Well I just bought my first 2nd gen 1996 3500 4x4. Its not my first 12v cummins, but anyway, I love the truck to death already. When I test drove it a few weeks before I got the truck the a/c was ice COLD. Lately its been hanging around a 100*F here in texas. So after I get the truck the a/c wasn't blowing cold any more. So I hook up my gauges and to my surprise the pressures were actually perfect. 35psig low and 250psig High. But the odd thing I noticed when the pressures get to the normal operating point, the compressor will cut off. After a few moments it will kick back on and a few moments after they level out it will cut off. The low line is sweating and cold, but the vents are just barely even cool. I dropped the truck off at a local mechanic to have them check the dash to be sure it wasn't the blend door. They called me this morning and they were just as dumbfounded and they said it was doing exactly what I said it was, but everything is working as it should. The only thing they found I didn't was that there is a switch that was being tripped on and off, they didn't mention if it was on the low or high side. I'll try to find out more details when I pick up the truck. They called a reputable automotive a/c shop and the shop told them they are sure there is a blockage and recommend redoing the system.

Ok so if you haven't fallen asleep yet, here is my obvious question, is there anything I can check before redoing the system? I'm planning on tackling the job myself, I've done other systems before so it's no BIG deal, but sucks either way. Any points or tips that might save me a few hundred dollars would be great. Hope someone can help me out. I'll keep yall posted.

 

almost sounds like to over full..just a guess.
did you try to measure the dash air temps and compare them to outside temps?

250 psi sounds a little on the high side to me. but I do know its in spec. the low is also on the low side..

do you have a vacuum pump. you'll need one if you open the system.

and if you really want cold air coming out..use propane instead of R134. its really nice having less than 40f blowing out when its in the upper 90's..
use at your own risk. it does burn..

-dkenny

 

I don't have exact specs , but I usually just set my mark around those pressures as ok since that's what I see on an average. As for the vacuum pump, I usually just rent one from slowzone (autozone) for free, but I have my local mechanic evacuate the system first. Which reminds me, when I picked up the truck, I got a little more details. He said they bumped up the low side to 40 but he didn't mention what the high side was reading. Then the compressor started cycling rapidly. I'm wondering if the high side is too high. But what is too high? If I get a chance I'm going to hook up my gauges again and play around with it. If something is gonna give, it might as well be now. If anything I'm leaning toward a bad high side switch, since it's the reason it's cycling. I didn't even think about it, but he said it was tripping on and off which was cutting off the compressor. I might bypass it and see what the pressures do. The only other thing it could be is contamination/ blockage, or a bad thermostatic switch in the evap core if it has one. As for the the propane.... I've heard so many bad stories in my a/c class when I was getting my diesel certification. I am licensed for handling automotive refrigeration by ASE but I really don't mess with a/c systems much. I just got it since I needed it to pass the course. As much as I don't like it, I can redo the whole system but the dash. I hate messing with dashes....it's my weak point. I'll keep yall posted on what I find.

 

Cycling compressor clutch, sweating suction line, good high and low side pressures? Sounds like the evap is cold but not getting any heat transfer through it...

We ARE talking R134a in there right? not something else with different boiling point curves? (aftermarket R134a substitutes can have a little bit different boiling point curves and MAY be freezing up the evap at the normally expected suction pressures.
Is the evap frozen up and NO air comes through it? if not, it's gotta be the blend door or something mechanical like that...
The blend door/vacuum actuator IS a major culprit in these trucks... ( my guess)

 

As far as I know it is 'supposed' to be R134a in it, but I'll take a quick sniff, it's the best I can do with out a tester. As for the blend door, that's why I took it in the shop to begin with. I could actually here the door slap close when you mess with the temp control. Unless there is a hole in it, it seems to be fine. Shop said everything in the dash is working as it should. I'll take an temp reading from everything. I got to do some hauling with it right now, so I'll try to dig in a little deeper later this afternoon. Thanks for all the help guys!

 

I'm having the same problem: barely cool air (it does blow thought) with lots of compressor cycling and high pressure when the system is off.

So I'm hitchin on!

