Massey Ferguson AC21 – Air Conditioning Control Module: X318 - Automatic air conditioning compressor relay output error.

The controller has detected an electrical fault (such as an open circuit, short to ground, or short to power) on the output circuit that commands the air conditioning compressor relay to engage. Because of this error, the compressor clutch will not receive power, and the AC system will fail to blow cold air.

• 7616
• 5612
• 7622
• Interim Tier 4 (2012-2015)
• 7614
• 5611
• 7615
• 5610
• 5613
• 7620
• 5609
• 7618
• 5608
• 7619
• 7626
• 7624
• 7600 Series
• MF 7614
• MF 7615
• MF 7616
• MF 7618
• 8600 Series
• MF 8670
• MF 8690

1. 1

   Locate the Air Conditioning Compressor Relay (designated as X318 or K3) in the tractor's main fuse/relay panel.

2. 2

   Swap the AC compressor relay with a known good relay of the same type (e.g., a work light relay) to see if the fault clears. Test the original relay's coil for an open circuit or internal short.

3. 3

   Inspect the wiring harness between the AC Control Module (X439 blue connector / X440 yellow connector) and the relay block. Look for chafed wires, corrosion at the pins, or a disconnected plug.

4. 4

   Verify that the wiring from the relay output to the actual AC compressor clutch is not shorted to ground, which could overload the relay circuit and trigger the output error.

1. 1

   Turn off the tractor and engage the battery isolator switch (disconnect battery power) to prevent electrical spikes.

2. 2

   Access the cab's electrical fuse and relay center.

3. 3

   If diagnostics point to a failed relay, remove the faulty X318 / K3 AC compressor relay.

4. 4

   If diagnostics revealed damaged wiring between the AC module and the relay panel, splice and repair the affected wires using proper heat-shrink connectors.

5. 5

   Install the new replacement relay into the designated K3/X318 slot.

6. 6

   Restore battery power, start the engine, and turn on the automatic air conditioning.

7. 7

   Verify that the AC compressor clutch engages and that the AC21 fault code changes to an inactive state or clears from the dash display.