Massey Ferguson 21 – Air Conditioning Control Module (AIRCON): Air Conditioning Compressor Relay Signal From AC Module Issue

The air conditioning control module detects that the AC compressor relay is not operating correctly, or the electrical signal to/from the relay is outside of its expected operating range.

• 7724S
• Interim Tier 4 (2012-2015)
• 7722S
• 7722
• 7615
• 7720S
• 7716
• 7718
• 7726
• 7715
• 7616
• 7618
• 7619
• 7626
• 7624
• 7726S
• 7614
• 7718S
• 7620
• 7719
• 7714
• 7716S
• 7622
• 7720
• Tier 4 (2015-2021)
• 7715S
• 7724
• 8600 Series
• 7600 Series (7614–7626 Tier 4 models)
• 7700 Series (7714–7726S Tier 4 models)
• Global Series (4700, 5700, and 6700 series tractors)
• Dyna-4
• Dyna-6
• Dyna-VT

1. 1

   Locate the AC compressor relay (K3 or K4) in the tractor's main fuse/relay box.

2. 2

   Swap the K3/K4 AC compressor relay with a known good relay of the same type (e.g., a work light or wiper relay) to see if the fault clears and the compressor clutch engages.

3. 3

   Inspect the relay socket for pushed-out pins, corrosion, or signs of melting due to high resistance.

4. 4

   Inspect the wiring between the AIRCON control module and the relay box for chafed wires or loose connections that could interrupt the signal.

5. 5

   Ensure the AIRCON module itself is receiving proper voltage, as undervoltage can cause relay output errors.

1. 1

   Turn off the tractor and disconnect the battery isolator switch

   This prevents electrical shorts.

2. 2

   Open the access panel to the main fuse and relay box

   Location varies by cab type, often on the right-hand console or pillar.

3. 3

   Locate the AC compressor relay (K3 or K4)

   Use the fuse box diagram printed on the panel cover.

4. 4

   Remove the faulty relay

   If the socket terminals are corroded, clean them with electrical contact cleaner.

5. 5

   Plug the new replacement relay firmly into the socket

6. 6

   Reconnect the battery isolator switch and start the tractor

7. 7

   Turn on the AC system and clear the fault code

   Use the AGCO Electronic Diagnostic Tool (EDT) to clear the inactive fault code 21 from the AIRCON module, or allow the system to cycle and self-clear if the physical fault is resolved.