The clutch draws bookoo current. You can't be timid when you run power to it. It will spark, then engage if it's working. Hold it on there a few seconds to see.
If it looks like it's engaging, try it again with the engine running.
If the compressor itself is frozen you'll know it - unlikely.

If that works, but bypassing the CONNECTOR on the low pressure switch doesn't, swap the relay with the horn relay. Check to make sure the wiring is OK too - And remove the top of the relay box and check the wiring underneath the sockets.

The low pressure switch signals the computer, then the computer closes the relay. The relay then activates the clutch.
It's a very simple system.

Even with no freon, bypassing the pressure switch should operate the clutch, if it's going to work at all.

 

Ok here's what I have figured out. My xj is a 99 4.0 automatic.240,000 miles. There was a grey plug which I think is a single wire ground, it was bad, so I fixed it. After doing that I have found that bypassing the low pressure switch does engage the compressor clutch. I went and bought a set of gauges off of Craigslist for $20(us general). Here are the readings. Jeep off, low pressure 30, high pressure 28. Jeep running compressor on, low -15, high side 52. I have found after putting in some freon the compressor cycles the clutch on and off every 15 seconds. Also when I took the readings, it was at idle( I forgot to rev the motor). I'm thinking that I have a blocked practice tube.? Does this sound right? Do I have to pull the dash to get to it?

 

Sounds like it's working.
Just one can can give you 30 static - just shows it has some in it. Can pressure is 30 lbs.

If the orifice was plugged, the low side would be much higher (cannot suck), and the high side lower (doesn't get through the orifice.) BTW - Orifice is not under the dash!

Suction is good, but high side isn't high enough.
How many 12 oz cans did you put in? Sounds like it needs more.

Put a thermometer in a vent inside, close windows, AC High and recirculating.
Watch the temp - as you SLOWLY add it will drop, when it gets low, then starts back up STOP! It's full. Depending on the ambient temp, it may cycle on and off a little - after it's down to temp.

 

I put in a half of a small can with a cheapy autozone charge kit but It seems like the can stops putting in freon, but still feels like the can is half full. Now that I have a set of real gauges should I try another can with it. I'm not real sure how though.

 

It seems you've had a leak at some point. When that happened, not only did you lose freon, but you introduced regular air into the system.

Before you put any more freon in, find the source of the leak. Once that is fixed, do not just dump a bunch of freon in there in hopes of having a properly working AC. To function correctly, the system needs to be vacuumed down to remove air and boil off damaging moisture that the air carried into the system. After it has been vacuumed down, you can then put in a proper charge of freon.

Failing to follow these procedures will lead to poor performance at best, and compressor/system failure at worst.

 

You can sometimes rent compressors and gauges at your local auto parts store to do this.

 

Also don't forgot to add oil not even freon can comes with oil 2oz is all you need most of the time and keep checking your gauges or high pressure spikes, you'll want to low around 35-40 and your high at around 200 but it also depends what the weather look like too, so what the temp is outside times is by 2.2 or 2.5 and that should be you high side reading

 

How do you add oil? After a vacuum, is all the oil out? If I use uv leak finder, will I see it if it's the parts under the dash? Sorry for all the noobness, I would rather do it myself and just buy the equipment.

 

Vacuuming the system down will generally remove moisture/water, but not affect the oil so much. The physics works like so:

Water boils at 212 degrees at 14 psi (atmospheric pressure). If you drop the pressure (by creating a vacuum) you also lower the boiling point of the water. Once it boils and turns to vapor, the vacuum pump will suck it out of the system. Now, the oil is subject to the same physics, but has a much higher boiling point at normal atmosphere. From what I've been told, you will lose some of the oil, but not all of it during a vacuum operation. However, you will have lost an unknown amount of oil just from having the leak. I'd ask an AC expert, but I'd think adding 2-4 ounces of oil back in wouldn't hurt.

You can use UV leak finder and a blacklight, but it may be easier to use a sniffer, especially if its leaking under the dash somewhere. Some auto parts stores will rent a freon detector. From my experience with vehicles of all kinds over the years, leaks are usually (in order) in the condenser coils in front of the radiator, at the compressor itself, or at a tube junction or sensor bung. These areas are most subject to flying rocks/debris, heat/vibration and pressures. It's certainly possible, but I've found very few leaks in the evaporator coils as a general rule. They are usually protected by the dash, and are on the low pressure side of the system, which works in their favor.

 

Ok so how do I check the oil amount? And, how do I put it back in?

 

There is no real way to check the oil level as it is spread throughout the system, and, depending on the location and severity of your leak, varying amounts of oil may have been lost.

The oil is generally added to the ports on the top of the compressor. You'll need to take the twin hose assembly of the top to put the oil in. Go very slowly as it can take a few minutes for it to work its way into the compressor.

While you cant directly check the oil level, there are some guidelines. Look at my profile under my Cherokee album. In there you'll see a screenshot from the factory service manual for my 98. It generally recommends adding one ounce of oil if you've had a leak (correcting my earlier statement) and gives a chart of how much to add if you actually replace different components. Also note that once you vacuum it down, the system freon charge is 1.25 pounds. I also put a screenshot of the FSM leak testing procedures in the album.

Hope this helps.

 

If the system was not opened, and even a little bit of freon was still in it - IT'S IMPOSSIBLE FOR AIR OR MOISTURE TO GET IN! The residual pressure, anything even as low as 1 lb, won't let air in.

The gauges read lbs ABOVE atmospheric.

 

Ok new dilemma! The pressure had previously dropped to zero on the low side. So it probly does have a leak, but it does hold some pressure the last couple days. So when I got home today I started the process of charging it again. I had the low pressure switch wired so the clutch would kick in. My low side had went from a -20 or so up to zero and the high side was about 90psi. Everything was going good then the compressor kicked off. The electic fan was still running. Now I can't get the clutch to start again$@@$&&! WHY? I checked the fuse and the relay. Does it do things like this if there is air in the system or maybe kick off cause of some other reason.

 

by passing the presssure switch clutch should stay engaged no matter what pressuers there, check for current at switch plug with ac on key in run position yet?

 

Yep, 12v at the switch. I went back out after it cooled for a bit and it worked for a minute then stopped again. Could it be week and needs a shim removed?