Bill, As I recall there is not a low presure switch on a 72. There is however, a fuse that is mounted near or on the tube comeing out of the evaporater core. You might want to take a look at the fuse.......Good luck.....Steve
Chris, that's what I was hoping to do through the low freon switch but evidently the 72's didn't have one. I did find the fuse Steve was referring to. It's called a thermal fuse - has 3 prongs in it and plugs into a plastic housing/harness. It was mounted on the vacuum line to my wiper door actuator. The fuse is inside the housing between the B & C posts. Now all I need to do is figure out how to test it. Hopefully, that's my problem. If not, it may be the heat sensitive switch going into the back of the compressor.
Spent some time reading through the shop manuals last night which helped my understanding of the system. You know what they say: When all else fails, read the instructions. I was looking for a regular old fuse like the fuse panel uses. Boy, was I humbled.
Anyway, if you can tell me how to jump power to the clutch without the low freon switch, it would be appreciated.
Speaking of jumping things, I caught Smokey and the Bandit last night. It's still a hoot.
"There is no way, no way, you could come from my loins. The first thing I'm going to do when we get home is hit your mamma in the mouth" Sheriff Buford T. Justice
If there is a switch lead on the back of the compressor, it is a superheat switch, a fancy word for a pressure switch. it is used to shut the system down in the event of a leak, or if you are running the system in the winter in defrost mode so you don't slug the compressor with liquid and bust a bunch of expensive parts.
If you do not have any pressure in the system now, or have evacuated it but gone no further, you need to put enough refrigerant in the system to get the compressor switch over 30 psi or so. The compressor should cycle until it pumps the refrigerant out of the low side and trips the switch again.
If this works, shut down the engine and add as much of the measured refrigerant charge through the high side (liquid or condenser) as possible. Start the engine up and finish the charging through the low side SLOWLY. Be careful not to slug liquid or you'll be buying a new compressor.
To check the clutch power, test for voltage at the clutch electrical connector to ground with the a/c on. If there is no voltage, it ain't the clutch or switch. Check the compressor clutch with an ohm meter.
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
The dreaded 3 pin Thermo Limiter
So it's a fuse... and the third wire is the input from the HI temp switch on the back of the compressor..... When you go OVER pressure (superheat) that switch closes to ground...POOF goes the fuse.
( Why they didn't use a circut breaker here is beyond me!)
The Thermo limiter ( 3 prong fuse )
One of the outside lines
and the middle pin are the fuse
The other outside line is a resistor. The point of it being a resistor.. is that it's like a slo-blo fuse.... so it SLOWLY MELTS the internal fuse.
So the two outside lines are a resistor.
The pins are refered to as
On the socket, for circut numbers.
The first 59 goes to the A/C clutch
( * Note check if the center green has power...fuse out.. when you call for A/C... then if it does jump the 2 #59's and the clutch should grab.)
The middle 59 comes from the "Ambient switch" in the blower cage
[Dark green with double white stripe]
A tan line goes out from the "Ambient switch" to the A/C controller panel in the center console.
The Ambient switch will not allow power to the compressor if it see's the air temp is below 37 degrees.....
The last line 202 is the lead to the Super heat switch [Black]
So In my 73 the actual switch in the console was corroded from sitting to long.
And NO there was no low freon switch..like in the newer cars.
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
Look over the blower cage area... and find the Ambient temp switch... that's the next piece of the puzzle...... I'll bet the TAN wire does have power to it..... if not.. then out comes the center console.
Problem solved.:cool After all the checking and diagnosing this and that, I found my problem pretty much by accident. Couldn't figure out why the thermal fuse was good but there was no power to the clutch. Was checking the connector on the clutch to make sure it was fastened snugly and it grabs! Turns out the wire had come loose in a clamp connector between the thermal fuse and the clutch. Now, Xena's cold as ice. Man, it's frustrating to me to spend so much time on something like this and it turns out to be so simple to overlook and fix. Ah, the joy of owning a 30 year old car.
Let me share my A/C story. We are frantically trying to get ready for Sharkfest. Our 95 3500 dually has about 95K on the clicker. We have owned it for about two years and have hardly put more than 3K miles on it. She is loaded down with all the goodies including a/c.
I noticed after driving for a while it would stop blowing air through the vents. Turn of the air for a few minutes, click it back on, and all was well. Considering I hardly use it, I would forget about it by the time I got home.
I finally popped the hood a couple of weeks ago after about a 1.5 hour cruise (air had stopped) and found the entire low pressure side of the a/c system covered in a layer of thick white ice. Therefore, my evap was freezing over not allowing any air to travel.
Most of the time this is a symptom of low freon (freezing up). I opened the system only to find somebody else had been there and it was full of glow in the dark green die. I went through the system finding nothing wrong other than the fact the a/c clutch was ALWAYS engaged when turning on the air. No obvious defects.
So, a new drier, orphis, oil, o-rings, and hours of my time later, she had a fresh full charge of freon. Low and behold, she was never cycling. I also installed a new cycle switch, and high pressure cut out.
At total frustration I accidently (just like you) found that some prior owner had gone down about 3" in the harness for the cycle switch and spliced the wires together. Thus giving the signal that all was well all the time to the system so it kept the clutch engaged regardless of the cycle switch position. ARGHHHHHH. I could have had it cycling for ten minutes, some solder and heat shrink!
To add insult to injury. All was well, system cycling, and life was good. On the way home from church yesterday she starts blowing HOT AIR! After doing all the normal checks, I find out that the outer clutch pull ring was too far away from the pulley. I closed the gap, and it seems to be cycling and working now.
BUTTTT, on the way into work with it this morning, the brake lights quit working (fuses and relay good-acts like a bad ground).
Man! Some things just won't work with me!
Glad you found it. Like with me finding the spliced wires, it is a relief to be a simple problem, but you kick yourself in the butt for not finding it sooner.