The intake air temperature (IAT) sensor is a thermistor (a resistor which changes value based on temperature) mounted in the intake manifold.

Low temperature produces a high resistance (100,000 ohms at -40°C/-40°F) while high temperature causes low resistance (70 ohms at 130°C/266°F).

The PCM supplies a 5 volt signal through a resistor in the sensor and the PCM then measures the voltage. The voltage will be high when the intake air is cold, and low when the air is hot. By measuring the voltage, the control module knows the intake air temperature.

The PCM uses the IAT signal to control fuel.

The mass air flow (MAF) sensor measures the amount of air which passes through it. The PCM uses this information to determine the operating condition of the engine, to control EGR operation. A large quantity of air indicates acceleration. A small quantity of air indicates deceleration or idle.

The scan tool reads the MAF value and displays it in Grams per Cylinder, Grams per Second (gm/s), and Hertz (Hz). Values should change rather quickly on acceleration, but values should remain fairly stable at any given RPM. When the PCM detects a malfunction in the MAF sensor circuit, the following DTCs will set:

The optical sensor has a 5V reference that powers the optics. The PCM supplies 5V to the optical sensor through the high res signal circuit and pump cam signal circuit. The optical sensor then generates a high resolution signal and a pump cam signal and sends these signals back to the PCM.

The APP (1) module contains three potentiometers (a device for measuring an unknown voltage or potential difference by comparison to a standard voltage). Each of the APP sensors send a varying voltage to the PCM. By monitoring the output voltage from the accelerator pedal position (APP) module, the PCM can determine fuel delivery based on the accelerator pedal position (driver demand).

The high resolution signal is an optical device which generates 64 equally spaced pulses per cylinder combustion stroke from the slotted disc mounted on the injection pump timing cam ring. The pulses are counted by the PCM and used to measure the angular pump displacement. Fuel is metered by using the angular indication and timing is measured by counting the angular pulses between the pump cam signal and the crankshaft position sensor signal.

The pump cam signal is an optical device which generates one pulse per cylinder combustion stroke from a slotted disc mounted on the injection pump timing cam ring. The pulse is used to locate the start of injection event for each cylinder (i.e. timing) with respect to the crankshaft position sensor. The number one cylinder is identified with a wider pulse (larger slot in disc). The PCM uses this information to adjust idle fuel, timing, trigger real time events and is used in the diagnostics of the crankshaft position sensor and the high resolution circuit.

The fuel temperature sensor is a thermistor (a resistor which changes value based on temperature) mounted in the injection pump.

The fuel temperature sensor is part of the optical/fuel temperature sensor. The resulting voltage is monitored by the PCM which uses this signal to minimize fueling error due to injection pump efficiency changes due to very hot fuel.

The fuel temperature sensor receives 5V from the PCM through the fuel temperature signal circuit. The fuel temperature sensor has a thermistor (a resistor that changes based on temperature) that causes a voltage drop across the circuit. The PCM then interprets the amount of voltage on the circuit to determine fuel temperature.

The crankshaft position sensor is a hall effect type of a device which generates one pulse per cylinder combustion stroke from a four tooth reluctor wheel attached to the engine crankshaft. The pulse is used to locate the cylinder reference event (top dead center) for each cylinder. The PCM uses this information to adjust timing, trigger real time events and is used in the injection pump cam signal diagnostic.

(1)	Electrical Connector
(2)	Manifold Absolute Pressure (MAP) Sensor

On vehicles equipped with EGR, the EGR control pressure sensor, mounted on the left side of the cowl, is used to monitor the amount of vacuum applied to the EGR valve. It senses the actual vacuum in the EGR vacuum line and sends a signal back to the PCM. The signal is compared to the desired EGR calculated by the PCM. On vehicles not equipped with EGR, the boost sensor is used to measure BARO. This sensor reads barometric pressure (BARO) under certain conditions, which allows the PCM to automatically adjust for different altitudes.

Boost (BARO) Sensor

The boost sensor measures the changes in the intake manifold pressure. The boost sensor converts engine load and speed changes, then converts the change in readings to voltage output.

The PCM sends a 5 volt reference signal to the boost sensor. As the manifold pressure changes, the electrical resistance of the boost sensor also changes. By monitoring the sensor output voltage, the PCM detects the boost pressure. A high pressure (high voltage) requires more fuel. A lower pressure (low voltage) requires less fuel. The control module uses the boost pressure signal to control fuel delivery.

The boost sensor also is used for BARO readings only on vehicles not equipped with EGR. This sensor reads barometric pressure (BARO) under certain conditions, which allows the PCM to automatically adjust for different altitudes.

Vehicle Speed Sensor (VSS)

The VSS is attached to the output shaft housing. This device contains a permanent magnet surrounded by a coil of wire producing a magnetic field which is interrupted by rotor teeth pressed on an output shaft. As the rotor interrupts the magnetic field, an AC voltage is generated in the circuit.

Vehicle Speed Signal (VSS) Buffer Module

The VSS buffer module (1) is an electronic device that processes inputs from the vehicle speed sensor and outputs various signals. The VSS buffer module outputs a 4000 pulse per mile signal. This signal is used by the PCM to determine vehicle speed. The PCM uses vehicle speed signal input for cruise control and fuel cutoff. The VSS buffer module is matched to the vehicle based on transmission, final drive ratio and tire size. The VSS buffer module is located behind the instrument panel.

Fuel Solenoid Driver

The fuel injector driver receives an inject command signal from the PCM and provides a current regulated output to the fuel solenoid that controls injection. It also returns a fuel solenoid closure time signal back to the PCM to inform it when the fuel solenoid has actually seated.

Brake Switch

The TCC normally closed brake switch supplies a B+ signal into the PCM. The signal voltage is opened when the brakes are applied.

The cruise control normally open brake switch supplies a B+ signal to the PCM when the brake is applied. These signals are used by the PCM to control transmission and cruise control functions. The brake switches are part of the stoplamp switch.

Glow Plug System

The glow plug system is used to assist in providing the heat required to begin combustion during engine starting at cold ambient temperatures.

The glow plug relay switches power to the glow plugs and is commanded on and off by the PCM. The glow plugs are heated before and during cranking, as well as initial engine operation. The PCM monitors the glow plug relay output voltage which indicates if the relay is following the PCM commands.

Cruise Control System

The cruise control switches are part of the multifunction turn signal lever. These switches enable the driver to control the cruise on/off, set/coast and resume/accel signals. These signals are inputs to the fuel control portion of the PCM and allow the PCM to maintain a desired vehicle speed under normal driving conditions.

A/C Signal

This signal indicates that the A/C compressor clutch is engaged. The PCM uses this signal to adjust the idle speed.