Service manual

90-898303

4-25

Manifold Absolute Pressure (MAP) Sensor

The MAP sensor is located on top of the air plenum. The ECM regulates fuel

flow, in part, based on manifold absolute pressure. The MAP sensor becomes

more critical in engine running quality as the engine is operated at higher

altitudes (i.e. mountain lakes). Should the MAP sensor fail, the ECM will

default to a value of approximately 14.7 psi.

The MAP sensor’s DDT readout will vary according to altitude, throttle plate

opening and barometric pressure. However, for a given location and weather

conditions (I.E. altitude and barometric pressure), the MAP sensor readout

between engines should be approximately the same when the ignition key is

turned to the “ON” position. For example, if two engines indicate 15 psi when

the key is turned “ON” and a third engine indicates 10 psi, the third engine’s

MAP sensor would be suspect. When the engines are started, MAP sensor

readout should drop. When the engines are initially accelerated, the MAP

sensor readout will drop momentarily and then begin to rise. Throttle plate

opening will also affect the MAP readout. Refer to Service Manual Section 2C

for correct throttle plate clearance.

The Digital Diagnostic Terminal (DDT) can be used to determined whether the

MAP sensor is functioning properly. As throttle is advanced, numerical value

on DDT display should increase. As throttle is retarded, numerical value

should decrease indicating MAP sensor is functioning. If numerical value does

not change as throttle setting varies, MAP sensor is defective.

NOTE: If MAP sensor is not functioning, #4 LED indicator light on DDT will be

illuminated.

a) Manifold Absolute Pressure (MAP) Sensor

Notes

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