I am doing an LS swap now and working on my harness. I am using the Holley brackets so I can keep my R4 compressor since I have a fully charged R12 system to put back in the car. I believe someone said you can use the 4th gen pressure switch on the 3rd gen hard lines, but I am not sure where the switch is supposed to go. Is it up top on the side of the dryer, or down on the lower hard line? I just need to know so I make the wires to the connector long enough.

 

From what I've heard is you remove the high pressure connection port on the lower line and screw on the high pressure switch from the 4th gen line. That is what I did but I don't have the car finished yet so I don't know if it will work. It should.

 

The 4th gen 3-wire pressure switch goes on the high pressure line circled here:



Then you wire it to the LS1 PCM with a relay like this:



I can explain how I did mine:
There is a mounting tab next to the blower motor relay for a second relay. I bought another GM oval relay for the AC compressor and mounted it there.

The "Hot in IGN/RUN" on the Compressor relay is connected to the blower motor relay output. So when the blower motor is energized, so was the new Compressor relay.

The Black wire from the Compressor relay goes to RED 43 on the PCM. The PCM grounds it to command the relay on/off, to turn the compressor (green wire) on/off

The Green wire gets voltage from the "hot in IGN/RUN" when the PCM trips the Compressor relay. It goes to the green wire on the stock compressor clutch connector. It also splits off and goes to RED 18 on the PCM, which tells the PCM when the compressor is on. The PCM will then bump the engine idle up to compensate for the AC load.

The Bright Green is the green (blue?) wire from the HVAC controls that originally went to the low pressure switch on the dryer can. Cut it, and extend it to the PCM. Now when you turn the AC on, instead of the voltage going directly to the compressor, it goes to the PCM, telling it "hey I want AC" and the PCM then grounds the black wire for the relay.

 

Thanks for the location.

When I remove the old pressure pressure switch, will it dump all the refrigerant, or is there a valve under it?

 

There should be a schrader valve in the fitting welded to the line. It opens with the switch fully screwed in place. Removing the switch quickly you should only lose a tiny amount of gas. Same with installing.

M

 

Thanks for the reply, I was hoping to just get some use out of it since it is still charged. If the answer was "no" I would have just run it with the stock switches and wire the clutch to send a signal to the ECM