Jumping the AC compressor

Last summer I switched my 89 4.0 FI to a new painless computer. In the process I lost AC as the stock computer controls the AC switch requests, so I need to wiring in a switch to run the compressor when I want AC.

According to this diagram it would seem I would just need to run a relay and 12power on the Blue-T wire?

From what I see I would think that 12v on the blu T should do it just replace the trigger from the ECM is all you are doing and it should work?

that's what I'm thinking.

It would work but the low side and high side switches are there for a reason. Evaporator icing prevention and overpressure protection.

actually it doesn't work. I think maybe it won't kick on unless the switch are also on?

How would one go about this?

The ECU would ground the blue/tan wire to fire the compressor up, not power it..

hmm ok so I just have it backwards, will give it a shot. would like to keep the switches but maybe more work then it's worth.

Here's an expanded diagram. I've removed the circuits not related to either the HVAC or 4.0l. Sorry for the size but it's a bit easier to read. Maybe the full diagram will help more.

The Tan-T, Blu-T and Lt Grn all connect to the ECU. The Lt Grn is the AC Select (Pin D-2). Tan-T is the AC Request (Pin C-2) and the Blu-T is the AC relay ground (Pin A-12). Power for the relay comes from the fuel circuit.

When the Ign and AC switch are on, 12volts are sensed at the ECU amd the ECU grounds the Blu-T wire energizing the AC relay and the clutch engages. Once the thermostat hits it's cutoff, it open the circuit to the AC Request, the ecu open the ground, and the ac relay de-energizies and the clutch disengages. Obviously, if the low pressure switch opens, the same thing happens.

So I suspect you should be able to mimic the ECU function with a relay (or possibly two). The ECU simply detects for voltage at the 2 pins and simply makes or breaks the ac relay ground.

hmm thanks. I just was going to post an update.

I ran the Blue-t to ground.

I ran the switched 12volt power to the Lt-green wire.

Compressor now runs when I flip the switch.

I'll have to give it some thought on how to use relays as you suggested.

Sorry to hijack but I jumped the low pressure switch and turned my compressor on. It sounded like it was working correctly but still was not taking in any r134. The Jeep has been retrofitted for it as well.