P0645 on 2012-2016 Lincoln MKT: A/C Clutch Relay Circuit Causes and Fixes

What's Unique About the 2012-2016 Lincoln MKT

For this specific generation of Lincoln MKT and its platform mates (Ford Flex, Ford Explorer), Ford/Lincoln issued a Technical Service Bulletin (TSB SSM-45815) that directly addresses an inoperative A/C accompanied by code P0645. This bulletin points technicians toward a specific diagnostic path, noting that the PCM's strategy may disable the A/C compressor and that replacing the PCM is often not the correct solution. It specifically advises using a Ford IDS scan tool to command the A/C clutch on; if this works, the PCM and its output driver are functioning correctly, and the fault lies elsewhere in the system. This makes diagnosis more direct compared to vehicles without such specific manufacturer guidance.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Air conditioning does not blow cold air or works intermittently.
• Check Engine Light is on.
• A/C compressor clutch does not engage (no 'click' sound when A/C is turned on).
• Cooling fans may run on high speed when the A/C is commanded on, even if the clutch does not engage.

Most Likely Causes

1. Faulty A/C Clutch Relay 🔴 High Probability Relays are common failure points due to constant cycling and exposure to engine bay heat and vibration. This is the most cited cause in owner forums for the MKT and its platform mates.
   How to confirm: Locate the A/C clutch relay in the under-hood fuse box (Power Distribution Box). For a 2013 MKT, this is relay #98. A simple test is to swap it with an identical relay from a non-critical system (like the horn relay) and see if the A/C clutch engages.
   Typical fix: Replace the faulty relay with a new one. The OEM part is a common Motorcraft relay.
   Est. part cost: $10-$30
2. Wiring or Connector Issue 🟡 Medium Probability Wires in the engine bay can become brittle, corroded, or damaged over time from heat and vibration. Spilled fluids, such as gear oil during a PTU service, can also damage wiring insulation near the fuse box. The control wire from the PCM to the relay is often a White/Brown wire (CH302).
   How to confirm: Visually inspect the wiring harness and connectors between the PCM, the A/C relay socket, and the A/C compressor clutch for any signs of damage, corrosion, or loose connections. Use a multimeter to check for proper voltage (greater than 10V) at the relay socket and for continuity/shorts on the control wire.
   Typical fix: Repair or replace the damaged section of wiring or the faulty connector. Clean any corrosion from terminals.
   Est. part cost: $5-$50
3. Failed A/C Compressor Clutch 🟡 Medium Probability The electromagnetic coil in the A/C clutch can fail, creating an open or short in the control circuit, which the PCM detects.
   How to confirm: With the engine off, check the resistance of the A/C clutch coil using a multimeter. A typical Ford clutch coil should have a resistance between 2.5 and 4.5 ohms. An out-of-spec reading (infinite for an open circuit or near zero for a short) indicates a bad clutch coil.
   Typical fix: Replace the A/C compressor clutch or the entire A/C compressor assembly, followed by an evacuation and recharge of the A/C system.
   Est. part cost: $100-$400

Rare But Worth Checking

• Blown Fuse: Always a good idea to check fuses first. For a 2013 MKT, fuse #70 (10A) in the under-hood box powers the coil side of the A/C clutch relay. A blown fuse related to the A/C system will cause the circuit to fail and is a very simple fix.
• Faulty Powertrain Control Module (PCM): This is the least likely cause. The PCM's internal driver for the A/C relay can fail, but all other possibilities should be exhausted first. The TSB for this vehicle specifically warns that PCM replacement often does not resolve the issue. Diagnosis involves checking the control circuit for shorts to power or ground before condemning the PCM.

Diagnosis Steps

1. Verify the P0645 code is present using an OBD-II scanner.
2. Check for the most obvious symptom: Does the A/C blow cold air?
3. Locate the under-hood Power Distribution Box. On a 2013 MKT, the A/C clutch relay is in position #98.
4. Swap the A/C relay (#98) with a known-good, identical relay (e.g., from the horn). If the A/C clutch now engages, the original relay is bad and needs replacement.
5. If the relay swap doesn't work, inspect the A/C system fuse. For a 2013 MKT, check fuse #70 (10A) in the same under-hood box. Replace if blown.
6. Visually inspect the wiring and connector at the A/C compressor clutch for any damage, fraying, or corrosion.
7. Use a multimeter to check for battery voltage at the A/C clutch connector when the A/C is commanded on. This confirms the relay's load side is working.
8. If voltage is present but the clutch doesn't engage, test the resistance of the clutch coil. It should be between 2.5 and 4.5 ohms. If it's out of spec, the clutch has failed.
9. If no voltage is present at the clutch, check for power and ground signals at the A/C relay socket to determine if the issue is on the control side (from PCM) or the power supply side.
10. Follow the detailed pinpoint tests outlined in Ford's service manual, which involve checking the WH/BN control wire for shorts to voltage or ground between the relay socket and the PCM connector.
11. If all wiring, the relay, and the clutch are good, the problem may lie with the PCM, but this is rare and requires advanced diagnostics.

