98 Grand Caravan AC problems?

I've been a HD diesel mechanic most of my life - and I'm not an automotive AC technician. I know the basics, and that's about it.

I recently bought a 98 Grand Caravan AWD with 125K miles on it. The AC system was dead. The previous owner told me she had it charged several times at here Dodge dealership - and it worked fine for a few weeks each time and then lost its charge and went dead.

This has the 3.8 engine and HVAC setup with dual temp controls - i.e separate driver's side and passenger. NO rear AC, just front.

So . . . when I got it - I charged it - and leak tested. It had a slow leak out of the compressor. I'm pretty sure it was blowing cold air at that point after I charged it - but I'm not sure of my memory on the matter. So, I installed a brand new compressor, pulled a vacuum for a few hours - and then charged to system specs with some added PAG oil. It took three 12 ounce cans. or 134A.

Now - it barely blows cool air and it gets warmer the longer it runs. I've run several hi-low pressure tests and all seems in-specs according to ambient air temps. Also, in the factory Dodge manual - there's a table for determining under or overcharge using low and high side pressures along with high-side line temperature. So, I attached a temp-gauge to it and it too checked out right in specs.

Even though all pressures were good - out of desperation, I added an extra 12 ounce can of refrigerant. It made no difference.

I then read that too much oil in the system can lessen cool air output. So, I pulled it all apart, took the new compressor out - and drained some oil. I did some careful calculations and figured it was maybe 4 ounces overfull. So, I removed 4 ounces of oil. Put it all back together, pulled a vacuum, recharged, etc. And . . . same as before. When I first use it - I get cool air - but not ice cold from the vents. Once it runs for awhile - the air from the vents - if anything - is a bit warm - yet the AC high and low side pressures are fine.

Last night - air temp was 65 degrees. I attached gauges - and before starting the engine - system pressure on both sides was 50 PSI. After starting and running the AC - at first it was low - 2 PSI and high - 100 PSI. Once it ran a few minutes - low came up to 15 PSI and high came up to 200 PSI. This seems about right since the outside air temps were cool. At this time - I put my hand on the two lines at the compressor - one was very hot and the other just barely cool (certainly no frost on it).

At this point - I'm kind of stumped. I'm no AC expert, but I've fixed plenty over the years and never came across anything like this. Could this have anything to do with the HVAC system? I've never had a car or truck with such a system before now.

Is there any way - that when I evacuated the system - and then recharged -that something changed in the computer settings for the HVAC controls?
My Dodge manual does have a section on HVAC codes and settings - but I've been reluctant to mess with it - so far.

I don't think your compressor should be engaged at pressures as low as 2-15 psi. The low pressure cutoff switch should activate in the 21-28 psi range - check the manual for the actual cutoff point. The high pressure side seems a little low at 200 psi, but you mentioned it wasn't real hot out when you tested it too, so that may be ok.

Any chance the old compressor spewed it's guts? If so, you didn't flush the lines when you replaced the compressor and the new one wouldn't last in that environment. I don't suppose you replaced the receiver/drier when you changed the compressor, did you? Should've been done since you probably had the system open for awhile.

You mention that it gets warmer the longer it runs - have you checked to make sure the mode doors for the vents are operating correctly? If it's leaving the door open to the heater core while in a/c mode, it won't cool very well. Not sure how difficult it is to check on these vans though.

I've heard of the climate control module "going out" on these vehicles, but don't really understand enough about them to comment on it further.

I don't think your compressor should be engaged at pressures as low as 2-15 psi. The low pressure cutoff switch should activate in the 21-28 psi range -

No - I don't think it works like that. The pressure-switch is hooked into the high-side discharge-line, not the low side. High side never gets below 50-60 when there's a proper charge in the system.

I don't suppose you replaced the receiver/drier when you changed the compressor, did you? Should've been done since you probably had the system open for awhile.

I didn't. I usually do when I work on a system - but since nothing was trashed in this one - I was hoping it'd be OK without. I taped all the fittings closed tight when the compressor was out - but it wasn't apart long.
Nothing was trashed in the system - the old compressor just had a slow leak.

You mention that it gets warmer the longer it runs - have you checked to make sure the mode doors for the vents are operating correctly?

I checked the mode and cool-down last night after reading through the factory manual and learning how to push the right buttons. Computerized mode-test checked out good. Cool-down test failed. Then - I added yet another 12 ounce can of refrigerent and then . . . it started to work. Not great - but good enough to pass the cool-down test and get rid of the error lights that were flashing in unison. When done - air temp was 72 F outide - and duct air was blowing 58 F. Shortly after, I pulled my wifes diesel Volkswagen into the shop to change-over the R12 system to 134A. When I got done - it was blowing a duct-temp of 34 degrees F which surprised me a bit. Maybe too cold? A heck of a lot colder than this Dodge.