Craig

 

I had this situation in my 01, rapid cycling of the compressor. I just added a can of 134A with oil & sealant from Wal Mart and it cured it. I know next to nothing about AC systems so it was pure luck... but it worked and this was several months of AC use ago...

 

Well I figured it out! I dug up a pdf dealer service manual that I had downloaded from another forum. I love this book now! It had every specs and even a chart for the ambient temperatures. Turned out I was way off on the a/c pressures. After confirming the ambient temperature to be 100*F outside, the chart showed the low side should be between 60psi-70psi low side and 270psi-330psi high side. A can of r134a later everything was to specs, I check the temps at the vent and was getting 15*F. I guess I just have a leak somewhere. I'll keep an I on it, and when it needs another recharge I'll put some of that leak repair stuff in, it actually worked on another vehicle that had a bad compressor body seal. Then I'll top it off with r134a that has the uv die and oil mixed in it. Hopefully should be good to go, I'll try to dig up the link to the manual in case anyone is interested in it. It is plenty helpful for most 2nd gen trucks. Thanks for the input guys!

 

Found the link, its for a 2001 but there is a lot of helpful info on it.

Here yall go http://www.rm-indy.com/wavefiles/2001ramsrvcmanual.zip

Hope this is useful to anyone that will need it. This is a pdf document!

 

I missed you for some reason, as you can see I figured out my problem. The rapid compressor cycling is either going to be because your low side is too low or your high side is too high. With the compressor off your pressures should equalize and should show about the same pressure on both sides. I just can't get over the fact of how high the running pressures are in these trucks!

 

I recommend putting the dye in the system FIRST. That is a neat way to find leaks w/o a halogen detector.
At least that way whenever you need to put any more 134a in, there SHOULD be some dye stains wherever the leak is.
I bought a compact fluorescent BLACK LIGHT bulb and put it in my droplight.
Found a leaking comp discharge hose right away...



At the ambient region of the scale, the boiling point-vs-pressure curve for R12 is known to be approx 1psig for each 1 degF . So, R12 at 70 degF was really close to 70psig, and @32DegF it was really close to 32PSIG.
R134a is reasonably close enough to that same slope that by rule of thumb you can tell approximately what the temperature is by its pressure too.

15Degrees F? @ 60-70psig... Huh?

15degF is WAY too cold... Hard frozen temps...
Did you possibly mean 15deg "C" which could be within the realm of possibility for ~60psig.

With the interior cooling down and the discharge air nice and cool, I would expect the comp suction pressure(depending on RPM) to be around 30-45psig, not 60 to 70 Psig.

 

Sorry, been busy the last couple of days, blew out my drivers side u-joint, that was a fun job. Anyway, thanks for all the info. It makes obvious since to put the die in first and I'll just keep an eye on the levels to be sure they don't get low again. Hate to starve the compressor of oil. The temp was measured in Fahrenheit. However, it was an elcheapo laser thermometer, and at the center vent, it drop down to 15F according to it. I left the truck running for a bit and when I came out, my windows were fogged up good when it was 100F outside...LOL

I was just as surprised about the low pressure readings. I'm just following what the book says. I usually see the low side around 35psi as normal. But this was the first. The book says on the low side 60-70psi at 1000 rpm's. But the thing to keep in mind is they do drop down a bit once everything is stable. It's the starting pressure you really have to watch. Once the cabin starts cooling down the low side will drop a bit, and once the fan kicks in the high side will also drop a bit too.

My friend was laughing at me because every time I opened the can to add some more refrigerant, I would take a few steps back and be at an awkward stance. I was in front of the condenser and seeing them high pressures and mentally realizing I'm standing right next to it doesn't make me feel to comfortable. Especially if this is the first time messing with them pressures like that. I just figured if somethings gonna give it might as well be now. But all seems good so far.

Edit: Sorry, missed read part of your post. I completely agree with you, but like I mentioned the dealer book says 60-70psi at 100F ambient is what is recommended. It was very frosty cold inside too! Now the low and high side did drop a bit once the cabin cooled down. The low was hovering between 50-55 and the high was about 340ish but was varying quite a bit, but nothing weird.