Deutz-Fahr I26 – Instrument Panel (Cluster) ECU: Preheating control relay is short-circuiting to battery positive or disconnected

The instrument panel indicates that the preheating control relay is short-circuiting to battery positive or disconnected.

• Agroplus 310, 320, 410, and 420 Profiline series tractors
• Agrofarm 85
• Agrofarm 100
• Agrofarm 115G
• Agrotron K90
• Agrotron K100
• Agrotron K110
• Agrotron K120
• Agrotron M600
• Agrotron 80
• Agrotron 90
• Agrotron 100
• Agrotron 105
• Agrotron 108
• Agrotron 118
• Agrotron 128
• Agrotron 215
• Agrokid 30
• Agrokid 40
• Agrokid 50

1. 1

   Visually inspect the contacts on connector '5K' of the instrument panel and the contacts on the preheating control relay (RL42) to ensure they are not oxidized and are firmly secured.

2. 2

   Perform a continuity test: With the starter key OFF and connector '5K' disconnected, test resistance between pin 2 of connector 'RL42' and pin 19 of connector '5K'. It should read 0 Ohms.

3. 3

   Bench test the preheating control relay (RL42) by applying 12V to the coil pins and checking for continuity across the load pins.

4. 4

   Perform an alarm reset test: Clear all alarms, turn the starter key to OFF and then to ON, and check if the alarm returns.

1. 1

   Disconnect Power

   Always disconnect the battery terminals before working on the electrical systems to prevent damage to the ECUs. Disconnect the negative (-) terminal first, followed by the positive (+) terminal.

2. 2

   Access the Relay Panel

   Locate the main relay/fuse board (usually in the cab console or control unit compartment) and identify the preheating control relay, labeled RL42.

3. 3

   Clean Connections

   If the diagnostic checks revealed oxidation on connector 5K or the RL42 socket, clean the pins thoroughly with electrical contact cleaner.

4. 4

   Repair Wiring

   If the continuity test between pin 2 of RL42 and pin 19 of 5K failed (open circuit), trace the wiring harness and repair the broken wire or shorted segment.

5. 5

   Replace the Relay

   If the wiring is intact but the relay failed the bench test (or if the internal coil is shorted to positive), replace the RL42 preheating control relay with a new OEM unit.

6. 6

   Reassembly

   Reconnect the battery (positive terminal first, then negative).

7. 7

   Verification

   Turn the ignition to the 'I' (ON) position. Verify that the preheating light illuminates normally and that fault code I26 is no longer displayed on the instrument panel.