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1984 Conquista
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768 Posts
You have to have some freon in the system or the clutch will not engage.
Or you can bypass the low pressure switch on the accumulator with 2 male spades and a wire to check the clutch.
hey drb930 , looks like we posted almost the same time. i was going to ask the same but he said its already charged...but that could be it too. maybe not enough freon for the pressure switch to kick in?.
 

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1979 El Camino (yellow), 1979 El Camino (blue)
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134 Posts
Discussion Starter · #22 ·
[Edited: I looked at the start of this thread, and I did talk a lot about pressure switches. Then I found that my G-body doesn't use a pressure switch to cycle the compressor. But it could.]

Thanks for the replies. I'm not using pressure switches yet. There are locations for a low on the accumulator, and a high on the compressor discharge line, but no switches installed. I was planning on adding the switches once the AC is working.
Currently I just have the request from the AC (some kind of thermistor on the liquid line to the evaporator). The AC request goes high (12v) when the thermistor gets too warm, and I have this running to a Bosch relay. The relay takes 12v battery in, and switches it to the compressor clutch when the AC request goes high. That's the extent of my AC wiring. I think the later G-bodies use a low pressure switch instead of the thermistor to control the relay. I've confirmed that the AC request goes high when the AC is turned on, and presumably goes low when the thermistor gets cold enough.
My question is really about the 4 feet of 16ga wire from the relay to the compressor clutch, and whether that has too much resistance to run all the current that the clutch needs?

On the bright side, I've been checking the pressure in the system, and it seems to be holding. So when I return I can evacuate and charge the system, once I get the compressor clutch to activate!
 

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1979 El Camino (yellow), 1979 El Camino (blue)
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134 Posts
Discussion Starter · #23 ·
The AC is finally working! Number one: do not use a relay to send current to the AC clutch -- it will melt down / burn out the AC clutch coil. It took me three clutches from the junkyard to figure this out. The current from the AC request wire that comes from the AC evaporator box is plenty.

Also I ended up using the low pressure switch in my original diagram in the first post. This kept the compressor from running as I was adding freon, until the pressure got to be high enough. It might be a good idea to use a trinary switch here, that cuts off when the pressure is too low or too high. Or I could use the high pressure switch on the liquid line.

The Silverado manual calls for 29 oz of freon, but I figure the G-body evaporator might be smaller, so I used two cans (12 oz). Low side is 45 and high side is 275, and there is cold air coming from the vents.

Hopefully it stays cold, and nothing fails. I think I did everything right:
  • found all the leaks using pressure testing
  • pumped down the vacuum for 90 minutes and let it sit overnight
  • new AC components throughout

I'm going to take it out today in south Florida in August, and not be hot.
Hood Automotive design Motor vehicle Automotive tire Engineering
 

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El Camino Central Founder/Co-Founder N.E.C.O.A.
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Over complicating the extremely simple.

There is no "Low Pressure" switch on system, there is a compressor pressure cycling switch on the accumulator, to
prevent the evaporator from freezing up. There is a high pressure cut-out switch on the back of the compressor.
The third switch you are refering to, as a high pressure switch in the liquid line is an automatic reciculating switch, that turns on the recirculation function on the controls to close the recirculation door if it is on inside air, if the system reaches 375 psi or greater. Since your 79 does not have that functionality, eliminate that switch entirely. Always always always, use a relay on the clutch circuit, I cannot stress that enough. If you have burned out clutches, you are wiring it wrong or some other issue. Your 79 has 43 year old wiring, which equals high resistance/low voltage. You do not want to supply low voltage to the compressor clutch. That will cause the clutch to slip and smoke. Use a relay to supply it with full battery voltage whenever energized.
You gave pressures and the amount of refrigerant you put into the system, but no ambient temperature. The 79 with an A6 compressor had 3.75lbs (60ozs) of R-12. Subtract 20% for HFC134a = 3lbs or (48ozs).
Then, since you are you are using a smaller, yet higher efficiency T/A condenser, subract another 20% = 2.4lbs (38ozs). You will also want to use an original pressure cycling switch, and turn the center screw counter clockwise 1/4 at a time until it cuts the compressor out at 21psi, idle up to 1200-1500 rpm to acomplish this. The evaporator in the El Camino is larger than that of the Silverado.
I am concerned that you only have 24ozs in the system with pressures 275 over 45, something not right. Are both fans running when a/c on? There is really only one way to nail the refrigerant charge on retrofitted, non OEM system, and that is to use a thermomcoupler or a decent clamp on thermometer. You will want to measure the evaporator inlet tube (after the orifice tube on the cold side) and evaporator outlet tube temperatures, you want them equal, but within 5* is sufficient. You may also test condenser inlet and outlets looking for a 25*-65* drop. Check pressure again and provide the ambient temperature as well. I have done several LS swaps and this is how I do them without fail.

Wire as per schematic below:







The AC is finally working! Number one: do not use a relay to send current to the AC clutch -- it will melt down / burn out the AC clutch coil. It took me three clutches from the junkyard to figure this out. The current from the AC request wire that comes from the AC evaporator box is plenty.

Also I ended up using the low pressure switch in my original diagram in the first post. This kept the compressor from running as I was adding freon, until the pressure got to be high enough. It might be a good idea to use a trinary switch here, that cuts off when the pressure is too low or too high. Or I could use the high pressure switch on the liquid line.

The Silverado manual calls for 29 oz of freon, but I figure the G-body evaporator might be smaller, so I used two cans (12 oz). Low side is 45 and high side is 275, and there is cold air coming from the vents.

Hopefully it stays cold, and nothing fails. I think I did everything right:
  • found all the leaks using pressure testing
  • pumped down the vacuum for 90 minutes and let it sit overnight
  • new AC components throughout

I'm going to take it out today in south Florida in August, and not be hot.
View attachment 149316
 

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