Operation - Section 3 10/00 OM3023 Page 3-10 Operating Instructions The continuous numbers on the chart indicate the combination of speeds and grades which the vehicle can safely negotiate for unlimited time or distance. Continue this procedure to downshift to the required gear range to maintain engine speed at 1800 - 2400 RPM and brake oil temperature below 248°F (120°C). When the proper gear range is attained, continue to use the retarder as needed to maintain a safe, productive speed. The short-length numbers listed on the chart indicate the combination of speeds and grades which the vehicle can safely negotiate for three minutes.
These speeds are faster than the continuous values, reflect- ing the thermal capacity of various system compo- nents. System components can accept heating at a higher-than-continuous rate for a short period of time, beyond which the system would become over- temperature. If brake oil temperature exceeds 248°F (120°C), the Brake Oil Temperature Warning light will turn on . As quickly as safety will permit , bring the truck to a complete stop away from traffic, move transmission range selector to "Neutral", apply the parking brake, and run engine at high idle. The short-length rating (sometimes called the "three- minute" limit) will successfully accommodate most downhill loaded hauls.
It is necessary to divide haul road grade segment length by allowable speed to determine actual time on grade. If actual time on grade exceeds the allowable amount, the grade will need to be negotiated at the continuous speed. Ambient temperature, as well as the prior tempera- ture of the brake cooling oil can affect this number (the brake cooling oil could already be above the nor- mal range from recent operating conditions). Continue to run engine at high idle until Brake Oil Temperature Warning light turns off and brake oil temperature cools to below 248°F (120°C).
If temperature does not return to this range within a few minutes, report the condition immediately to maintenance personnel and wait for further instructions before moving truck. ENGINE OVERSPEED If the truck and engine speed is not controlled, the rear brakes will be automatically applied at full pressure. The operator and any passengers may be subjected to severe deceleration forces and the truck may be difficult to control. DO NOT exceed these recommended MAXIMUM speeds when descending grades with a loaded truck. 3. For Efficient Retarder Operation when descending a grade, the operator should:
If the engine is allowed to run into an overspeed con- dition, the rear brakes will be automatically applied at 2600 RPM. Maintain engine RPM between 1800 - 2400 RPM, and Apply the retarder lever while observing both the Tachometer and the Brake Oil Temperature Gauge . The rear brakes will not be released until engine speed falls below 2350 RPM. The engine RPM must be maintained at 1800 - 2400 RPM and the Brake Oil Temperature must be maintained below 248°F (120°C).
If the operator observes that either the maximum engine speed of 2400 RPM or the Brake Oil Temperature of 248°F (120°C) are about to be exceeded, the operator should immediately move the transmission range selector to the next lower range and use the foot-operated brake pedal, to apply more brake pressure until the truck is slowed to a speed which will permit the transmission to downshift to the gear range selected.