It all started with the A/C seeming to be shy of freon, not blowing that cold of air and seeing bubbles in the sight glass. Grabbed a can and slowly added until there were no more bubbles in sight glass, air was blowing colder inside , all seems well. Shut engine off, grab wife and dogs to take a putt/ test drive, start Jeep and the A/C doesnt work no fan either. Fiddle with controls for a minute, no dice, so I shut it off and go to check the fuses in the power distribution center, but before I get the cover off something goes bang about as loud as a firecracker. Since the engine was off and keys out of the ignition when it happened I quickly pulled the ground cable, thinking something shorted out. Ended up being the coolant sensor on the back of the head.Since it's a 94 with 180k on it, I figured it was just the sensors way of telling me it's old. I get a new one from Napa, start it up, and in less than 2 minutes I smell something HOT. The new sensor was starting to melt the top off again and the A/C still doesnt work. Any ideas where to start?

 

It sounds like the gauge may be fried and causing too much current thru the sending unit. To test the gauge pull the wire off the sending unit it should read lowend of scale. Short the wire to the block and it should read near full scale hot. Otherwise, there could be a short to 12V somewhere on the wire between temp gauge and sending unit.


Switch should be in the Run position to test temp gauge

 

The power for that single wire is jumped from the gas gauge IIRC, if the issue was coming from before the voltage regulator in the gas gauge you would have both gauges fried, if not more.

What part of the sensor melted? You said the top, but did the connector with the single wire also melt, or just the top of the sensor with the single metal contact?

 

Just the sensor seems to be melting . Here's a pic of the first one

 

Side question; are bubbles in the sight glass an indication of low coolant?

 

The sight glass is for the ac coolant is for the engine, 2 unrelated systems. as for the sensor and lack of ac operation, as far as I know when the vehicle registers excessive operating temp it may disable ac to prevent extra strain on the engine, if your sensor is shorting out it may cause the ac to shut off as a result of temps showing higher than they are. Just my thoughts may be wrong, if they are I apologize.

 

Since the topic was about AC, my question was regarding the same topic and low AC coolant. Question still up in the air. Sorry for the sidetrack OP. Does the indication of bubbles in the sight glass (receiver/dryer) indicate a low level of AC coolant (ie. r12 or r134)?

EDIT: Since nobody here knew, I got my answer from Yahoo; http://au.answers.yahoo.com/question...1202232AAUwe7o The OP may want to review as well.

 

That may be correct if it was the sensor on the tstat melting, but the sender on the head is for the gauge only and is not wired to anything else.

 

Thank you for the input guys! I wont have a chance to test anything until tomorrow. Something diffidently has a short. It just seems weird that it happened right after charging A/C but aren't directly related to one another. My brain thinks "Cause = Effect"

 

I put the multi meter on the wire that goes on the sending unit, and it reads 12 + volts. I turn the ign key off, still have 12+ volts. I didn't try grounding the wire as told because I feared doing more damage might occur. I thought taking the dash apart to get to the gauges and check for a short was the next good idea. Can't see anything obvious, so I disconnect the cluster and set it aside. I then checked the purple wire with yellow stripe at the connector( same color as the sending unit wire) and with no surprise at this point I have 12+ volts there too. I think it's safe to say the gauge is good, or am I wrong and that wire is supposed to be hot with the key off Does anyone have a diagram that shows if the wire is a straight shot from sender to cluster, or does it make a stop somewhere along the way?

 

Resolved . There did end up being a connection with the A/C, and a few other things too. After scouring wiring diagrams from a FSM that I downloaded I noticed that the cooling temp sensor wire makes a stop at the ignition switch. I pull the switch and test it per the FSM and discover one terminal is bad, the same terminal feeds a few fuse's on the fuse block. A/C, radio, and IIRC turn signals too. When that one terminal in the switch shorted it sent power to the temp sensor when the key was off and no power to the fuse block. Luckily the dealer near me had a ignition switch and coolant sensor on the shelf and my wife's XJ is up and running again

 

Glad to here you resolved your issue and thanks f

 

Glad to here you resolved your issue and thanks for sharing how you fixed it.