Air Conditioner not working... Need your electric smarts!

OK, 97 XJ, 4.0, lifted etc. A/C has worked perfect for last 2 years I have owned with no freon added. I have abused her fairly hard off-road this winter.

Problem. A/C not working.

I assume it's electrical since when I used to turn on the a/c or defrost (I know that is the same) the fan would run and the a/c clutch would cycle as needed. And last fall it was ICE cold. Now, it does NOTHING when the A/C is turned on, in fact, the fan will NOT come on even if I get her hot (hit 215-220 other night never came on).

I checked the fuses in the pass. kick panel and Power distrubition center, even bought a new "little" relay for the a/c in Power distrubition center, didn't work.

What to try or test next?

Am I correct in assuming that it has to be electric, and not a freon leak?

Thanks for your help!
Jason

If your system is not filled properly, the low-pressure switch will not let the clutch engage, and the AC will not work. That switch could be bad, the clutch could be bad, or you could be low on refrigerant.

Concur. Start by checking continuity through the LPCO (Low Pressure Cut-Off) - which should be mounted on the receiver/dryer. It looks like an oil pressure switch.

Problem is, if there's no continuity there, it's either that the switch is shot, or that you're low on refrigerant - and you can't check that without manifold gages. If your refrigerant is fine, then you'll likely need to purge they system to replace the switch - which means that a retrofit may be in order, if you run R-12 (there's usually a sticker somehwere underhood if you run R-134a. Since you have a 1997, that is a possibility.)

If you run R-12, the cost to refill will be prohibitive. If you run R-134a, it should be rather cheaper. There's another option if you run R-12 - but I don't recall what it is offhand. However, it's a refrigerant that is supposed to be compatible with both oils, and run a lower head pressure, and be somewhat less than R-12 and comparable in proce to R-134a. I haven't looked in a while, but five minutes' work with Google should turn it back up (I just don't have a need to at the moment.) However, it would significantly reduce the cost of the conversion, because you wouldn't have to replace half of the system and flush out the other half...

5-90

I have a 97, I assume it 134.

I will check the pressure... I have walmart gauges! LOL, they work.

Check the LPCO switch first - if it's closing to allow current to close the compressor clutch, the refrigerant fill is fine (and the LPCO is easier to get to, and you don't have to touch the valves.)

Check the pressure if the LPCO isn't closing. Like I said, there should be a sticker saying you use R-134a - and the service valves are also different. R-12 gages will not directly fit R-134a service valves, and the other way 'round.

5-90

OK, what is the best way to check the LPCO switch?

Yes, I have a voltage tester.

Thanks allot, I have a feeling this will save me LOTS of money from what the shop will charge!

Hmm, I've got a 1997 FSM here somewhere...

There are apparently TWO switches - a LPCO and HPCO (High Pressure Cut-Out.)

The LPCO is the one with Light Green and Dark Blue/Orange wires going to it, and the HPCO has the Dark Blue/Orange from the LPCO and a Dark Blue/White wire for its output.

Both switches have to be closed to "make" the circuit - meaning that there's a range of pressures, and having too much will probably blow a compressor head seal.

The Light Green wire should be powered with the AC Selector Switch in any AC position. The Dark Blue/Orange wire will be HOT at the HPCO switch is the LPCO is closed, and the Dark Blue/White wire will be HOT if the pressure is below the maximum (usually, this is done to protect the compressor from failure due to system overheat.)

In summation -
LTG HOT w/AC ON - Select Switch CKT OK
DBL/ORG HOT w/AC ON - LPCO Closed, system pressure above minimum (not underfilled)
DBL/WHT HOT w/AC ON - HPCO Closed, system pressure below maximum (system not overfilled or overheated.)

5-90

5-90,

OK, I really appreciate your help. I don't really understand how this A/C system works! I have built entire cars (Factory Five racing Cobra replica W/O A/C!) and rebuilt engines, installed lift, etc, etc... but never messed with A/C.

I did not start jeep, but turned ignition to on. Turned A/C knob to on.

I unplugged each connector to test, here is what I found:
Light green wire, no power.
Dark blue/orange wire has 12v.
Dark blue/white wire has 12v.

Now that I typed it out, I am wondering if it should be running, and then I am wondering if the connectors need to stay connected and checking voltage by piercing wire???

Sorry you have to dumb this up so bad, but please tell me what I need to test and how.

Again, thanks for your help! I hope I can some how return favor someday!
Jason

That's interesting.... (Translation - Bollocks! Something's blown up good, or I've misread the manual!)

I find it interesting that you're getting power everywhere BUT the feed - unless I misread the wiring diagram (which is always a possibility.) I'm not entirely sure where the power feeding into the circuit is - but it looked like it was from the AC Mode Select (the knob in the dash) and read by the controller.

Assuming that I've gotten things the right way around...

You can check for voltage at the LTG wire just by disconnecting it and probing with a meter - unplug it from the LPCO switch.

Check for the next step (+12VDC at DKB/ORG) with the LPCO connected, and test at the HPCO with the unit disconnected - probe directly, the same way you did for the LPCO.

The third stage (DKB/WHT) will either require piercing the wire, or just going to the other end of that wire and disconnecting/probing it there. Note that it may not actually BE necessary to pierce the insulation tho - "backprobing" is an option if you can get to the wire terminal somehow. For instance, most MOLEX and Delphi Weatherpack terminals have open or semi-open (not moulded shut) backsides - you can either use a backprobe adapter (essentially a very stiff, short bit of steel wire/rod) or a straightened paper clip, or even a straight pin - as long as you can get it into the back of the connector to touch the wire under test, and NOT have it touch anything else (like the ground path, for instance... SHORT CIRCUIT!)

