A/C Compressor not coming on - 1970 Jimmy
 

I am having a problem getting the A/C compressor to engage. All original A/c Equipment appears clean and in good condition. I am assuming 12 freon

I have rebuilt the dash control unit and all of the ducts. All OEM and working fine. The switch turns on the blower at all levels just fine. Blower blows strong.

But the compressor is not coming on to make cold air.

The pulley spins. I have pulled the green and black connector off and when I ground the black spage, and put power to the green side, the clutch clicks just fine. So I think that is engaging.

Is there a way to test the green and black wiring to see if that is good?

There is a strange looking little jumper in the plug of the black and green connector that looks like a little resistor? (Picture posted) Cold this be fouling something up? Does not look original.

I see posts to jump the terminals on the pressure switch to see if that engages the compressor possibly indicating low Freon, but I don't know where to look for that. I have posted a picture of one switch looking think but not sure if that is it or not.

I have researched all the threads I can find as well as the internet on testing low and high pressure switch. I am not an A/C guy so I don't know where to find the switches. I see nothing anywhere under the hood, and I had the A/C blower completely out of the dash and only saw one connector on the back of the blower box that I assume has something to do with the heater?

Can anyone tell me where the low pressure switch is? There is a 5/8 ish plug on the back of the compressor but I pulled that off and it has no wires to test like I see on other internet posts.

Ideas?

Sammy

Re: A/C Compressor not coming on - 1970 Jimmy
 

The extra wire in the connector is a diode so the engine will shut off with the AC on. There is a switch under the dash by the blend door with 2 green wires, check that.

http://www.lmctruck.com/icatalog/CB/full.aspx?Page=177

number 23

Re: A/C Compressor not coming on - 1970 Jimmy
 

About that switch -- kwmech, you beat me to it! Resto-Rat, that LMC page says it's behind the glove box door, but that's a bit misleading. It's actually located behind the heater box under the dash, slightly to the right of the cig. lighter. The switch mounts with the connector prongs down and the plunger on top. The plunger is operated by the A/C and Heater outlet slider at the center of the HVAC control. Here's a pic of the switch:

https://www.classicparts.com/images/68-961-th.jpg

Re: A/C Compressor not coming on - 1970 Jimmy
 

Here's the diagram I promised Rest0-rat in his other thread. The switch is called the anti-dieseling switch and it's purpose is to prevent engine run-on as was stated.


Attachment 1799308

Re: A/C Compressor not coming on - 1970 Jimmy
 

I saw that switch when I had the blower box out installing ducts. I thought it was heater. And after I posted I thought about a LMC diagram and saw the ole behind the glove box location. It may be possible that when I put all the ducts in that plunger is jut not engaging

This looks like just a switch not a pressure switch as no Freon to it. So to bypass the switch and test it can I just jump the two prongs temporarily? Assume I can just manually push in the plunger and test for continuity?

Also the diagram of the AC circuit shows an anti-dieseling relay not a diode. Does that mean the relay went bad and someone put the diode in rather than fix the relay? If so where should the relay mount as I would prefer to put it back correct.
Posted via Mobile Device

Re: A/C Compressor not coming on - 1970 Jimmy
 

Right, it's just a simple normally-open electrical switch. When the plunger is pressed, the contacts close to send 12vdc to the compressor to engage the clutch. So yes, you can test with a jumper across the prongs. Also correct, if you push in the plunger you should read continuity.

My switch was only working part time at best. I played with it and made it a bit better, but ended up replacing it.

I'm not familiar with the anti-dieseling relay (or the diode) so will let others reply to that.

Re: A/C Compressor not coming on - 1970 Jimmy
 

Status update. After all day testing and wiring, got the compressor to engage, blower blowing (again) after a few blown fuses. Took A/C switch off and tested. All good. Perhaps just unplugging and re plugging and cleaning up some wires helped.

New it seems that the compressor is working and I heard hissing. Hose coming for the condenser is dry rotted and cracked spewing hissing noises. So leaking. Obviously no Freon. And therefore no oil in Compressor.

Looks like I can buy all new hoses for about $175.

Another $40 for Expansion Valve and Another $40 for Accumulator.

Should I just get all of it new and then find someone to vacuum it down and drive new Freon in it? Assume it is R12.

Little bit closer and thanks for all the input.

Sammy

Re: A/C Compressor not coming on - 1970 Jimmy
 

As a Professional Mechanic, I feel the need to make a post with several key notes.

1. FREON is a registered trademark of The Chemours Company, which uses it for a number of halocarbon products. They are stable, nonflammable, moderately toxic gases or liquids which have typically been used as refrigerants and as aerosol propellants.

2. FREON is illegal in many jurisdictions and may not be sold to an individual unless they are a certified and licensed. Freon MUST be recovered and stored in approved containers and MAY NOT be re-installed in automotive air conditioning systems.

3. An R134A is the current approved automotive air conditioner refrigerant, and replaces R12. An R12 to R134A conversion kit costs about $60.00 retail and should take about an hour for a qualified journeyman (with all the correct tools) to covert an automotive AC system.

4. All converted R12 to R134A ac systems use ESTER OIL for lubrication. It is also suggested to add a UV-Dye to the AC system during refrigerant recharging.

5. REQUIRED BY LAW IN MOST JURISDICTIONS - Prior to installing refrigerant in air conditioning system. The system must be tested by a fully qualified and licensed refrigeration mechanic with a nitrogen gas (N2) test, where the system holds 200psi for a minimum of 20 minutes AND the system must also pass a vacuum and leak test of 30 inches mercury (HG) for a minimum of 20 minutes. Upon completion of these test, the Mechanic will apply a certification sticker in a conspicuous location.

Good luck with your project.

Re: A/C Compressor not coming on - 1970 Jimmy
 

Not sure about the regulations in Canada, but R134A in 12 oz bottles is available over the counter at auto parts stores here. You can also get a product called Freeze 12 in similar size cans for older vehicles using R12.

Re: A/C Compressor not coming on - 1970 Jimmy
 

Quote:

Originally Posted by Stocker (Post 8295811) It's actually located behind the heater box under the dash, slightly to the right of the cig. lighter. The switch mounts with the connector prongs down and the plunger on top. Here's a pic of the switch: https://www.classicparts.com/images/68-961-th.jpg

Would anyone be able to post a picture of this switch in the installed position?
Thanks!

Re: A/C Compressor not coming on - 1970 Jimmy
 

I guess it ain’t legal but I’ve seen r12 at lots of swap meets and even flea markets. It’s still around and it works a lot better in r12 vehicles than 134. I don’t know maybe it’s just me but I really could care less about the regulations I want my truck to work as intended and don’t really care to please regulators over stupid things like this.

Re: A/C Compressor not coming on - 1970 Jimmy
 

I buy r12 on CL for around $10 a can and have quite a stock pile. Both my factory systems have been restored to original and have been blowing ice cold air for YEARS now. No reason to monkey around with anything else as you will spend more money in the end and have a marginal system at best.

Re: A/C Compressor not coming on - 1970 Jimmy
 

This is out of the manual. It's screw to the vent housing on the back side that attaches to the heater vent. You have to adjust the cable to make it push down on the switch to get power to the compressor. :chevy: