Hey all,

I noticed lately that either my ac is not as cold as it used to be, or I just hadn't ever used it when it was really hot (I normally use windows when my wife isn't with). Its still cool, just not ice cold. I have also noticed in the past that at idle my ac compressor will lock and unlock regularly. I was reading about ac issues now to try and see whats up and discovered that the locking and unlocking probably means my system is low on pressure. Now I'm looking for advice on how to deal with this. Should I try a do-it-yourself can from autozone? Ive been told in the past they are dangerous and not the correct way to service the system, but I've been reading and a lot of guys say they use them with great success. Is it worth a try or not?

I'll gladly try that, but doesn't the low side pressure need to be high enough to keep the compressor from cycling more then time per minute in all positions? 

And what about the low side like getting a little icy....

leather,

 

I hope this helps a bit in your visualization.   if we hold a bunch of other variables constant, refrigerants have a very defined table of temperature of the refrigerant to the pressure of the refrigerant.   The chart IBM posted is good, but it is ESTIMATING an air temperature given an ambient temperature and pressure ranges. 

 

Here is a chart of refrigerant temperature at a refrigerant pressure.  (it is useless EXCEPT it may help you see why a high "low side pressure" is not what you want.  It will be what you get, but not what you want.)

 

Notice that at a refrigerant pressure of 26.1psi  the temperature in F is 30.  That is below freezing.   So in our cars we try to stay above the point of freezing on the evaporator coil.    This is why the "low side"  switch can bounce between operation and not.  

 

IF you have a 2001 FSM  look at pages 24-20 and 24-21  that explain the operation of the low and high side switches. 

 

To have low temperatures in the cab you want low pressures on the suction side (low side) of the system...

 

You may have let the refrigerant out too quickly, and that is why you missed your "perfect" point. 

 

I can't find it in the FSM,  but GM vehicles during this type of testing (oh btw, FSM says 1,000rpm not 1500 on our trucks) The compressor is allowed to cycle  once every 2 minutes or so.  (in the CCOT system (Constant Cycling orifice tube) pressure control usually comes from the low side switch.) 

 

I hope this helps your endeavor.   But from where you are at, I would get the low side to keep the compressor locked for more than a couple minutes and call it good.  (as the interior temperature goes down, your ambient is going down so harder to keep the low side from going too low.)  Add the freon slowly.   stop occasionally and let it balance out.    and like IBM says, don't use MAX.   It should be more stable when the "ambient" temperatures are high.

Thanks @Haggar. I think I'm starting to understand the principles of this fairly well. My only remaining questions at this point are, 1) are you saying I should only go up to 1000 rpms for testing? By this I mean that is the highest point at which the compressor needs to stay locked? Or should it stay locked at all rpms up to 2000? My reason for that is, if I'm cruising down I81 my rpms are at about 2000. I wouldn't think the compressor should be cycling more than once a minute or so at that point, if that much. And 2) is that why I was seeing some ice on the low side then? Because pressure was too low and so the freon was cold enough to freeze the condensation? 3) why is it that I shouldn't use max? I'm still a little confused on that one....

Edited August 11, 20168 yr by leathermaneod

1)  It is a testing rpm from the factory manual.  They didn't pick it from science completely but a way for the technicians to get similar results everytime they tested.  (remember that was a guideline for hundreds of technicians doing it all over the country.)  Going down the highway at 2000 rpms, it probably will lock and unlock occasionally.  Some cars you can feel this (my honda is a prime example.... no guts and less guts when compressor running.)  (our trucks we notice the alternator more than the compressor, due to the load of the grid heaters lol.)   once settled down, I would not expect to have it happen more than once every few minutes.  If it begins happening more than that, I take it off of MAX for a few minutes.   It happens to me more than most people as I try and run a bit lower suction pressure (running the risk of freezeup) for a potentially lower duct temperature.  but sometimes I don't notice or forget, and freeze the evaporator up... (have not on the dodge yet, but one of my suburbasaurus is wicked for that....) 

 

2)  YES.  when the low side pressure is that low, its physical temperature is nearly the theoretical temperature.  Any pressure below 28 is basically guaranteed to create ice.  (this is why the low limit of the low side pressure switch drops the compressor out at 23  and doesn't let it re-start until 38 or so.)

