The Wiring page for the air conditioner function answers three questions: is A/C control on, where does the request come from, and how does the ECU turn the compressor on.
| Setting | Options | When to use each |
|---|---|---|
| Air Conditioner Output Enable | On / Off | Turn it on to let the ECU control A/C at all |
| Input Mode | Digital - Switched | A simple switch on the dash asks for A/C |
| CAN Input | The request arrives over CAN, for example from a Haltech CAN keypad or retained factory climate controls | |
| Compressor Mode | Digital - Switched | A digital output (DPO) switches the compressor clutch, normally through a relay |
| CAN Output | Supported vehicles where a CAN-based module switches the compressor |
Why the ECU should control A/C
Letting the ECU control the compressor means it can raise idle before the compressor load hits, drop the compressor at wide open throttle and protect the system using pressure limits. That's much better than a switch wired straight to the clutch, which leaves the engine to cope with a sudden load it didn't expect.
Our advice
On a swap with factory climate controls, check whether your vehicle is supported over CAN before running extra wires. Otherwise, a switched input and a DPO driving a relay is the simplest reliable setup. Next, configure the air conditioner function, the pressure sensor and the max pressure limit.
