A/C thermal switch hook up?

[N2TRUX]

I had to replace the Thermal switch on my 77 a/c system. This is the little control box that goes next to the dryer canister and hooks to the pressure switch.
I got everything hooked up but the two wires that feed it. Since this switch is totally different from the old one, and there is no markings I have no idea which wire go where.

Since the switch was 28.00 bucks, I don't want to fry it. Does anyone have a diagram of this switch? I have searched the net, and this site but can't find what I need.

[N2TRUX]

Anyone?

[Grande Rojo]

Ken, sorry I'm no help but look at the bright side. You have a 50/50 chance on getting it right.

[staggerd86c-10]

usually those switches have a positive and a negative, i dont think you can get it wrong. thats what i would do but dont quote me on it.

[f.monroe]

I may be wrong but isnt the switch just a make or break switch ?I think it is just designed to pass through the switch so it shouldnt matter which one goes where (2 wires right). Is your AC working without the switch hooked up ? If so are they just jumpered ? If the last two questions are answered yes , polarity is not a problem . If I am correct , the box you are referring to is just a thermal switch to kill the compressor when the evap temp gets too cold .Same as a low pressure cycle switch on newer systems which are inline switches and not polarity specific.
Frank

[N2TRUX]

Frank- I think you are on to it. I have tried to locate a diagram, but no luck. It's been many years since I did A/C installs and just can't remember the electrical part of the system. I do know that it is a normally open switch and one side goes to the pressure switch. I haven't traced the other side because it disappears in a taped up harness.

The compressor does not come on with out the wires hooked up. I'll just hook it up and see what happens.

[f.monroe]

Hook em up quick . Its getting hot in Texas.
Frank

[westofb]

So that is what that little thingy-ma-bob is called, I think mine may be not long for this world. I have replaced almost all the a/c parts on my 75 and a/c works great in town, get it up to highway speed and the compressor kicks out. My truck is running 2400 rpms at 65 mph just fyi. I looked all over the net and could not figure out what it was called, does it kick out the compressor if it gets too cold? I think this is what you are talking about, it is near the drier mounted on the side of the case where the evaporator is located....it is wired in series with the high pressure cutout switch, the in and out wire are the same color, green I think. I need to get to the shop and hook up a set of gauges to it and see what kind of head pressure I have at elevated rpms...you can be sitting still and rev the motor to a constant 2400 and it will also kick out the compressor.

Sorry about thread jacking, but I agree with f monroe, for what its worth, (if we are talking about the same switch) mine is wired with the truck side of the harness to the prong closest to the firewall, and the farthest one from the firewall runs to the the high pressure switch, like I said above the wire is the same color on both sides of the switch and remains same color to both sides of the high pressure switch.

[Desert1957]

Show me some pictures, If the box has a thermal bulb that hooks to the evaporator outlet line it keeps the evaporator from freezing up. if it screws in a pressure port anywhere on the accumulator its a low pressure cut out switch. If the system gets low on freon the low side pressure can sometimes go into a vacumm. this switch senses low pressure and will either cycle the compressor out or not let it engage at all.

PS: the switch should be a normaly closed , wiring should go either way , still would like to see a pic...

Desert

[westofb]

Originally Posted by Desert1957

Show me some pictures, If the box has a thermal bulb that hooks to the evaporator outlet line it keeps the evaporator from freezing up. if it screws in a pressure port anywhere on the accumulator its a low pressure cut out switch. If the system gets low on freon the low side pressure can sometimes go into a vacumm. this switch senses low pressure and will either cycle the compressor out or not let it engage at all.

PS: the switch should be a normaly closed , wiring should go either way , still would like to see a pic...

Desert

I don't have a pic available, but its mounted in the side of the evaporator case, not in the accumulator. I thought about adding a low pressure switch in the drier to replace this switch, it is wire in series with the high pressure switch just like a low pressure switch would be. When I re-did my a/c system, I had to get a drier that was a universal fit so I have an extra low pressure port for the low pressure switch. By the way, you should see the orifice tube in these old systems, it looks like the old quadrajet fuel filter, a perforated brass piece, I never saw anything like it before. I wish I had taken a pic of it!

