SENSOR AS-SPEED 1072675 - Caterpillar

All EnginesBecause of the type of information in this article, it is not written in Caterpillar Fundamental English.
There has been considerable confusion in the field concerning the correct definitions, usage, and applications of the following nomenclature.
1. Inlet Manifold Pressure2. Bring The Engine To Normal Operating Temperature Before Measuring Inlet Manifold Pressure3. Engine High Idle RPM4. Altitude Consideration In Horsepower RatingThe following explanations should help clear up this confusion and lack of understanding.
Inlet Manifold Pressure
Inlet manifold pressure (frequently referred to as "boost") is the air pressure being delivered to an engine. This pressure is most commonly measured in inches of mercury gauge, but sometimes you will find it specified in absolute pressure or PSI. It has been common practice to compare measured pressures on engines with published figures in the Rack Setting Information Book to try to determine if an engine is producing its rated horsepower. These two quantities, manifold pressure and horsepower, are not always related, therefore, inlet manifold pressure by itself can not be used as a definite indicator of the horsepower being produced. Other factors have to be considered.
To use inlet manifold pressure as an indicator of the horsepower being produced, the guidelines listed must be followed. If there are deviations from these guidelines, the inlet manifold pressure measured is not a true indicator of the horsepower being produced by the engine and should not be used as such.
1. The engine is in good working condition.2. Proper adjustments have been made to the governor, fuel system, timing, etc.3. Correction factors for barometric pressure, ambient air temperature and fuel API have been taken into consideration.4. The engine has been brought up to proper temperature.5. Special attention has been given to any qualifications explained in the Rack Setting Information Book.6. There are no leaks in the inlet or exhaust systems.7. Air inlet and exhaust restrictions are within the published limits.8. Adherence to information in Special Instruction Form FEO36044-06, Engine Performance Evaluation and Adjustments with 4S6553 Instrument Group.If the guidelines are followed, the measured inlet manifold pressure should fall within the specified range published in the Rack Setting Information Book. A comparison can now be made between measured inlet manifold pressure and horsepower to get an indication of whether the engine is performing properly or not.
It should be pointed out though, that inlet manifold pressure can vary from engine to engine. Therefore to assume that a measured pressure at the low end (or high end) of the range is an indication that the horsepower is at the low end (or high end) of the allowable tolerance is not necessarily correct.
Bring The Engine To Normal Operating Temperature Before Measuring Inlet Manifold Pressure
An engine being checked for inlet manifold pressure should be at normal operating temperature before this measurement is taken. Also, if the inlet manifold pressure is to be checked at the torque converter stall speed, the torque converter must be at normal operating temperature before this check is made. This is necessary to stabilize all systems for obtaining valid