OK..I have a 93 XJ with the R-12 system. Bought the adapters and new freon/oil to run R134A in it also a new drier and pressure switch..Anyway..The compress will only kick on if you unplug the low pressure switch and jump the 2 terminals in the plug. When this happens the electric cooling fan also comes on. But more importantly when I attempt to put any freon in it the pressure gauge on the can instantly goes up into the red zone saying the pressure is way to high even though there is no freon in the system at all.

So....Is it just acting wacky because there's no freon and I should continue to pump it in? Or is it possible the compressor is bad even though the clutch engages if you jump the pressure switch plug? And why is the cooling fan coming on when I jump the ac low pressure switch?

 

You should be able to jump the low pressure switch and continue to fill. The fan comes on because it's always on when the compressor clutch is engaged. You should flush the entire system first, especially since you're switching over from R-12. This removes the old oil, which is not compatible with the new oil, and cleans out any gunk accumulated over the years. It is also advisable to draw a vacuum on the system for at least 1/2 hour to remove any moisture in the system. You can do it without drawing the vacuum, but your just asking for trouble down the road.

 

Are you using a/c gauges with high and low side reading or just low side? Did you pull a vac to the whole system? It the freon actually drawing in or is the can pressure forcing in in?

There is a possible of things, one is low vac and leak or no vac at all. Two pressure switch bad but when jumped fan turns on so possible not. Three compressor is bad , smack the top of it slighty and see if it cycles on that's if you have a can into the system already , if not your just hitting it. The clutch should cycle with one can in the system no matter what even if its for a couple seconds.
To me its seem like its a possible no vaccum and compressor is starting to crap out but with out the gauge reading it hard to understand.

 

Did you open the low side valve on the compressor so refrigerant can flow into the system? About one turn or so clockwise, no more. Too much and you will close it at the other extreme.

As stated, if you haven't already, you really need to vacuum the system first. I had a shop do that when I converted mine.

 

Well that also depends on which compessor and what the valve look like too. Are they the older style one where you use a 1/4 wench to open the valve. But yes you are correct all the way in the isolate the compressor from the rest of the system, half to fill, and all the way out for normal function.

 

I don't claim to have an overview of systems by year. My '85 had the valves as does my '93. I have no idea whether all '93s have them. I know they did away with the valves at some point.

My first post wasn't clear: '85 XJ was what I converted a long, long time ago. '93 was already converted when I got it. Just have to add a bit of 134a every year and a half or so.

 

hbracing, you can probably answer the basic question relating to what the single ended gauges that come with the 134a kits will show if you attempt to fill the system with the valve closed (assuming that there is a valve).

I have the idea that the gauge will show the high pressure reading the OP mentions but may be wrong about that. I've spent more time than I would have preferred messing with those damnable cans of refrigerant through the years but am not absolutely sure about this point.

 

Well it all depends if he has the valves or no. It could be that they are closed, or the compressor is isloated, where it not even entering the system or there is a block in the system thru either the orfice tube, dryer or even compressor. Need more into possible pictures of the high and low pressure side, that will show if the valves are present.