New Holland TRANS 79 – Transmission Controller (TRANS): Engine RPM from the alternator is measured too high

The transmission controller detects that the engine speed signal has exceeded 3,000 RPM. The engine speed signal is generated by the AC frequency output from the 'W' terminal of the alternator.

• TG210
• TG230
• TG255
• TG285
• T7030
• T7040
• T7050
• T7060
• T8000 series
• T9000 series
• MX210
• MX230

1. 1

   Check for a recent operational over-speed condition (e.g., running downhill with a heavy load), which may also log fault code TRANS 80.

2. 2

   Verify the alternator hardware, ensuring the correct model is installed and the pulley is the correct diameter (an undersized pulley will spin the alternator too fast).

3. 3

   Perform an Instrumentation Display Test by driving the tractor at different throttles and monitoring the RPM speed on the display to see if the signal is normal.

4. 4

   Isolate the signal source by disconnecting the wire at the alternator's 'W' terminal, clearing faults, and running the tractor to see if TRANS 79 reappears (indicating a harness bleed-over).

5. 5

   Inspect the wiring harness from the alternator's W terminal to pin 33 of connector 351 on the transmission controller for shorts, loose connectors, or bent pins. Pay special attention to areas where the engine RPM signal circuit crosses paths with the wheel speed sensor circuit or the PTO speed signal circuit.

1. 1

   Clear Codes

   If the fault was caused by a downhill over-speed event, simply clear the code and resume normal operation.

2. 2

   Repair Wiring

   If the isolation test proves the harness is shorting to another speed signal wire, locate the chafe or melt point in the harness (specifically between the W terminal and pin 33 of connector 351) and repair the wiring.

3. 3

   Replace Alternator

   If the wiring is intact, the pulley is correct, and the Instrumentation display shows an erratically high RPM, the alternator's internal rectifier/regulator is likely failing and outputting an incorrect AC frequency on the W terminal. Replace the alternator.