[westofb]

I played with my a/c system a couple days ago, and it is definitely the thermal switch, if I put a jumper wire in it, it cools great, high side pressure max's out at around 260 lbs and the low side bottoms out at 20 lbs. I went to carquest, o'reillys, and chevy dealer to get a replacement, but could not find one.

N2 where did you find one at, the parts stores could not even find it in there systems and the dealership found it but said it has been discontinued and was unable to locate one. Thanks.

[gchemist]

A/C systems use an electrical series path. The low pressure switch and high pressure are on the same power line. If either switch trips, the compressor is shut off.

[Pyrotechnic]

I believe 77 and under trucks had the thermal switch, and 78+ trucks had a low pressure switch. You can use a low pressure switch and leave the thermal switch disconnected.

When I had the lines made for my AC system, the guy sold me a low pressure switch that screwed onto a port on the dryer. Works great.

[N2TRUX]

Originally Posted by Pyrotechnic

I believe 77 and under trucks had the thermal switch, and 78+ trucks had a low pressure switch. You can use a low pressure switch and leave the thermal switch disconnected.

When I had the lines made for my AC system, the guy sold me a low pressure switch that screwed onto a port on the dryer. Works great.

This is correct. I had an automotive A/C expert tell me the same thing. The reson I didn't go this route was purely cosmetic. The switch, plug and related wiring were going to be much more visible than the wires, etc for the Thermal switch.

westofb- I found the replacement unit at CarQuest Auto parts. The one near me was once part of a local parts store chain that was bought out by CarQuest. One of the original counter guys still works there, and really knows his stuff. He was able to cross-reference my switch to a later version.

The appearance and connectors were nothing like the old one, but the internals were correct. I finally got it connected, and my local hotrod mechanic added 1.5 pounds of freon. It blows cold and cycles correctly so I am somewhat pleased.

On a side note, I need to spend more time with the system. The foam insulation has deteriorated from my ducts, so I get a loss in flow pressure. Something is wrong with my controls as well. The fans blows harder on "Normal" and high than it does on "Max"....

[westofb]

Originally Posted by N2TRUX

westofb- I found the replacement unit at CarQuest Auto parts. The one near me was once part of a local parts store chain that was bought out by CarQuest. One of the original counter guys still works there, and really knows his stuff. He was able to cross-reference my switch to a later version.

The appearance and connectors were nothing like the old one, but the internals were correct. I finally got it connected, and my local hotrod mechanic added 1.5 pounds of freon. It blows cold and cycles correctly so I am somewhat pleased.

On a side note, I need to spend more time with the system. The foam insulation has deteriorated from my ducts, so I get a loss in flow pressure. Something is wrong with my controls as well. The fans blows harder on "Normal" and high than it does on "Max"....

N2, you wouldn't happen to have the box laying around would you? I have a local carquest store, and they couldn't find it...maybe a part number will help.
I too have the problems with the foam getting sprayed all over the interior of my truck, when I had the dash apart, I cheated and used duct tape at all the joints, it blows air just great (I just hope the foam is about out of it's system, having to vaccum it out everytime I drive it).
I jumped my thermal switch, and last weekend I went to a show about 1/2 hour drive and had my a/c system act up. I think it actually froze up in the area of the orifice tube causing a blockage and the compressor to lock up. After it thawed out all was good again. I am still running the old style (original) orifice tube and suspect it maybe the cause, this week I will evacuate the system again and install a newer style oriifice tube hoping to solve the freeze up problem. I have messed with a/c systems in the past and never seen one freeze up in that area.

Thanks for the info.

[N2TRUX]

Originally Posted by westofb

N2, you wouldn't happen to have the box laying around would you? I have a local carquest store, and they couldn't find it...maybe a part number will help.

I can't believe I didn't take a pic of the box as usual, but I didn't. If you need me to call my local CarQuest and get a number let me know.