So my wife's 91 Volkswagen is blowing 34 degrees after I did a 15 minute retrofit job on it - and this damn van is blowing 58 degrees (and it's the best so far).

I'm still stumped. Especially since the book says the entire AC system takes 34 ounces. At first - I'd put in three 12 ounce cans - i.e. 36 ounces and it didn't work at all. Then added a fourth can and it still didn't work. Then added a fifth can - and now it DOES work. Something here does not make sense. After adding that fifth can - and running the AC for awhile - low side pressure was 40 PSI and high side was 275 PSI. Center duct temp at 58 degrees F.

I then got thinking maybe my cans were not totally full? Some I've had some my shelf for over 10 years. So, weighed one - full and then empty. 15 1/4 ounces full and 3 ounces once empty. So, seems they are correct.

As I said - at this point - I'm stumped. I can leave it as-is with what should be an over-charge condition. Or, I was thinking of pulling it apart, flushing it, replacing the expansion valve and the receiver-drier - recharging - and see what happens. That's not usually the way I do things - but at this point - I don't know what to do.

I'll add that I drove over to a shop 40 miles from here that is supposed to be pretty good with AC. I explained my problem and the guy said it sounds like my orifice-tube is plugged. Now I'm wondering if I'm being told some mularkey. As far as I can tell, my Dodge does not have an orifice-tube. I've replaced several in my GMs - but I don't see one shown for this Dodge. I wonder if the expansion valve has a screen/filter - and if it does - what are the symptoms if it gets plugged?

There should be pressure switches on both lines, but I think on the high side it's more of a pressure release valve that should vent if pressures go above about 375 psi or so (IIRC). On the low side, it should shut off the compressor if it gets below 21-28 psi to avoid freezing up the evaporator. Of course, I could be mistaken on this particular van. I'll make a point to look at my manual tonight.

That is somewhat puzzling about how the overcharge seems to be working. Give it a couple weeks and see if it's still cooling I guess. Could be leaking under pressure, but not under vacuum. That happened to me the other day with a bad o-ring being the culprit. It was holding down to 500 microns for over 10 minutes under vacuum, but had a very bad hissing type leak when I pressurized with nitrogen to 120 psi. Didn't even need soapy water, much less a leak detector to find it.

A clogged orifice tube could yield a low side pressure that is too low - but being low on refrigerant exhibits these same symptoms as well. Too bad these vans use a thermal expansion valve (TXV), and not an orifice tube, LOL.

Logic would say that a clogged TXV (or one that is stuck closed) would result in excessive high side pressures - kind of like blocking off the high side port and seeing how far the compressor can pump up. Of course, they can stick open as well, leaving you with a low high side pressure.

Glad to hear your VW retrofit is going well. That mineral oil isn't supposed to be compatible with R134a, so I'd be curious how long before it leaks. I'd also be curious to know how well it performs in 110°F weather.



Chad

I was wrong about the pressure sensor being on the low side. You're right - it's on the high side. I was thinking of an orifice tube system I guess since that's what I've mainly been working on. It has a temperature probe on the evaporator to shut off the compressor when the evaporator gets close to freezing, and cycles it back on when the evap gets warmer. That's in addition to the low pressure cutout switch (a/c pressure transducer).

I hate how the service manual talks about testing the pressure transducer by checking the voltage. Granted that's the signal it sends to the PCM, but the least they could do is provide some kind of indicator of how that correlated to high side pressure. A little research got me my answer though - a very crude approximation is about 100 psi/volt on that transducer*. I certainly wouldn't use this in place of a gauge on the high side, but a decent way of verifying the output voltages are in the ballpark given the high side pressure reading.

I would still tend to suspect that you have a leak that only presents itself with pressure (not vacuum), seeing as how you've put in 60 oz, which is 12 oz more than needed even if you had rear a/c. Assuming the condenser, lines, and evaporator aren't clogged, the only other thing to impede refrigerant flow in the system is the TXV, which could be malfunctioning if the temperature sensing bulb was not properly attached to the evaporator inlet line with insulating tape (prestite tape), but that'd be lower on my list than pressure testing the sytem. If you can, pressurize the system to 100-150 psi with nitrogen and either soapy water or leak detector to find the leak. This all assumes you have the stock setup. I suppose if you have an updated condenser, then PERHAPS that could increase the required charge, but it's a pretty excessive difference.

*Techically, I was given that 0.45V corresponds roughly to 30psi, and 4.5V corresponds roughly to 450 psi. One can assume and generate the equation for a line where the slope is about 104 psi/volt, but the pressure at zero volts corresponds roughly to -17psi, or in math-speak, y=mx + b, where y is pressure, x is voltage, m is the slope, and b is the zero voltage pressure, or -17 psi.