AC Compressor Clutch Won't Engage - Wrong Water Temp Switch (Sender vs Switch)

Electricalintermediate✓ OP Confirmed

Problem

After installing AC components on a 1989 MK3 Supra, the AC compressor clutch would not engage when the AC switch was pressed. The AC amplifier was not providing the ground signal to the AC clutch relay coil.

AC compressor clutch does not engage when AC is turned onAC clutch engages when jumped directly to 12V+AC clutch relay closes when manually jumped to 12V+ and ground12V+ present at relay connector pins when blower is onDual pressure switch tests closed with proper continuityGround pin on low current side of AC clutch relay is not groundingAC amplifier not outputting ground at pin 7 (BR wire) to relay coil

Root Cause

The AC water temperature switch (mounted in the engine, connected via light green with red stripe wire to AC amplifier pin 8) was actually a temperature SENDER (variable resistance) rather than a temperature SWITCH (simple open/close). The sender was always providing a ground signal to pin 8 of the AC amplifier, which told the amplifier the engine was overheating (above 234°F). This caused the AC amplifier to block the ground output at pin 7, preventing the AC clutch relay from energizing. The AC water temp switch must be a Normally Open (N.O.) switch that only closes when coolant temperature exceeds 234°F.

Confidence

High confidence — Original poster confirmed this resolved the issue

View Original Thread →Posted February 2022 on SupraForums.com