AC troubles

nursecosmo · **Posted:** Mon Jul 02, 2012 12:41 pm

Hi folks, long time no post. I've having some trouble with my AC for the past month or so. When I first start the Jeep in the morning and the engine is cool, the AC will cycle on and cool just fine but as the engine approaches operation temp, the AC compressor won't engage. The engine temp does not go over 170 but the the auxiliary fan stays engaged regardless of whether or not the AC button is on. I checked the pressure on the low side and it is right in the green when the clutch does engage but shows high when it disengages. I have a feeling that there is a temperature or pressure sensor somewhere in the AC system which is wonky.

Where should I look?

Sir Sam · **Posted:** Mon Jul 02, 2012 1:45 pm

There is a pressure switch on the AC line by the drivers side boost hose, unplug it and jump it with a piece of wire between the appropriate pins. Look at the pinout diagram in the FSM to determine the correct pins.

With the switch tested you will know if its that or the clutch that needs to be tested next.

The pressure cycling sounds correct.

geordi · **Posted:** Mon Jul 02, 2012 4:51 pm

Wrong pressure switch - The one by the AC compressor on the driver's side is the HIGH pressure switch, and has more than 2 pins. The low pressure switch is on the dryer canister, over by the passenger side firewall. The high pressure switch might be working just fine and protecting your system - pressure varies with temperature, and it has been REALLY hot lately. With temps over 95, the high pressure can easily approach 220 psi on a full system, even more on an overfilled system.

The high pressure "might" be too high when the compressor is engaged, but what is the actual pressure number for the low side when it is running? When the compressor is off, the pressure SHOULD be higher on the low side, the system is equalizing pressure and static will be somewhere around 100 psi.

If the low side drops below 20 psi, then the low cut is doing its job properly to shut the compressor off. This usually happens due to a very very slow leak. If you tell me what your low pressure number is (green zone doesn't mean anything - low pressure varies with temperature too) I can tell you whether the system is running low or if it is something else.

Sir Sam · **Posted:** Mon Jul 02, 2012 8:37 pm

geordi wrote:

Wrong pressure switch

No, its not.

The fact that his AC works at first and cycles leads to me think the high pressure switch is getting hot and cutting out. Hence why I recommend to test it.

My experience with low pressure switches is that they either work or that they dont.

The fact that it works and first and then later cuts out leads me to believe that the problem is not with low pressure but something else.

nursecosmo · **Posted:** Mon Jul 02, 2012 10:15 pm

Thanks both Sam and Geordi. I guess I should have read the Service Manual (thanks again Sam) before posting my question. I did jump the low terminals(per the FSM) to rule out the low pressure switch and that apparently isn't the problem. I only have a cheap-o low side gauge so I can't compare sides for a truly diagnostic reading but I'm getting about 55-60lb when I jump the clutch relay which jumps up to about 150lb when it disengages. I don't know how reliable this gauge is since it came packaged with a can of freon.

I am still a little unclear about the function of the high pressure switch since the FSM states that it has 2 terminals, however, on this Jeep, it has 3. I can't find the connector pin out diagram anywhere in the manual but I have 5V coming in to 2 of them. The manual also says that both the low and high are connected in series back to the ECM but I get 12V coming in to the low side. So what gives? Why 12V on the low switch and 5 on the high switch? As usual, the manual doesn't seem to be very faithful to reality.

This is making me wonder if I have a wonky ECM now. I guess I'll pay to take it in to a shop with proper tools and have it diagnosed.

geordi · **Posted:** Mon Jul 02, 2012 10:36 pm

My high pressure switch has 3 pins also, and I don't know which ones are which to try and jump them - Hence why I don't suggest that.

The pressures you offer DO tell me quite a bit however, even given the dubious accuracy of those gauges - Did you use that can of gas in your CRD? It seems like the system could be overpressurized, depending on your ambient temps. Wisconsin temps shouldn't be the same as in Texas / Phoenix / outer circles of Hell... Which is what that low pressure is telling me. I would expect a low side (engaged) of 55-60 in 100 degree ambient. Milwaukee is in the low 90s during the day, so you could be 45-50 psi... But that pressure when it is off is the real indicator - Too high at rest!

The highest I've seen on a small system is about 130psi, and that is an RV air conditioner. Our system at static should be around 100-110, MAYBE as high as 125 in the heat.

An AC shop should be able to diagnose this quick for you, but if you want to give it a quicky try (since they will probably want to refill it anyway) you can connect your gauge back to the port without an empty can attached (If this is the type that has a trigger control) to bleed a little back off.

With the engine off, hook everything up, then try slowly unscrewing the can - This is to check the seal on the trigger, so you have some control. If it opens to loose with no large blasts of pressure... Good. Give the trigger a quick squeeze, and release - Check the pressure when the gauge settles. It will probably take 5-10 times before the pressure comes down to the 130 mark, but if you get a reading of anything other than a pegged needle... I would start by trying to bleed about 20 psi off of it, and give it another shot in the daytime heat.