Basically, you should end up seeing +12VDC at all three stages. If you don't get +12VDC at one end but at the other and in the middle, that switch (the last one in order as you test) is open, find out why. That's when you get out your gages to back-check, but at least you'll know what you're looking for, then.

Battery power is fed IN to the blower motor housing, through the blower motor relay contacts, the blower motor itself, THROUGH the selected resistor segment in the "speed pack", into the fan switch, then the AC Mode Select switch, through the LPCO, the HPCO, and then to the PCM, C22.

Here are the wires in order, if you'd like to check. I'm starting iwth the PDC exit, and going through the circuit...
RED/LTG (12) from PCS, fuze 6 (40A) to the HVAC unit housing, CKT 8. The blower motor relay seems to be internal.
Either one of the four following wires, depending upon the speed selected -
BLK/TAN (12) - HIGH Speed Ground Path
LTB (14) - #3 Speed Ground Path
LTG (14) - #2 Speed Ground Path
TAN *14)- LOW Speed Ground Path
Through the Fan Speed Select and AC Mode Select Switches (internal path) to a LTG (20) into Splice #143 (Keft rear of engine bay, above brake booster)

The three wires going into splice #143 are LTG (18), LTG (20), and LTG (20).

Branch out of splice #143 into a LTG (20) wire to LPCO switch.
Through LPCO switch via DKB/ORG (20) to HPCO.
Through HPC switch via DKB/WHT (18) to A/C Switch Sense Input, PCM, ckt C22, connector C3 (grey shell, bottom right looking INTO connector at terminal end, bottom LEFT looking into back of connector - like it's still hooked up.)

I'd probably start at the PCM sense signal (the last one I gave you) and work backwards until you see +12VDC - the you'll have a better idea what to check. Or, you'll be able to PM me with more specific information, and I can take what you gave me and have a shufty at the FSM again, with some more information. The numbers in parenthesis after the colour codes are the wire gage/size - to give you a better idea if/when you see more than one wire of that colour.

5-90

stick a paperclip in the l.p.s. and see in compressor runs if it does u need r134

5-90 Not to say they are the same however on my 2000 the dash knob compleats the ground path to actvate the compressor. I found out when adding a switch to alow me to run the vent/floor setting in the winter months with out having the compressor cycling on & off.

Red97XJ said:

I don't really understand how this A/C system works!

Click to expand...

A/C is the process of the evaporation of a liquid into a gas, which removes heat with it. Ever wonder why if you have two boiling pots of water, one with a lid and one without, the one with the lid boils faster? It's due to the fact that the one without a lid allows the evaporated water to carry heat away, thereby lowering the temperature, or in this case slowing the rate of heat buildup. Swamp coolers work well in dry climates, and not so well where it is damp and humid.

In theory, it works the same way in all A/C systems. A refrigerant gas is compressed and drawn through a condenser coil (the radiator looking thing just behind the grill) where heat is exchanged and pulled away by a fan. A cooler gas reforms to a liquid and is carried to an evaporator coil (inside where you sit). As if passes, the cooled liquid absorbs heat and reaches its boiling point and evaporates, carrying heat with it. The low pressure gas returns to the compressor to be compressed and the cycles begins again. A/C is more involved than that, but you get an idea now. In buildings, the system must be sized correctly, taking into account the occupancy (how many people and how much heat all the bodies give off), windows, walls, ceilings, insulation, orientation to the sun, etc. to work correctly. See, unlike refrigeration, A/C, or air conditioning, does exactly that- it conditions the air by making it cool and dry. Too much A/C and the air cools too quickly before the system can dry out the air leaving you feel clammy. Too little and you are wasting energy not conditioning the air to feel cooler.

There's plenty of related sites on the web that explain this better than I can. Maybe someone else can explain this better.

I would test for continuity with the power off. Both switches should be closed. LP switch opens at pressure drop and HP switch opens on pressure rise.

Which will check the switches (and that should be done anyhow!) but not the wiring or the rest of the circuit. That's why I'd offered up a pattern trace - so he can find the fault - in case it's not a switch - and nail it down fairly well.

Irritating when a component tests sez everything's working fine, and you still can't figure out what's going on...

5-90

I wanted to pick this thread back up. It seemed like a pretty good Aircon thread but did not have a ending.
my Jeep is a '98XJ SE 2wd, used to have a 6cyl auto, now it has a Kubota with a '94-AX15 with a '99clutch pedal set. I have a pretty bastarded electrical system with a slightly hacked up harness for a little Kubota Turbodiesel. I had AC for a good bit during the winter. I just recently lost power to the compressor. Not sure when it happened, maybe after I had the compressor loose and moved for Engine mount work last week. But now No AC.
E-fan is working with the temp sender disconnected. LP switch show open, I have not tested the HP switch. I have 12vdc to the low pressure switch, it feeds thru to the HP switch. with the aircon switched on and both HP and LP switches jumpered the Efan comes on, but no power to the compressor. If I jump the compressor to 12volts the compressor comes on just fine and the engine loads up.
I don't have a good set of AC gauges anymore. but my little walmart Freon refill shows about 25-30psi on the low side with the engine shut off.
I tried to jump pin02 on PCM connector C3 to ground, at the connector, not the PCM. that did not work either.
I am about to go to HF to get a cheapy set of AC gauges and some more Freon. I am hoping that I had a slow leak over the winter, and a Freon top off will get me cold again. or at least help me trouble shoot the electrical problem that is keeping the compressor from turning on. it is air-conditioning time in South Texas

I got sorted out, I had a compressor signal interference from tapping into the aircon circuit to power some other "ignition on" components

Is there a build thread on your Frankenstein XJ? sounds pretty cool...