Komatsu ecot3 GALEO HM300-2 Operation And Maintenance Manual page 148

OPERATION OPERATION
1. Before starting to travel downhill, release accelerator pedal (1)
and operate retarder control lever (2) to slow the machine
down.
2. Move the shift lever to a position (5, 4, 3, 2) that matches the
maximum permissible speed
performance.
3. When traveling downhill, operate retarder control lever (2), run
the engine at a speed of at least 1800 rpm, and travel so that
the retarder brake oil temperature gauge is in the white range.
for the retarder brake
https://www.besttruckmanuals.com/
OPERATION OPERATION
3 - 97