2nd Gen Non-Engine/Transmission What cycles the A/C compressor?

[BDaugherty]

2001 Ram 2500. I'm just beginning to troubleshoot my evaporator and lines freezing up. First, the A/C works great. Then it blows frost from the vents. Next, airflow decreases and air temperature increases. Under hood, all the A/C lines from the orifice back to the compressor will be covered in ice.

My question: What cycles the A/C compressor on our trucks? Is there a thermostatic switch in the evaporator? Or does it just cycle off the low pressure switch? If it's the switch, is it the one mounted to the accumulator/dryer?

 

[Forrest Nearing]

could be overcharged, but you could have a high pressure or low pressure switch malfunctioning



undercharged can cause the same freezing. what do your pressures look like?

 

[BDaugherty]

The problem came about very suddenly. I took the family on a day trip in North GA. A/C worked great most of the day. On the ride home, it just stopped working. That was a couple weeks ago. I checked it out this morning and the pressures were low. I recovered the system, vacuumed, added 1/2 ounce dye, and recharged the specified 0. 85 kg R134a. A couple hours later, I drove across town. It worked like a champ for the first 10-15 minutes, then blew frost from the vents, then started freezing up. In the AM, I'll check the charge and if it's low, I'll look for leaks. I really just wanted to know what cycled the compressor. What I'm used to is a thermostatic switch in the evaporator that cycles the compressor by giving or taking away ground from the compressor clutch while power comes from the A/C switch through a trinary switch on the back of the compressor that will disengage the compressor clutch in case of either too much or too little pressure.

 

[Walking Catfish]

If your truck is like mine (mine is a 1999), there is a round canister mounted to the firewall on the passenger's side of the engine compartment. This tank is called the accumulator. On the top of this tank there is a switch that is screwed into a threaded fitting. There is a two-wire connector plugged into this switch. It is this switch that turns the compressor on or off when the A/C is running. It is normal for the compressor to cycle (at 3 to 5 minute intervals if memory serves me right) when everything is "right". This switch is probably GOOD, however, since the compressor is cycling so do not remove it! Doing so will release ALL of your refrigerant!



From the symptoms you describe, I think your problem is that your system is low on refrigerant. This could be caused by a leak somewhere in the system. If it is a very small leak, you can probably "fix" your problem temporarily (for anywhere from 1 month to a year or so) by adding R134a refrigerant to your system.



You can get small kits to do this at Wal Mart or your local auto parts store. They usually include 1 can of R134a and a "cheap" gauge set with instructions for $25 or so. This kit will be sufficient for your needs. 1 or 2 of these small cans of R134a is all you will need (as I recall the full charge, from empty, for my 1999 truck is 12oz).



Follow the instuctions in the kit for hookup and charging and you should be OK.



I find the following suggestions helpful:



1. Add refrigerant SLOWLY. you don't want to put too much in or you will be overcharged and things will still not work right.



2. LISTEN to the compressor. As you add the refrigerant, the compressor will start cycling more slowly. This is a valuable indicator as you want it to cycle at 3 to 5 minute intervals. This system is designed to cycle the compressor and, if memory serves me right, 3-5 minute intervals is normal.



Once you have recharged the system, your A/C should work good again... for a while. As I said, you probably have a leak somewhere. The size of the leak will determine how long the "fix" lasts. This can be anywhere from 1 week (for a large leak) to a year or two (for a very small leak.



If you have a large leak -seek professional help.



If your fix lasts for a year or more, it is pretty small. In this case, the next time you need to recharge it, get the R134a cans with the "leak sealer" additive (which will NOT fix a large leak). There is no guarantee that they will seal up your leak, but if they do it will save you the cost of professional help.



I am a refrigeration tech and I used these techniques to fix the small leak on my 1999 RAM 3500. Mine would leak down about once per year. I experienced this for about 3 years. The first time I took it to a Dodge dealer, who said they couldnt find any leak, recharged the system and added some leak detection dye. I had to recharge it again a year later -and again the year after that. The third time I recharged the system using the leak sealer stuff and my A/C has worked flawlessly since July 2005.



Hope this helps and Good Luck!

 

[Walking Catfish]

Sorry... I just saw your post #3... sounds like you have a HUGE leak.



Good Luck

 

[BDaugherty]

My feeling is that I have a leak either at the compressor shaft seal or the condenser. It seems to leak down to a certain point pretty fast, but then hold steady. I added the dye today and ran it, so tomorrow I can look for traces of the dye leaking from somewhere. I have a couple 30lb cans of refrigerant and a White Industries A/C machine so gauges are not a problem. For me, it's more a matter of knowing how this particular system operates than it is having the tooling to work on it.

 

[Walking Catfish]

Sorry... when I was writing my first reply the only post I saw was your initiall question.



Upon reading your second post I realized that you are refrigeration savvy...



Anyway, unless they redesigned the A/C between 1999 and 2001, the switch is mounted on top of the accumulator tank which is located in the engine compartment on the passengers side of the firewall near the cowl.



The expansion of the refrigerant takes place in this cannister just before it goes to the evaporator. This system is designed for evaporation temperatures of about 40 degrees F so this switch cycles the compressor on and off in order to prevent the whole thing from freezing up.



In my truck, Chrysler used "Quick Disconnect" fittings at most connections. I have heard that these can leak pretty badly at times also.

 

[BDaugherty]

Thanks for you replies. I'll post what I find wrong with it.

