Got a gauge, connected it up the the low pressure port and see a fluctuating pressure. When compressor is not engaged pressure rises up to 75 PSI, compressor kicks on until pressure reaches about 25-27 PSI, then rises, repeat...

Now the line is nice and cold and all seems visually normal, but not a lick of cold air coming out in the cabin...

So, should I be seeing this large range (25-75 PSI) in pressure?
Anybody got the specs for a '96 Grand Cherokee in regard to the A/C system?

Just a stupid question - the changeover flaps are air powered, can you hear the flap moving? might be stuck and not routing cold air.

Your system is low on Freon. The low pressure cut off switch is turning the compressor off.

Here is a chart: http://www.ackits.com/index.cfm?fuseaction=Chart

Your system is low on Freon. The low pressure cut off switch is turning the compressor off.
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Can you give me a little more detail on why you think this? Reason being the chart lists reasonable pressures being in the 25-50PSI range, yet it increases to 75PSI before the compressor kicks in. I did add a small amount of 134 to see the effect, and all I got was a range of 25-75PSI instead of 25-70PSI, anything over 65 indicates a danger warning...

Like I said, new to the A/C system stuff..

Okay, after a little more poking around, I see that an engine off PSI reading of 125-130 is what I should be seeing on the low side, so I'll add refrigerant and see what the results are. So readings above 65 are only a bad sign if the compressor does not lower them to the 25PSI range when it kicks on.

Just a stupid question - the changeover flaps are air powered, can you hear the flap moving? might be stuck and not routing cold air.

Good point, I don't hear a flap "slap"...how to get to the flap tho? Have to remove the dash?

fraid so, you could trymoving switch whilst revviing then decellerating (up to say 2000rpm's) as vacuum greater on overrun. had mine stick once and that is how I fixed it. Also check housing not full of water due to drain blockage, mine ran warm when the drain blocked. I also found when my r134a was low, that the cooler pipe id not get cool. If one pipe is hot and one cold then ac is probably working enough to at least feel cold air. If you put on recirc does it get any colder?

OK OK OK, I have been having the same problem in my 94 ZJ. It's been a few weeks and I have not done anything like add more freon due to the fear of harming the syste,. I am not an HVAC guy, so this is not one of my strongpoints. S

G vs. J - you say that as long as the pressure doesn't remain in the RED/DANGER zone when the compressor kicks in it is OK, is this true? My reading are 28 then builds up to 58, then compressor kicks in and drops back down to 28.

is there anyway for some with a working A/C to check there Low Side Pressure Readings and post them....PLEASE.

ok I found this website http://dodgeram.org/tech/repair/HVAC/R134a.htm
with the outside tempature and what the LOW SIDE PRESSURE should be at. If I understand this chart correctly the LOWSIDE Pressure reading is what it should be at when the Compressor Clutch kicks in, right?

Guys...the pressures only matter when the compressor is running. You WILL go into the red on the low side if the compressor isn't running.

The chart I posted a link to is your guideline. Pressures change with temperature and load. If I post my pressures with 110 degree outside temp and a hot interior, they wont be anywhere near the pressures at 80 with a cooled down interior. Like I said, use the chart as a GUIDE.

Steve, thanks for clearing that up, thats exactly what I wanted to know.

G vs. J - you say that as long as the pressure doesn't remain in the RED/DANGER zone when the compressor kicks in it is OK, is this true? My reading are 28 then builds up to 58, then compressor kicks in and drops back down to 28.

Looks like this is the case. I ran a pressure check again this morning (after a little side excursion to replace the water pump yesterday) and with temperatures being lower than earlier this week and got reading between 25-65, added R134 to the system and got a nice reading which stays in the 25-40 range with the compressor running like I remember it should. Figure it to increase on the high end when temps go up.

Thanks for the input SteveB05!!

fraid so, you could trymoving switch whilst revviing then decellerating (up to say 2000rpm's) as vacuum greater on overrun. had mine stick once and that is how I fixed it. Also check housing not full of water due to drain blockage, mine ran warm when the drain blocked. I also found when my r134a was low, that the cooler pipe id not get cool. If one pipe is hot and one cold then ac is probably working enough to at least feel cold air. If you put on recirc does it get any colder?

Thanks chrisewers - worth a try, I'm getting colder air now after charging the system, but wonder if the flap isn't stuck partway maybe, so I'll give it a go.

ok I got another Q. On the chart I posted earlier (http://dodgeram.org/tech/repair/HVAC/R134a.htm
, it says at 85 degrees Farenhiet there should be a reading of about 95 PSI, is this true or am I missing something, I added some more refrigerent to my system today to try to bring it up, well the compressor clutch spins all the time now and my pressure reading is around 70 PCI, this is in the RED/DANGER zone on the pressure guage, is this OK or am I reading the chart wrong....

Looks like this is the case. I ran a pressure check again this morning (after a little side excursion to replace the water pump yesterday) and with temperatures being lower than earlier this week and got reading between 25-65, added R134 to the system and got a nice reading which stays in the 25-40 range with the compressor running like I remember it should. Figure it to increase on the high end when temps go up.

Thanks for the input SteveB05!!

Is your A/C Working now? Clutch spinning all the time?

Is your A/C Working now? Clutch spinning all the time?

Yes, got to enjoy nice cold air as of yesterday. Pressures looked good.

Do you have a steady 70 PSI, or just a max of 70 before the clutch engages?

steady 70 PSI, the clutch is engaged all the time now. My AC is working now, but I think it might be too high. I let a lil out yesterday so bring it out of the red it's at about 60 PSI now and it's in the ALERT zone.

Also, that chart I posted said it should be at 95.3 PSI when outide temp at 85 degree, but that would put it in the red, I wonder if that is OK.

Outside temp affects high side pressures. Low side pressures affect inside temp. Charge state affects both. The chart you posted is cute, but you cannot cut the system into little pieces like that. You have to see all four pieces 1) Low Side Pressure 2) Evap Temp (vent outlets) 3) High Side Pressure and 4) Cond Temp (outside temp). Then, you need to compair that to the chart I posted. If everything falls into the proper ranges, then your good, if something is out, you need to determine what the issue is...over / under charged, blockage, weak compressor etc.

Generally if you have a 134 system running on High and stable (meaning the inside is cool, not 110 degrees) and an outside temp of 85 degrees, you should see a low side of about 30 to 35 PSI and a high side of about 180 - 210 PSI. And your outlet temp should be in the low 40s to high 30s.

This is such a difficult DIY topic. There are so many variables...humidity, ambiant temp (outside), load (inside temp), volume, condensor cooling efficiency, evaporator size...etc, etc, etc. Use the chart I posted, if your outlet temp is 40 degrees or more less than ambiant and your pressures are NOT outside the chart boundaries, you're good to go.

BTW 95 PSI on the low side is NOT ok :)