Not good at ac I need help

[Golden1973]

I've been restoring my grandpa's 1973 Chevy c20 slowly as a senior in high school since my freshman year when I pulled it out of the trees. Everything works swimmingly as it is built square body tough except for the AC. I've looked everywhere for help and replaced the resister, rely, and switch. The cool box was rebuilt and working, but the AC blower and refrigerator won't turn on anymore. I don't know where to look and just need help at a place to look at.

 

[Radiohead]

What are the pressures? If you jumper the low pressure switch on the receiver drier the compressor clutch should engage. Don't run it in a low charge condition for long or the oil won't be moving properly. Off the cuff it sounds like a leak developed, or if recently serviced perhaps over charged. AC is science but not that complicated, just a few hard physics rules and the equipment peculiarities.

 

[Snoots]

If it was in the trees, pull the blower motor, clean all the junk inside and hook the blowermotor straight to the battery. If it works, You can troubleshoot backwards. The '73 model IIRC, had a glass fuse near the junction terminal on the firewall.
You going to change to R134a I hope. More work and parts will be needed.
Keep us posted.

 

[Golden1973]

What are the pressures? If you jumper the low pressure switch on the receiver drier the compressor clutch should engage. Don't run it in a low charge condition for long or the oil won't be moving properly. Off the cuff it sounds like a leak developed, or if recently serviced perhaps over charged. AC is science but not that complicated, just a few hard physics rules and the equipment peculiarities.

The pressure is fine I think when I am switching the AC switch you can here the pressure move or sounding and when adjusting the HOT/COLD switch. Is there wiring that goes to start the cool box too? I know the blower motor should always run but it runs when I put it straight to a battery so I dont think the motor is the problem on the no air end of things. Is there anyplace online I could look for the wiring diagram.

 

[Turbo4whl]

Was your truck built with AC from the factory? If so it would have a POA system, truly a very cold system when operating correctly. Being a 50 year old truck, pictures of the AC pump and pressure lines will help us, to help you. The system could have been changed or added on.

The wiring to the pump has a diode in the power wire (at the plug) to protect the switch contacts. They fail. Testing with a jumper lead to the battery to the clutch and a second wire to ground the clutch.

 

[Jgonick]

Wiring manual LINK

Service manual LINK

 

[75gmck25]

My '75 has a CCOT system, so I'm not as familiar with the POA systems they used 73 and 74.

You may want to check the discharge pressure cutout switch, which should be mounted into the hard line near the A/C box and blower motor. This switch prevents the A/C from turning on if pressure is too low. You can pull the rubber switch plug up and you will see two terminals inside.

If you use a couple of spade terminals and a short wire you can make a jumper and bypass the switch, and the A/C will compressor will come on even if the pressures are low. You don't want to leave it that way for long, but if you do jumper it and you have the A/C controls on, you should hear the A/C compressor click on. Note - It's common to bypass this switch when recharging a system that is very low.

- Leave the A/C fan switch in one of the positions lower than max, and you should have multiple fan speeds.
- When you switch the fan to max, or switch the A/C to recirculate, it activates the A/C relay under the hood and powers the fan's high speed directly from the power junction on the firewall. That fuse may be blown, or the relay may be faulty.
Note - The lower fan speeds are controlled by the blower resistors, which are mounted near the fan relay.

On my '75 the recirculated air function and max fan speed are controlled by the temp slider. If you slide it almost all the way to the left you will feel a small indent where it sticks. That is normal A/C at max cold. When you slide the temp lever a little more to the left (1/4"?) so it is as far as it can go, that makes the system use recirculated air, and also sets the fan speed to high. If that control is working properly you should hear the fan go to high speed, and if you look at the passenger side kick panel you will see that flapper open up. This is where it pulls in air to recirculate.

The evaporator temp control has a thin metal probe that sticks through the evaporator. If the evaporator gets so cold that it might start forming ice on the evaporator, that temp switch turns the A/C off.

 

[Golden1973]

Thank you all for the help I've followed the wiring diagram from the post and I have 12v up to the ac relay that I have already replaced. There is power coming in but I cant read any power leaving.

 

[Turbo4whl]

Thank you all for the help I've followed the wiring diagram from the post and I have 12v up to the ac relay that I have already replaced. There is power coming in but I cant read any power leaving.

So the switch in the heater/AC control needs to switch power to turn on the relay. Fuse for AC/heater? Next pull the dash bezel and loosen up the control at the dash. Move it out enough to check power in and out from there.

 

[Radiohead]

I love it when someone more eloquent than I can walk someone through something I'm having issues just putting into words. There's a few of you here and you are appreciated.

Thanks for the effort.

 

[Radiohead]

Thank you all for the help I've followed the wiring diagram from the post and I have 12v up to the ac relay that I have already replaced. There is power coming in but I cant read any power leaving.

Lol, I'm stealing that power to fire the switched electric fuel pump under the hood, so I can waddle Pearl around as needed until I redo the fuel system proper.

 

[Toad455]

My '75 has a CCOT system, so I'm not as familiar with the POA systems they used 73 and 74.

You may want to check the discharge pressure cutout switch, which should be mounted into the hard line near the A/C box and blower motor. This switch prevents the A/C from turning on if pressure is too low. You can pull the rubber switch plug up and you will see two terminals inside.

If you use a couple of spade terminals and a short wire you can make a jumper and bypass the switch, and the A/C will compressor will come on even if the pressures are low. You don't want to leave it that way for long, but if you do jumper it and you have the A/C controls on, you should hear the A/C compressor click on. Note - It's common to bypass this switch when recharging a system that is very low.

- Leave the A/C fan switch in one of the positions lower than max, and you should have multiple fan speeds.
- When you switch the fan to max, or switch the A/C to recirculate, it activates the A/C relay under the hood and powers the fan's high speed directly from the power junction on the firewall. That fuse may be blown, or the relay may be faulty.
Note - The lower fan speeds are controlled by the blower resistors, which are mounted near the fan relay.

On my '75 the recirculated air function and max fan speed are controlled by the temp slider. If you slide it almost all the way to the left you will feel a small indent where it sticks. That is normal A/C at max cold. When you slide the temp lever a little more to the left (1/4"?) so it is as far as it can go, that makes the system use recirculated air, and also sets the fan speed to high. If that control is working properly you should hear the fan go to high speed, and if you look at the passenger side kick panel you will see that flapper open up. This is where it pulls in air to recirculate.

The evaporator temp control has a thin metal probe that sticks through the evaporator. If the evaporator gets so cold that it might start forming ice on the evaporator, that temp switch turns the A/C off.

I had a 73 with CCOT sytem also, I think POA was the previous body style.

 

[Toad455]

I've been restoring my grandpa's 1973 Chevy c20 slowly as a senior in high school since my freshman year when I pulled it out of the trees. Everything works swimmingly as it is built square body tough except for the AC. I've looked everywhere for help and replaced the resister, rely, and switch. The cool box was rebuilt and working, but the AC blower and refrigerator won't turn on anymore. I don't know where to look and just need help at a place to look at.

What part of Texas are you in?

 

[roundeyed sqbdy]

1) order Vintage air complete system.
2) remove entire old antiquated and constantly failing factory air, box, evaporator, condenser.
3)install complete vintage air.
4) say Ahhhhhh!

 

[roundeyed sqbdy]

Cool and clean.

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