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TL960 SERVICE MANUAL

Tamper Resistance

One of the features of the ECM is a programmed tamper resistance. This prevents the vehicle from exceeding its preset maximum road speed in the event the vehicle speed signal is lost. Example: Assume the rear axle ratio would allow the vehicle to run 72 mph at 2400 rpm; however, management wants a maximum of 60 mph. This is programmed into the ECM by the factory, which means the engine would be turning over only 1600 rpm. (These are only figures used for the purpose of explaining how the Vehicle Speed Limit works.) IF the speed signal is lost, the bus would run 72 mph without the tamper resistance feature. With this feature, however, it will run only 60 mph, and under greatly reduced power.

Caterpillar Hydraulic Electronic Unit Injector System

The Caterpillar Hydraulic Electronic Unit Injector (HEUI) system uses the ECM to control the injection of fuel, using high pressure engine oil to provide the "muscle" instead of a cam shaft. In a conventional mechanical fuel system fuel injection pressures increases proportionally with engine speed, whereas the HEUI is electronically controlled independent of engine speed. Both the start and end of injection are electronically controlled. Plunger movement timing is NOT limited to the speed or shape of a camshaft. The next several pages describe how the HEUI system operates.

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