant. If I am not mistaken, these trucks hold 1 lb 14oz in the system. If you only put in two 12 oz cans, then you are short by 2 oz, and that is if the entire contents of the can went in. Check the little blue sticker under your hood (near the belt routing sticker). It will tell you exactly how much a full charge of R-134A refrigerant is (by weight) for your vehicle.

EDIT: Corrected R134a capacity based on R134a sticker under the hood (1.875 lbs)

 

I re-read your post and didn't get what you said the first time around... the compressor will run in all positions except OFF and VENT, all other positions the compressor should run.

 

Correct, the first three positions (Max A/C, fresh air A/C, and split level), and the last two (floor/defrost and defrost only) use the compressor. The two spots in between those two groups (dash vent only and floor vent only) are fresh air only, no compressor.

I don't think I worded that too well the first time around

 

Thanks for the suggestions guys, I will check things when I get home. As for leaks, it held a vacuum for over an hour with no change. Also, I probably should have worded my condition a little better. The compressor is cycling like it should, just as HVAC settings I wouldn't expect. As for amount of coolant, the FSM and the strut under the hood gave an amount of refrigerant (32oz I believe) and two cans of the non-freon R-134 refrigerant say 1 can is equivalent to 16oz of original coolant, so I put 2 cans and watched the low side pressures. The pressure is right in the middle of the 'properly charged zone' of the pressure gauge that came with the refrigerant.

I knew the compressor would run on the two defrost settings and the two A/C setting just didn't expect it on any of the others. I will give things a look over the weekend.

It is cooling amazingly, better than it has since the truck was new, just cooling at times I wouldn't expect.

 

Getting the proper amount in there is crucial to any refrigeration system. What exactly did you put in there? Can you provide a link?

Those quick fix cans aren't the best thing to use for a recharge for a number of reasons: The gauge on them is of questionable accuracy. Sometimes they have stop leak included with the refrigerant (not good long term). If there is oil mixed in with it, it's easy to overcharge the system on oil (more on that in a minute).

Undercharged systems won't cool very well at all since the liquid refrigerant changes to gas too early in the evaporator. Slightly undercharged actually cools well but you run a risk of freezing (icing) the evaporator coils and reducing or blocking the airflow to your vents. Overcharged is bad since the refrigerant stays in its liquid phase too long in the evaporator and won't pick up heat as well. Plus there is a bigger risk of hydrolocking and blowing out seals in the compressor if liquid gets into it. This is why getting the right charge in the system is very important. I'm really interested to see what went your system.

For the oil, PAG oil is what is specified, but you can't use just any PAG oil. The are several grades of it, just like motor oil. There is PAG 46 (thin), PAG 100 (medium), and PAG 150 (thick). I believe our Rams use PAG 100. You'll have to check your cans and see if there was oil mixed in, and if so, what type. With that said, there is also a specified amount of oil that goes in the system. Too little and the compressor will get oil starved. Too much runs the risk of hydrolocking the compressor. The oil migrates around the system as the compressor runs. Any time a major component gets replaced, the oil in it gets poured out and measured, and the same amount of new oil goes into the new part.

You don't happen to have a set of manifold gauges do you?

 

Good info above.

I want to add check the AC relay in the underhood box. I've had these fail three times. Twice this year. Both times the truck sat over night and had dead batteries the next day. I replaced batteries the first time they were 4 years old and probably would not make another winter. When we connected the jump start unit the AC clutch clicked. A quick check showed the power was still on even with the AC control off and in vent positions. So the relay was stuck on. It was very hot too.

Relays are 18 bucks+ at the ******* or about $4 on line.

I've already used all the spares so I ordered 6 new ones.

Now I don't trust these so I'm going to install a battery cut off switch. I can't be having dead batteries. Jump start get expensive after you use up AAA.