Ac trigger signal to ecu

[ottawa coupe]

I have a very weird question regarding the function of ac in b3 and 4 and how it interacts with the ecu.

I am using b4 manual ac in my b3 coupe 4.2 swap. In the b4, there's a control module (relay with logic lol) that controls the cycling of the compressor. It takes up 2 relay positions. In my digging thru diagrams, i noted that the connections to the ecu, diag port and compressor speed sensor are the only ones in the second spot. So, i found the correct "dumb" relay that will work without that extra stuff. The reason is the the compressor on my 4.2 does not have the speed sensor and experience in my s6 is that it doesn't like working without it.

I would lose any diagnostic capability for the ac system but being manual... it either has refrigerant or it doesn't lol.

What i DO consider an issue is that i lose the trigger to the ecu to tell it the ac is on. I would like to retain that for smooth running. Will that just be a simple 12v signal thqt i can steal from some other means?

[johne]

The "trigger" to the ECU is just a signal to raise the idle speed by ~200RPM, nothing more. It doesn't affect any other of the ECU's running parameters. If you wired the trigger input permanently high, the only consequence would be that your idle RPM would rise.

Equally, if you leave it unconnected the only consequence is that when the AC compressor kicks in the idle will drop momentarily but then return to the normal 650RPM or so and you'll have less cooling from the A/C when stuck in traffic.

Your call but it's a complication you can easily live without.

[ottawa coupe]

Yeah, i am aware it's a minor thing. That said, if i's just a 12v signal, i can simply wire it into the ac button.