1998-01 Explorer PCM switching behavior on A/C relay, bypassing PCM due to removal, need a guru

[corerftech]

I am converting a 2001 EXP 5.0 to Carb and HEI ignition.
Reasons dont matter, Ill do a full write up of all the hurdles to make it happen once it is driving.

I have gotten to the harness work now that fuel, air, spark are viable/alive and PCM is gone. There is little to address in the conversion as 90% of the systems are gone, including narrow band HO2 sensors, etc. Engine bay is devoid of STUFF, quite nice. And all the Solid State for the various peripheral systems are autonomous, sans VSS signal sharing. But I missed one interface....

See the page below for reference. The A/C control switch in cabin, sends 12v to the PCM (high/ON) and the PCM then flips a switch internally to link the A/C relay COIL to ground through the A/C pressure control switches.
At least that is what I see, but the manual doesnt directly explain that function in a little info bubble. I make this assumption from the 3 wires merging at the PCM, 331, 347, 348.

Since I no longer have a PCM, I will need some form of SWITCH (relay) to act on the HIGH state from the panel control in cabin. That should short circuits 331 and 347 to allow the AC relay coil to energize, then be interrupted by the A/C pressure switches. So circuit 348 (Violet wire from cabin panel) should activate a new added relay to make that happen.

It appears the PCM paid close attention to the A/C relay status and modulated the TRANS EPC circuit.

I am using a Microsquirt for TCU which handles all EPC control independent (more bluntly!).

Looking for someone to verify this line of operation (as shown in the schematic), I want to get a second set of eyes to confirm a relay is in order for A/C relay activation from the VIOLET, Cir 348.

I have not included the PANEL switch, C146, page 54-2, (C) as I am hoping that you trust the fact that it is a HIGH state panel switch when A/C or MAX A/C is selected. Panel switch routes a 12V output from a 7.5A fused buss to the PCM input.

I had reviewed all of the circuits for A/C operation, Idle Stop for carb, etc...... transferring everything cleanly. But I missed this PCM switch completely. No big deal really. I think I even have a spare relay in the Power Dist box that has been put to sleep, that can be rerouted internally, easily, to add the relay. I am not an A/C whiz so need a guru on the switching logic ninside the PCM to fill in my blank.

Thanks in advance for the help.

 

[JoshT]

The AC itself is a simple system, just provide power to the right places and it works. The PCM/ECU is used to provide for things like WOT and engine load cutout. With the two extra cylinders and HP that it brings, the little extra engine load from the AC at WOT isn't going to matter much. Simply put, it sounds like you are trying to bypass the PCM/ECU to control the AC.

To straight up rewire and bypass the PCM, you would need to reroute and connect a few wires. At the relay, wire 348 VT going from control panel into PCM would need to replace wire 361 RD going into the relay, otherwise your AC would be always on. At the PCM, wire 331 PK/YE would need to be connected directly to wire 347 BK/YE.

Alternative you could stick a relay in place of the PCM and keep all the connections right there. It would need to be wired something like this.

 

[corerftech]

Appreciate the confirmation. The PCM switch behavior is as expected. A relay will be deployed.

Many thanks for the second set of eyes.