A/C Idle Fluctuation with AEM EMS - Compressor Clutch Relay Mod

When using an AEM EMS on a MK3 Supra (7M or 2JZ swap), the A/C idle-up compensation does not work properly because the AEM receives its A/C signal from the dash control unit rather than from the compressor clutch. This means the idle-up stays on constantly whenever the A/C fan button is pressed, even when the compressor clutch cycles off. The idle learned value then compensates downward, and when the compressor clutch re-engages, the engine stalls or drops RPM severely because both the learned value and the load comp are fighting each other.

The AEM EMS A/C request input (pin 10D on the 1130 box for 89-92 Supras) is wired to the A/C control unit in the dash, which sends a constant ground signal whenever a fan speed button (low/medium/high) is pressed. This signal does not reflect the actual state of the compressor clutch, which cycles on and off based on refrigerant pressure, water temp, and cabin temp as controlled by the A/C amplifier. The idle learned value compensates for the constant idle-up when the clutch is off, then there is insufficient idle support when the clutch re-engages.

Moderate confidence — Multiple community members agreed on this solution