Parts You'll Likely Need

• A/C Compressor Clutch Relay (OEM #8T2T-CA (or equivalent Motorcraft part)) — This is the most frequent and simplest failure point in the A/C clutch control circuit. It is a standard 4-pin relay used across many Ford/Lincoln models.
  Trusted brands: Motorcraft, Bosch, Standard Motor Products, Four Seasons
  OEM price range: $20-$40
  Aftermarket price range: $10-$25
• A/C Compressor with Clutch — If the clutch coil itself has failed, the entire compressor assembly is typically replaced. This repair also requires evacuating and recharging the A/C system.
  Trusted brands: Motorcraft, Denso, Four Seasons
  OEM price range: $300-$500
  Aftermarket price range: $150-$350

Technical Service Bulletins (TSBs) & Recalls

• SSM-45815: AIR CONDITIONING (A/C) INOPERATIVE - DIAGNOSTIC TROUBLE CODE (DTC) P0645

Platform-Specific Known Issues

• TSB SSM-45815 was issued for 2011-2017 Ford and Lincoln vehicles, including the MKT, for an inoperative A/C system with or without code P0645. It advises technicians to use the Ford IDS scan tool to command the A/C clutch on. If the clutch can be commanded on with the scan tool, the PCM is not the fault, and standard diagnostics should be followed.
• On platform mates like the Ford Explorer, owners have reported intermittent operation where honking the horn could temporarily engage the A/C clutch after swapping the relays, pointing to a poor connection at the fuse box or a failing relay.

Mechanic-Grade Diagnostic Values

• A/C Compressor Clutch Coil Resistance — expected: 2.0 to 5.0 Ohms. Failure: A reading below 2.0 Ohms indicates a shorted coil; a reading of infinity (O.L.) indicates an open coil. Ford's service documentation often uses a tighter range of 2.5 to 4.5 ohms.
• Voltage at A/C Clutch Relay Socket (Power Feed) — expected: Greater than 10 Volts (with ignition on). Failure: Less than 10V indicates an open or high resistance in the power supply circuit (CBB77, White wire) to the relay.
• Resistance of PCM Control Wire to Ground — expected: Greater than 10,000 Ohms (with PCM and relay disconnected). Failure: Less than 10,000 Ohms indicates a short to ground in the control circuit (CH302, White/Brown wire).
• Continuity of PCM Control Wire — expected: Less than 5 Ohms. Failure: Greater than 5 Ohms indicates an open or high resistance in the control circuit (CH302, White/Brown wire) between the PCM connector and the relay socket.

Scan Tool Commands That Help

• Ford IDS (Integrated Diagnostic System): Active Command: A/C Clutch (ACC) — As recommended by TSB SSM-45815, this command directly tells the PCM to engage the A/C clutch. If the clutch engages with this command, it proves the PCM driver and the circuit to the clutch are functional, pointing the diagnosis away from the PCM itself.
• FORScan: Control PID: ACC_CMD (A/C Clutch Command) — This is a widely available tool for Ford/Lincoln vehicles that can perform a similar function to the dealer's IDS. Activating this PID commands the A/C clutch on, allowing for the same diagnostic verification as with the IDS tool.
• Ford IDS / FORScan: KOEO (Key On, Engine Off) Self-Test — Running the KOEO self-test will cycle through all PCM-controlled outputs, including the A/C clutch relay. The technician can listen for the relay and clutch to click, providing a quick, non-invasive check of the entire command circuit.

Wiring & Ground Locations

• PCM Connector C175B — This is one of the main connectors at the Powertrain Control Module (PCM), which is typically located in the engine bay near the firewall or battery.. The A/C clutch relay control wire, Circuit CH302 (White/Brown), terminates at a specific pin in this connector (Pin 2 or Pin 25 depending on the specific vehicle wiring). Pinpoint tests require checking for voltage, shorts, and continuity at this pin.
• Circuit CH302 (WH/BN) — This White/Brown wire runs from the PCM connector (C175B) to the A/C clutch relay socket in the under-hood fuse box.. This is the specific wire the PCM uses to ground and activate the A/C clutch relay. P0645 is set based on the voltage the PCM sees on this circuit. Almost all electrical diagnosis for this code involves testing this wire for opens, shorts to ground, or shorts to voltage.
• Ground G103 — On many Ford platforms, G103 is a primary engine bay ground located on the passenger side (right-hand side) inner fender or near the hood prop rod mount.. While not directly part of the P0645 control circuit, G103 often serves as the ground for the engine cooling fan motor and other front-end components. A poor ground here can cause a variety of electrical issues in the engine bay, and ensuring all major grounds are clean and tight is a critical step in diagnosing any electrical fault.

OEM Part Supersession History

• 8T2T-CA (Engineering Number) → HU5Z-14N089-B (Service Part Number) — Ford often updates service part numbers over time. The original number is an engineering ID, while the supersession is the official replacement part.
  Heads up: The original 8T2T-CA is a standard 4-pin micro 280 SPST relay. The service part number HU5Z-14N089-B replaces previous numbers like 5L3Z-14N089-AA and 8T2Z-14N089-C. As long as the replacement is a quality 4-pin micro relay with the same amperage rating (typically 15A-20A), it should be compatible.