BOLT 4S5565 - Caterpillar

All Caterpillar-Built Diesel EnginesThe purpose of this article is to explain high idle rpm, full load rpm and set (balance) point rpm. The article also describes the use of the high idle adjustment screw. It has recommendations on when the high idle screw can or can not be adjusted. It also tells why there can be differences between the high idle rpm setting on the engine information plate and the actual measured rpm of the engine.
HIGH IDLE RPM
The high idle rpm of an engine is not an adjusted specification. It is the result of three factors:
1. Engine set point rpm (which is adjusted with the high idle screw).2. The differences of governor springs and weights.3. Parasitic (accessory) loads on the engine (fan, alternator, compressors, pumps, BrakeSaver, etc.).The high idle rpm is shown on the engine information plate or listed in the fuel setting charts. The engine information plate must be used for all measurements if it is available.
FULL LOAD RPM
Full load rpm is the point at which the engine will develop the rated horsepower. If the engine is equipped with a torque spring, this may or may not be the rpm at which the engine develops maximum horsepower.
SET POINT
The engine set point is an adjusted specification and must be maintained. High idle rpm is not an adjusted specification. Set point (formerly balance point) is full load rpm plus an additional 20 rpm. Set point is the rpm at which the fuel rack just starts to make contact with the stop bar or the first torque spring. The rack at this rpm still moves back and forth a small amount. When additional load is put on the engine, the rack will advance and become stable against the torque spring or stop bar. Set point is controlled by the fuel setting in the high idle adjustment screw. Measurements have shown that the former balance point was not a point, but a general area.
The governor springs and weights control the rpm of the engine. Because of the normal production variations in springs and weights of the same part numbers, it is possible to have engines with the same set point but with a different high idle rpm. This is why set point must be adjusted and not high idle rpm.
The parasitic loads on an engine will also cause a change in the measured high idle rpm. These loads will decrease the high idle rpm according to the amount of power used by them. Normally, the parasitic loads on an engine will cause the measured high idle rpm to be 15 to 20 rpm less than the original bare engine high idle rpm with no parasitic loads. The decrease in the high idle rpm caused by the parasitic loads does not change the set point of the engine.
Because set point, governor springs and weights and parasitic loads all have an effect on the measured high idle rpm on an engine, it is not possible to accurately adjust the governor by only measuring