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Note: All circuits powered by the ignition switch will remain HOT after the timer shuts OFF. To avoid draining the batteries the ignition switch must be turned OFF at that time.

Note: Excessive coolant temperature will not shut down the engine.


time of writing the specifications for the bus and is programmed into the ECM by the factory. When the programmed speed limit is reached the fuel to the injectors is limited, and will remain limited until the bus slows to a speed below the limited speed. Provisions have been made in the 3126B HEUI furnished by Thomas to allow for the fuel to be gradually reduced as maximum speed is approached, or about 2.5 mph before. This process, called Soft Vehicle Speed Limiting, prevents a sudden reduction of fuel at the top limit.

The soft vehicle speed limiting causes the engine to operate at a reduced rpm while maintaining the desired maximum road speed - a must if the best fuel economy is to be realized.

Engine Fast Idle

The engine is set to idle at 700 rpm by the factory; however, provisions can be made for a fast idle of 1000 rpm on buses operating in cold climates. This requires a dash mounted ON/OFF switch with an "Enabled Indicator Light" on the switch.

Idle Shutdown Timer

The idle shutdown timer is designed to shut the engine OFF after a predetermined period of idling. This can be between 3 minutes and 60 minutes, depending on the initial programming.

The "Check Engine" light on the dash will begin to flash rapidly 90 seconds before the engine shuts down. The driver can override the timer by depressing the brake pedal, if necessary, by doing so while the light is flashing. This is only if idle shutdown timer has been programmed to be overridden.

Engine Water Temperature Monitoring System

One of the features of the ECM system monitors the temperature of the coolant. At 2250F (1070C) the "Check Engine" lamp will come ON. If programmed, at that point the engine will gradually "derate" in horsepower to a minimum of 120 hp, and road speed will gradually reduce to 45 mph. This gives the operator ample time to get off the road and prevent damage to the engine. All drivers should be advised of this feature.

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