3126E Truck Engines Caterpillar

Sensor Locations

Illustration 1 shows the typical locations of the sensors for this engine. Specific engines may appear different from the illustration due to differences in applications.

Illustration 1	g00851903
Left side view (1) Injection actuation pressure control valve (IAPCV) (2) Rail pressure sensor (3) Coolant temperature sensor (4) Oil temperature sensor (5) Inlet air temperature sensor (6) Boost pressure sensor (7) Engine control module (ECM) (8) Oil pressure sensor (9) Speed/Timing sensors

Failure of Sensors

All Sensors

A failure of any of the sensors may be caused by one of the following malfunctions:

• Sensor output is open.

• Sensor output is shorted to "- battery" or "+ battery".

• Measured reading of the sensor is out of specification.

Boost Pressure Sensor

Boost pressure sensor (6) provides a signal which corresponds to turbocharger outlet pressure to the ECM. The air/fuel ratio control utilizes the actual engine speed and the turbocharger outlet pressure in order to control the transient smoke level. The ECM can control injection timing and the amount of fuel that is injected. When the throttle is increased and when the engine demands more fuel, the fuel limit is controlled in order to reduce overall smoke levels of the engine exhaust.

Failure of the Boost Pressure Sensor

The ECM will detect failure of the boost pressure sensor. The operator will be warned of the problem through the diagnostic lamp. The strategies that are related to the boost will be disabled. Engine power will be severely reduced in the event of a failure of the boost pressure sensor. A failure of the boost pressure sensor will not cause a shutdown of the engine.

Coolant Temperature Sensor

Coolant temperature sensor (3) monitors engine coolant temperature. The output of the ECM can indicate a high coolant temperature through a relay or a lamp. The coolant temperature sensor is used by the ECM to determine initiation of the Cold Start Strategy.

Failure of the Coolant Temperature Sensor

The ECM will detect a failure of the coolant temperature sensor. The diagnostic lamp will warn the operator about the status of the coolant temperature sensor. The cold start strategies will be disabled if failure occurs. A failure of the coolant temperature sensor will not cause a shutdown of the engine or any horsepower change.

Engine Oil Temperature Sensor

Engine oil temperature sensor (4) monitors the temperature of the engine oil. The ECM uses the information from the engine oil temperature sensor in order to adjust the timing of the fuel injection and the pressure of the fuel injection.

Engine Speed/Timing Sensor

If the ECM does not receive a signal from primary speed/timing sensor (9), the "DIAGNOSTIC" lamp will indicate a diagnostic fault code which will be logged in the ECM memory.

If the ECM does not receive a signal from the primary speed/timing sensor, the ECM will read the signal from the secondary speed/timing sensor. The ECM continually checks in order to determine if there is a signal from both sensors. If either sensor fails, the faulty sensor should be replaced.

Intermittent failure of the sensors will cause erratic engine control.

Inlet Air Temperature Sensor

Inlet air temperature sensor (5) measures the inlet air temperature. A signal is sent to the Electronic Control Module (ECM). The inlet air temperature sensor is also used by the ECM to determine initiation of the Cold Start Strategy.

Throttle Position Sensor

The Throttle Position Sensor (TPS) sends a signal to the ECM. The signal from the TPS is required in order to govern engine speed. The throttle position sensor signal is used by the ECM in order to calculate the desired engine speed. The TPS is calibrated during the initial installation of the engine.

Note: The throttle position sensor is not shown in the illustration.

Failure of the Throttle Position Sensor

An intermittent failure in the TPS causes the engine speed to vary erratically. The ECM will perform the following process:

• The ECM will detect the failure of the TPS.

• The ECM will warn the operator of the failure through the diagnostic lamp.

• The ECM will set the desired engine speed to low idle.