 

[ed9824v]

Not sure about the dodge diesel but a lot of autos have a freeze limit switch that will cycle off the compressor when it senses a freeze up the closes and when ice is gone and lets the compressor recycle as normal need to see if they have the freeze cycling switch Ed

 

[Bob V]

Frost from the vents might be from the HVAC plenum drain being pluged up.

 

[DMcLeod]

Interesting thread. I think I have a slow leak as rbR described. I added R134a and the pressures were about right. After about 6mo the compressor begins to cycle way too often again and AC is not as cool as it should be. What baffles me is why the compressor cycles in the vent mode and heat mode as well. So this compressor control switch is independent of the mode (heat/AC/vent) switch? Why would the compressor begin to cylcle when I turn the heat on? Maybe this occurance could help somebody answer BD's original question.

 

[fest3er]

2001 Ram 2500. I'm just beginning to troubleshoot my evaporator and lines freezing up. First, the A/C works great. Then it blows frost from the vents. Next, airflow decreases and air temperature increases. Under hood, all the A/C lines from the orifice back to the compressor will be covered in ice.
My question: What cycles the A/C compressor on our trucks? Is there a thermostatic switch in the evaporator? Or does it just cycle off the low pressure switch? If it's the switch, is it the one mounted to the accumulator/dryer?

As far as I know, ('98 12V), the control circuit consists of:

• wire
• fuse in PDC
• your key
• ventilation control switch in dash
• PCM
• low pressure cuttoff switch
• high-pressure cutoff switch
• PCM switch

The compressor clutch is powered from an key-on-powered fuse in the PDC, through the dash-mounted control, the PCM, the low pressure switch, the high pressure switch, then the clutch relay, and finally ground.

If the low-pressure side pressure falls too low (under about 23 PSI), the low pressure switch opens and stays open until the pressure reaches about 45 PSI; this is the most common 'cause' of compressor cycling. If the high-pressure side pressure gets too high, the high-pressure switch opens. If the go-pedal is put to the wood, the PCM switch opens. If the HVAC control is set to a non-A/C position, its switch opens. If the key is not in the ON position, the circuit is not powered at all. Any open switch in the circuit causes the A/C clutch to disengage.

That's it. That's the entire circuit. It's really a simple system.

 

[AllianceDiesel]

if you need some help, I can get you all you need, I am ase/dodge certified in a/c

 

[Missouri Mule]

Your evaporator could be leaking. Try to get hold of a sniffer.

 

[DodgeWarlock]

Had the same problem on our 01. Turned out to be the pressure cycling switch on top of the accumulator. Unforunately it's about $40-50. the Good news it there is a schrader valve underneath, so it can be changed by hand without breaking the system seal and losing refrigerant. Simply unplug and unscrew it. Then install the new switch. Three shops failed to find this and after reading the factory service manual I fixed it in 5 mins. Worked trouble free ever since.

 

[BDaugherty]

So here's the update. I took the low pressure switch off the accumulator. With the switch removed, it was open. Once I installed in back onto the accumulator, it closed. I ran the truck with gauges hooked up. It seems to have held the charge from last week. The pressures were about normal. I could not find any flourescent dye and my sniffer didn't pick up anything. However, when I first started it, the compressor cycled a few times over about a 5 minute time period. After that, it stayed engaged and the system began to freeze up. I also checked the condensate drain. I blew air back through it and could see condensate coming out before freezeup and during the thaw out.

What I want to be sure of is that it's the switch on the accumulator that cycles the compressor and not a temperature switch in the evaporator. Also, could said pressure switch be sticking? It opened when I removed it and closed when I reinstalled it... and it just now occurred to me that I could have removed it and tested it right away during the freeze up period... . but that might have been enough to open it. I guess if the concensus is sure that that switch cycles the compressor, I'll replace it. I guess I can always return it if it doesn't change anything. I just can't figure out why the compressor would cycle at first, but stay engaged after a few cycles.

 

[fest3er]

When the system is quiescent, the pressure throughout the system is the same, especially in the evaporator. It's possible that it could be sucking a vacuum on the low side real quickly until enough liquid builds up before the evap to ensure a steady supply to the compressor.

And yes, the pressure switch can stick. With it off, you should hear it rattle when you shake it. If not, you can try cleaning it out with carb cleaner or ether (something that evaporates without a trace).

If the system is cold enough to blow chunks (of ice, I mean), don't touch it! There are plenty of us who wish ours would run cold enough to be effective on hot, humid summer days.

 

[BDaugherty]

I ordered a new pressure switch from RockAuto.com. It should be here by the end of the week... I hope. I'll post up the results. Right now, blowing cold is great, but when it freezes into a solid block of ice, it's useless.

 

[BDaugherty]

I got my new low pressure switch from Rock Auto today. Even after the shipping and handling, it was less than half of what parts stores wanted... if they could get it. I installed it, but didn't have time to test it or the old one. However, the old one did not "click" when it was removed and really doesn't rattle like the new one, so I have a feeling that my A/C will work well. I'll try it out this weekend and post the results.

 

[BDaugherty]

Update 10/19

I drove the truck across town today for the first time since replacing the low pressure switch. A/C seems to be fixed. The old switch tested good, but I think the spring was getting weak. It would open when removed from the truck, but if it had a few psi on it, it would stay closed. That's my theory, anyway. Thanks for all the help I received on this site.