It really shouldn't take much, and you can certainly do it in a smaller increment (bleed only 10psi) with the engine already at temperature to see if it is *just a bit* above the cut-off-point for the switch. Once the compressor stops cycling and remains locked-on while running... You have the right amount of pressure.

nursecosmo · **Posted:** Mon Jul 02, 2012 11:20 pm

Thanks Geordi. No I did not add any freon. I got the can out of my basement to charge it last week but when I hooked up, It appeared to have an adequate charge already. The gauge can operate independently of the can it came on. When I checked a couple weeks ago, the reading was in the green (15-45LB) but today the temperature is a lot hotter and I'm assuming that this is why it is reading a higher pressure today. I am certain the problem isn't related to overcharging, since it hasn't had freon added since it came out of the factory. A couple of weeks ago, I thought that perhaps the evaporator, radiator or intercooler might of been clogged with bugs or something (even though I always run with a Fia screen over the grill), so I soaked it down really good with engine cleaner and sprayed it out with a hose. No difference.

Either I can buy some decent gauges for better diagnostics or I can pay my shop. I'm really liking box number two with the temperatures lately.

geordi · **Posted:** Mon Jul 02, 2012 11:39 pm

15lbs on the low side is a bit low, the low side sensors generally cut the compressors off below 20 in my experience. It does seem that the temperature is the cause here, and just pushed the high side over that sensor's limit.

If your shop tests it and thinks that the high side pressure is acceptable, then they probably will suggest to replace the sensor.

Sir Sam · **Posted:** Tue Jul 03, 2012 1:01 am

The service manual has the connector pinouts mislabeled. The "pressure" Transducer is the high pressure switch.

Pin #1, K310 20BR/DG should be the ground.
Pin #2, K301 20BR/LG, should be 5V supply.
Pin #3, K305 20BR/LB, should be the signal.

All three of these circuits run to the Gateway/Front Control Module. This sits vertically behind the drivers side headlight. As far as I can tell its sole purpose is to pass some information along on the vehicle BUS. Including AC information.

geordi · **Posted:** Tue Jul 03, 2012 2:44 am

Sir Sam wrote:

The service manual has the connector pinouts mislabeled. The "pressure" Transducer is the high pressure switch.

Pin #1, K310 20BR/DG should be the ground.
Pin #2, K301 20BR/LG, should be 5V supply.
Pin #3, K305 20BR/LB, should be the signal.

All three of these circuits run to the Gateway/Front Control Module. This sits vertically behind the drivers side headlight. As far as I can tell its sole purpose is to pass some information along on the vehicle BUS. Including AC information.

So which ones would he cross, if he was to try that? This doesn't seem like a good thing to mess with, honestly. I realize it is a dumb switch, probably shunts to ground when in the "too high" position... But from the signal?

Sir Sam · **Posted:** Tue Jul 03, 2012 10:15 am

geordi wrote:

Sir Sam wrote:

The service manual has the connector pinouts mislabeled. The "pressure" Transducer is the high pressure switch.

Pin #1, K310 20BR/DG should be the ground.
Pin #2, K301 20BR/LG, should be 5V supply.
Pin #3, K305 20BR/LB, should be the signal.

All three of these circuits run to the Gateway/Front Control Module. This sits vertically behind the drivers side headlight. As far as I can tell its sole purpose is to pass some information along on the vehicle BUS. Including AC information.

So which ones would he cross, if he was to try that? This doesn't seem like a good thing to mess with, honestly. I realize it is a dumb switch, probably shunts to ground when in the "too high" position... But from the signal?

Maybe none of them, there might not be a way to test it. Check out this video:
http://www.youtube.com/watch?v=mqMk6ILyceE

You can see the AC pressure switch volts and PSI change, none of which would be in an on/off 0 or 5V range. Best way I can think of to test it is swap it out with another. So much for that idea.

The other thing to check would be to make sure the relay and clutch are working correctly.

DarbyWalters · **Posted:** Tue Jul 03, 2012 12:54 pm

Also check to see if there is any "residue" on top of your fan shroud at the Schraeder Valve...mine had some and I found it was leaking...

Joe Romas · **Posted:** Thu Jul 05, 2012 6:26 am

How is your fan clutch? Sounds like the AC is working correctly and the ECM is protecting the engine even though it's not showing overheating. My electric fan runs long after the AC is shut off even when the engine has only been operated for five minutes and is no where near warm.
Our jeeps don't have a conventional "expansion valve" and instead is simply a restriction built into a aluminum line. These can get plugged up with gunk causing a high pressure also. So if you open the system be sure to replace that line and the dryer :JEEPIN:

AC troubles

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