e30 A/C signals to DME

[zaq123]

Does anyone know why stock DME needs two inputs from A/C system? It gets two inputs:
1. DME pin 40 from pressure switch in AC compressor - makes sense, compressor on / idle up
2. DME41 from snowflake button which also turn on aux fan..

Why AC button signal to DME? Could it be DME primitive logic to get a signal that button AC was left on (pressed ) during the key ign. ON adjust idle valve wider to compensate for the possible increased in engine load during the start up? Looks like e30 doesn't have any DME signal out to delay the AC during the start up..

Any thoughts? I'm wiring Haltech ECU pnp harness and would like to use stock harness with minimum modifications (separate sensor grounds etc)

[Panici]

Any thoughts? I'm wiring Haltech ECU pnp harness and would like to use stock harness with minimum modifications (separate sensor grounds etc)

Since you asked for thoughts, I'll mention that I used this circuit on my MS3X to kick on the SPAL Aux fan from the ECU. (Since I don't have functioning A/C in my car)
May have involved a relay for isolation, I don't recall at the moment.

Kind of nice to have ECU controlled electric fan, and also hit the snowflake button for manual fan control.

[e30davie]

I have found on my megasquirt ecu that if I tune the PID to be quite aggressively on the idle control valve it has no trouble dealing with the additional load of the AC, so no idle up necessary.

but I do have a modern AC compressor that i suspect is less effort on the engine than the 30yr old item.

Adding the AC on as an input and increasing the idle would also be quite easy at the stage you are at, and probably worth doing if you have the option. Tell it to add 50rpm to the idle when its on, and it will probably be good.

[zaq123]

I have found on my megasquirt ecu that if I tune the PID to be quite aggressively on the idle control valve it has no trouble dealing with the additional load of the AC, so no idle up necessary.

but I do have a modern AC compressor that i suspect is less effort on the engine than the 30yr old item.

Adding the AC on as an input and increasing the idle would also be quite easy at the stage you are at, and probably worth doing if you have the option. Tell it to add 50rpm to the idle when its on, and it will probably be good.

that's basically what I'm thinking to do with that input from the pressure switch. I also thinking to convert now useless 02 sensor relay to ECU controlled AC relay where it can be programmed to cut off AC compressor for the start up delay, at high rpm, WOT etc etc.. o2 relay is already wired back to ECU 55 pin witch will control its coil, just need to run 2 wires from it to brake up AC clutch wire after the pressure switch to control relay's coil + and relay contacts