 

3) The MAX setting changes nothing about the actual air conditioning.  It re-routes the duct work inside the cab to "recirculate" the air we have cooled. (it brings in NO outside air)    While we are testing, we want the evaporator to do WORK.  There is very little work involved cooling already cool air.... so the AC system doesn't really know what to do, you may not get all the vapor/liquid refrigerant to turn to a vapor... you don't get the proper phase change and heat transfer going on. etc... in other words it will make your pressures act a bit funny.   So I think for where you are at, you will get better pressure results using more outside air.  (Max setting in daily use is a must for you if you live in a humid area...  you will NEVER be happy if the AC has to constantly remove the water from outside air..... there is too much water stealing your potentially cool air.)

 

HTH

 

Hag

 

 

Thanks again! You and everyone else have been very helpful! I picked up another can of freon to hopefully wrap this project up. I got a cheapo autozone one this time to use with the manifold gauge I have rented from them. Definitely should have gone that route the whole time instead of using AC pro. Would have spent a lot less money. 

Wally world had the cheapest down here at $4.88 a 12oz can. Made my job much more cost effective since some venting occurred.

Just now, dripley said:

Wally world had the cheapest down here at $4.88 a 12oz can. Made my job much more cost effective since some venting occurred.

One more learning point lol. Rent gauges from autozone, buy freon from Walmart.

I saved enough money on the freon for my truck and the wifes car to almost pay for the gauges. Saved enough on labor to probably pay for the vacuum pump. Now as long as I did everything right and the ac does not explode I will just about break even. Thats a win in my book.

How much were you quoted to get your AC fixed?

I never went to anyone for a price. I pretty much made my mind up a while back to do it myself. Thats why I bought the vac pump and gauges. I had to replace the high pressure line and the drier. I replaced the evaporator a while back. When i had the ac serviced a good while back they knew there was a small leak somewhere and best they could figure it was in the evaporator. They told me that would cost about $900 due to the dash having to come out. That was out of my budget and I replaced it myself along with the dash pad and heater core. The evap was the leak and I had the fluorescent green oil in the hvac box.

This is what I have in the repair

 

                   parts

$70   high pressure line, drier, gasket kit

$30   freon

$50   evaporator

I did not use Mopar replacements. 4 Seasons at Rock Auto. Hope they last.

 

                   tools

$50    gauges

$150  vacuum pump

$10    misc

 

total of $360

 

labor was quite a few hours of my time. A lot of that was learning since I had never messed with ac before except squirting in a little freon here and there. The last time anyone serviced my ac it cost me about $180 for the freon charge and leak hunting. Still think I came out good on the deal and now I have the tools to do it again. Pretty sure the wifes car has a leak and it will have to be done next year.

got another question for you guys. I was just reading over on cummins forum and someone mentioned that a weak fan clutch can cause the AC compressor to cycle on and off excessively. Is this true? If so how does it work?

When the fan clutch looses it's silicon fluid it will not lock up and spin fast enough to draw cooling air over the AC condenser and radiator.  When this happens the high side pressure can reach a point that the high pressure switch, located on the discharge hose between the compressor and condenser, will open circuit and unground the AC switch sense at terminal 22 of the Powertrain Control Module. This will cause the compressor clutch to disengage. The high pressure switch opens at 450-490 psi and resets at 270-330 psi.   

Edited August 17, 20168 yr by IBMobile

oh ok. I just wanted to make sure that couldn't have been part of my issue. Doesn't sound like it. Do you know how many psi this temps would be on the high side?

21 minutes ago, leathermaneod said:

Do you know how many psi this temps would be on the high side?

I've seen high side pressure well over 300 psi when the air temp was 105-108°F.  I've seen a compressor relief valve open on an overcharged system with pressure over 450psi. and temp at about 

 80°F.

Oh ok well I should have been able to see if my high side switch was cycling with the gauge manifold too I suppose. And my highest pressure was about 280 or 290 I believe. 

Well I finally got around to retesting my system pressures after letting out too much freon, adding more, and then having not stay locked up and I just wanted to post the results for future readers. The next day the clutch was staying locked so I guess maybe these things just need a little time in between adding or removing freon to get an accurate idea of whats going on. Anyway, now everything is about as perfect as i could possibly get it.

 

80* outside

 

Idle:

Comp locked:

High-193psi

Low-30psi

 

 

1500:

Comp locked:

High-225psi

Low-23psi

 

 

2000:

Comp locked:

High-230psi

Low-20psi

 

It did not unlock at all :-)

How is your air temp?

I haven't check again yet, but last I checked, with the pressures higher, it was around 45-48°.