First off, I have no experience testing or troubleshooting automotive A/C systems, but I'm a quick study. Today is a sweetheart of a day -mid-60s, sunshine, and very light breezes - a perfect Spring day (finally). I'm cleaning up the cab of my truck, trying to get the smoke smell out of it left by the previous owner. I'm spraying the soft stuff with "frebreeze", and decided to spray some into the air intake for the HVAC system. Decided to check the A/C for operation - no dice. The compressor is cycling - runs for about 3 seconds then off for 10-15. Read up on the A/C system in the 2001 FSM (thanks again Mike). Last year I bought a set of A/C guages - haven't used them yet - perfect opportunity. I follow the gauge hook-up directions & get some readings. Static reading (engine not running) is ~60 psi. Started up the truck & put the control on Max A/C. Go around to the front & read the gauges. When compressor is running the suction side drops to about 15 psi when the low pressure switch opens up, then it resets at a ~ 40 psi. So the system is holding some pressure, the compressor is doing it's job, the pressure switch seems to be operating correctly, I presume it is just low on charge. Perhaps there is a slight leak somewhere, but not bad enough to tear it apart looking for the leak. I figure I should buy a can of 134A refrigerant & charge it up. This is the point where I need some guidance. All the bloody details; evacuating the hose that leads to the gauges from the refrigerant can? What side to charge-the low side or high side? How much, or when to stop - pressures or temps or both? Thanks in advance, Joe in St Louis

Slow leak for sure... Start adding freon on the low side till the low side pressure is 35-40 PSI compressor running solid. Then rev the engine to 1,500 RPM and hold and check the pressure during this time to be sure its not pulling lower if so add a bit more freon till the compressor hold tight at 35-40 PSI low side. But more here... http://mopar.mopar1973man.com/cummins/2ndgen24v/hvac/recharge/recharge.htm

Yep, system is low. The way it works is the compressor will cycle on the low pressure switch to keep the evap from freezing up. The pressure climbs in the suction line with the compressor off and allows the low pressure switch to close. That connects the clutch on the compressor and when the pressure drops the low pressure switch shuts the compressor off. You might be able to find a slow leak by spraying the lines with a soapy water solution and watch for bubbles. Chances are you won't find any, but you never know! The most common problem seems to be the evap leaking, and that is a bunch of work........

Yep, system is low. The way it works is the compressor will cycle on the low pressure switch to keep the evap from freezing up. The pressure climbs in the suction line with the compressor off and allows the low pressure switch to close. That connects the clutch on the compressor and when the pressure drops the low pressure switch shuts the compressor off. You might be able to find a slow leak by spraying the lines with a soapy water solution and watch for bubbles. Chances are you won't find any, but you never know! The most common problem seems to be the evap leaking, and that is a bunch of work........

Yeah... I'm getting ready to do my evaporator the second time now...

What yo can do is get the UV dye pack and add it to the system and keep a close eye on the under hood plumbing with a blacklight looking for the dye. But in my case I can never find any dye pack under the hood so the only play left is the HVAC under the dash which hold the evaporator. Thankfully NAPA evaporators have a lifetime warranty!

Follow-Up... I tell you; a man with tools & little knowledge is dangerous. I purchased some freon & serviced my A/C. Problem was I did it on a cool day, like around 50 deg F. The system sucked up the first can likity split & the compressor was still cycling real fast, so I loaded up a second can. Then it dawned on me to check the conditioned air temp, which was around 37 deg, so I stopped there. A couple days went by without me running the truck. I kept thinking about how much freon I had shoved into the system & convinced myself I had overcharged it. Yesterday the temps got up to 80 (27% humidity) so I hooked up the gauges again to see how it looked. At first the pressures looked ok -around 30 psi on the low & 175-190 on the high. Then after about 3 or 4 minutes the high side started to climb. I started to get nervous when the high side was approaching 350 psi & the compressor was not cycling so I shut it down. I figured I had verified that the system was over charged so I vented some out & checked again, but I got the same results. I vented some more & checked with no change. I noticed that the low side was going up slightly, like 40 psi when the high side was over 300. At this point I went back to the FSM. I read the diagnostic chart - symptom: normal/slightly high low side, very high high side - problem#1: clogged condenser/ low airflow through condenser. I recall noticing that the radiator fan was free-wheeling. I decided to monitor the pressures with air passing through the condenser. I positioned the gauges up on the windshield (held in place by the wipers), closed the hood to the safety latch & took a drive. Instantly the high side went down to 150 & would drop as the low side went below 30 & the compressor cycled. Conditioned air temp would run around 45-46 deg on the normal setting (cooling outside air) & down to 41 on max cool. But the compressor was still cycling pretty frequently, like 5 or 6 times a minute, and since the humidity was so low I figured it could use a little more juice. I came home & set up a fan in front of the condenser; it is a squirrel cage blower removed from a home furnace which blows some serious air. I let the truck & A/C run for a while with the fan running & the pressures and operation was similar to what I was seeing on my drive. I added about a half can of freon which brought the high side up to around 175 which was on the low side of specs for 80 deg, then I took another test drive. The compressor still cycled but not quite as often. The high side would get up to around 160 while driving before the compressor would cycle. But when stopped at a stop sign the pressures would start to climb to about 200-225. Conditioned air temps were a couple degrees cooler, around 43-45 with A/c control on normal. At one point I pulled over & let it idle for a couple minutes & high side pressure seemed to stabilize around 220-235. I switched over to max cool when I pulled over & temps got down to 39 deg. I drove home (about 5 blocks) & parked in the drive, leaving the truck idle. For a minute or so things looked as they did when I had pulled over, but then the high side started to climb again. I was out of time so I shut it down.I'm thinking I have it pretty close to the right charge, but I will monitor the conditioned air temp as ambient temps & humidity increase. I'm concerned with what happens with the high side pressures when airflow stalls through the condenser. I wonder what will happen in 100 deg temps when stuck in traffic. The FSM says the high pressure switch opens at 450-490 psi & resets at 270-330. I don't have any overheating problems - in fact my engine cooling system seems to be working well - I don't think there is a problem with the radiator fan clutch. A lot of cars these days have electric fans on their condensers that operate whenever the compressor is running - anybody ever try that? Maybe this is all normal. Any input appreciated.Thanks for your time,Joe in St Louis

