P0645 — Air Conditioning (A/C) Clutch Relay Control Circuit

Causes

• Blown fuse for A/C clutch or accessory circuits
• Faulty A/C clutch relay
• Open or short in the relay control wiring between the control module and relay
• Poor or missing ground at relay or compressor clutch coil
• Corroded or loose connector pins at relay, compressor, or control module
• Failed A/C compressor clutch or clutch coil

Symptoms

• A/C compressor clutch does not engage or engages intermittently
• Reduced or no cabin cooling
• Malfunction Indicator Lamp (MIL) or service light may illuminate
• No audible click from the A/C relay when system is commanded on
• Possible blown fuse or repeated fuse failures

What to check

• Scan for P0645 and any related codes; record freeze-frame data and attempt to command A/C clutch with scan tool
• Visually inspect A/C relay, fuse(s), relay socket, wiring harness, and connectors for damage or corrosion
• Listen for relay click when A/C is commanded on/off
• Check battery voltage and system voltage stability
• Backprobe relay coil terminals and measure voltage when A/C is commanded
• Measure continuity and resistance of wiring between module output pin and relay coil terminal and to chassis ground

Signal parameters

• Control signal type: On/off (PCM/BCM drives relay coil via ground or +12 V depending on vehicle architecture)
• Expected voltage at relay coil (when commanded ON): ~12 V at one side and controller switching to ground (or vice versa) — measured across coil
• Expected voltage at relay coil (when OFF): near 0 V across coil or one side at battery voltage and control side same (depending on switching method)
• Typical compressor clutch coil resistance: commonly ~2–10 Ω to a few dozen ohms (varies by model) — compare to service spec
• Control module output resistance to ground when OFF: open/high; when ON: low (module internal transistor should switch to ground or source)

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