TRANSMISSION AR 20R4400 - Caterpillar

Illustration 1.Illustration 2 shows the effect of a parasitic load on an engine and how the measured engine high idle rpm is lowered from Point B to Point P. A difference of approximately 15 to 20 rpm.
If an adjustment is made to change high idle rpm of an engine with parasitic loads (Point P) to the bare engine specification (Point H), the set point will not be correct. An adjustment to high idle rpm will affect the set point.
Illustration 2.Illustration 3 shows how the parasitic loads lower the overall tolerance range. The rpm range between Points B1 and B2 is approximately 15 to 20 rpm higher than P1 to P2.
Illustration 3.During production of an engine arrangement there will be small changes in the engine components due to changes in machine tools, materials, etc. The changes in engine components can cause a change in the average engine high idle rpm that is measured at the factory. This average high idle rpm number is put on the engine information plate. If changes in engine components cause the average engine high idle rpm to be outside the acceptable tolerance, then the rpm number which is put on the plate at the factory is changed to a new number. If there is a difference between the high idle rpm specification on the engine information plate and the fuel setting microfiche, use the specification on the engine information plate.
Remember that the high idle rpm is not an adjusted specification and is only to be used to check governor operation.