Joe, you are getting exited over nothing. The pressure of the high side will climb when there is no air flow over the condenser. TOTALLY NORMAL. There is a pressure switch on the high side to shut down the compressor when the pressure gets to high. Just get the suction pressure to be where the book says. What happens in 100* stop and go traffic is the compressor runs and either the low pressure switch OR the high pressure switch stops the compressor to allow the evap to warm and keep it from freezing up (low pressure switch) or the high pressure switch shuts the compressor off to keep the pressure in the safe zone (high pressure switch).

OK, I'll not worry about the pressures - thanks. So what do you think, does the A/C performance sound about right? Still cycling while driving around town (~30 mph) in 80 deg ambient, low humidity, conditioned air on normal mode (cooling outside air), fan on high, with output running 44-46 deg?I noticed when the pressures wet up while stopped, the conditioned air temps went up too. Still think a condenser fan might improve performance while stuck in traffic.Thanks again,Joe in St Louis

It sounds like your system is still a slight amount low. Far as the air temp going up when stopped, that is because there is no air flow over the condenser and the amount of liquid refrigerant is decreasing. Another normal thing to have happen. Electric fans will sure help that situation.

Yeah he's slightly low if the compressor is cycling. The low end pressure should be 40 PSI. I've also seen my jump high in pressures for the same reason but eventiually the high side switch will cycle the compressor off again...

OK, well thanks guys. Guess I shouldn't be a wimp when I see the high side heading north of 350 psi - damned the torpedoes! I'll shove in some more juice. But I think I'll do it with a fan in front of the condenser. What I observed is when the high side climbs up so does the low side - enough to keep the compressor from cycling. Having a fan on the condenser simulates normal driving conditions.Appreciate your help & patience,Joe in St Louis

OK, well thanks guys. Guess I shouldn't be a wimp when I see the high side heading north of 350 psi - damned the torpedoes! I'll shove in some more juice. But I think I'll do it with a fan in front of the condenser. What I observed is when the high side climbs up so does the low side - enough to keep the compressor from cycling. Having a fan on the condenser simulates normal driving conditions.

Appreciate your help & patience,

Joe in St Louis

Yep, that is what happens and when you drop the high pressure the low pressure drops too!

Yeah... I'm getting ready to do my evaporator the second time now... What yo can do is get the UV dye pack and add it to the system and keep a close eye on the under hood plumbing with a blacklight looking for the dye. But in my case I can never find any dye pack under the hood so the only play left is the HVAC under the dash which hold the evaporator. Thankfully NAPA evaporators have a lifetime warranty!

You know, if you can you post up a total step by step that would help me out.This spring i have to do it.

You know, if you can you post up a total step by step that would help me out.This spring i have to do it.

Real easy. Just get yourself some R-134 with the UV dye (leak detector) and put it in. Run the A/C for a bit (15 min or so). Then try to get the truck into the shade and use a black light to look at all the hoses and fittings you can. If you can't find anything that way there is a good chance the leak is in the evaporator. Try to find some place that has a hand held leak detector and have them put the sensor in the drain tube and that should tell you.

I figured Mike ment he had to pull his dash out again.Thats what i have to do in spring.