The throttle body contains a single throttle valve which controls the amount of air delivered to the engine. A coolant passage under the throttle valve heats the throttle body.

The throttle position (TP) sensor and idle air control (IAC) valve are mounted on the throttle body. The TP sensor and minimum air stop are not adjustable.

The TP sensor is a potentiometer that is mounted on the throttle body and provides the PCM with information on throttle valve angle. The PCM provides a 5 volt reference signal and a ground to the TP sensor and the sensor returns a signal voltage that changes with throttle valve angle. At closed throttle (close to 0 degrees) the TP sensor output signal is low (below 1 volt) and at WOT (greater than 80 degrees) the TP sensor output signal is high (above 4 volts). Because the TP sensor is not adjustable, the PCM must account for build tolerances that could affect the TP sensor output at closed throttle. The PCM uses a learning algorithm so that it can correct for variations of up to 6 degrees of throttle angle.

The PCM uses TP information to modify fuel control based on throttle valve angle. For example, power enrichment occurs when the throttle angle approaches WOT. Acceleration enrichment occurs when the throttle angle increases rapidly (similar to an accelerator pump on a carburetor). A faulty TP sensor may cause various driveability conditions and should set a DTC.

The purpose of the idle air control (IAC) valve is to control engine idle speed, while preventing stalls due to changes in engine load.

The throttle plate when closed allows a small amount of air into the intake manifold. However, most of the air for closed throttle engine operation passes through the IAC valve, bypassing the throttle plate. The IAC valve, mounted in the throttle body, controls air flow with a conical valve or pintle. By moving the pintle in towards the seat, air flow is decreased. By moving the pintle out away from the seat, air flow is increased.

The PCM moves the IAC pintle out if engine speed is too low or in if engine speed is too high. The PCM controls IAC valve movement in small steps called counts. The IAC valve position, measured in counts away from the valve seat, can be displayed on the scan tool.

The scan tool displays IAC position in counts. Zero counts indicates the PCM is commanding the IAC to be driven all the way into a fully seated position. This is usually caused by a vacuum leak.

The higher the number of counts, the more air is being commanded to bypass the IAC pintle. 320 counts indicates a fully open position.

Prior to starting the engine and at idle, the PCM calculates the desired position of the IAC valve based on coolant temperature, engine load, engine speed and battery voltage. While the throttle position is off idle, the IAC valve position follows or tracks throttle position to allow for smooth transitions from open to closed throttle.

If the IAC valve is disconnected or disabled with the engine running, the PCM may loose track of the IAC valve position causing erratic or incorrect idle speed. If this occurs, reset the IAC valve by doing the following:

1. The coolant temperature must be above 0°C (32°F).
2. The vehicle must be in Park (P) not Neutral (N).
3. Turn the ignition switch to Lock for 20 seconds.

The PCM receives data from various information sensors, determines the required fuel and idle speed values and then controls the IAC valve, eight fuel injectors, and spark advance to maintain the optimum performance and driveability under all driving conditions.

The fuel pump relay allows the fuel pump to be energized by the PCM. When the ignition is first turned On, the PCM energizes the fuel pump relay for two seconds. This allows the fuel pump to run for two seconds and build up fuel pressure for cranking. The PCM then waits for ignition reference pulses from the ignition control module (ICM). Once the PCM sees references pulses, the PCM energizes the relay to run the fuel pump.

A faulty fuel pump relay may cause long cranking times and should set a diagnostic trouble code.

The ECT sensor is a thermistor (a resistor which changes value based on temperature) mounted in the engine coolant flow. At cold coolant temperatures, the ECT sensor resistance is high and at hot coolant temperatures, the resistance of the sensor is low. The PCM supplies a 5 volt reference signal and a ground to the sensor and measures the voltage signal as it is changed by the sensor. The voltage is close to 5 volts at cold temperatures and close to zero volts at hot temperatures.

The PCM uses ECT information for fuel enrichment, ignition control, canister purge control, idle speed control and Closed Loop fuel control. A faulty ECT sensor may cause various driveability conditions and should set a diagnostic trouble code.

The electronic ignition system uses an individual ignition coil for each cylinder. There are two separate ignition module assemblies located in the camshaft cover of each cylinder bank. Each ignition module assembly contains an ignition control module and four ignition coils. Each ignition coil connects directly to a spark plug using a boot. This arrangement eliminates the need for secondary ignition wires. The ignition module assemblies receive power from a fused ignition feed. Both ignition module assemblies connect to chassis ground. A Reference Low and four ignition control (IC) circuits connect each ignition module assembly to the PCM. The PCM uses the individual IC circuits to control coil sequencing and spark timing for each ignition coil. The IC circuits transmit timing pulses from the PCM to the ignition control module to trigger the ignition coil and fire the spark plug. The PCM controls ignition system sequencing and timing events.

This ignition system produces very high energy to fire the spark plug. There is no energy loss because of ignition wire resistance, or the resistance of the waste spark system. Also, since the firing is sequential, each coil has seven ignition events to saturate as opposed to the three in a waste spark arrangement.

The MAP sensor is mounted on top of the intake manifold and measures the changes in intake manifold pressure which result from engine speed and load changes. The PCM supplies a 5 volt reference signal and a ground to the sensor and the sensor outputs another voltage signal that is proportional to the pressure signal. A high pressure (low vacuum) will produce a signal close to 5 volts and a low pressure (high vacuum) will produce a signal under one volt. The PCM scales the MAP sensor signal in kilo-Pascals (kPa). The MAP signal is the Inverse of what is read on a vacuum gauge. When the MAP value is high, intake vacuum with a vacuum gauge will be low.

The PCM uses MAP information for EGR control and diagnostics. MAP information is also used to determine barometric pressure and for ignition control. A faulty MAP sensor may cause various driveability conditions and should set a DTC.

(1)	Cylinder Head
(2)	Front Exhaust Manifold
(3)	Oxygen Sensor - Bank 2 Sensor 1
(4)	Cylinder Head
(5)	Oxygen Sensor - Bank 1 Sensor 1
(6)	Rear Exhaust Manifold
(7)	Y-Pipe
(8)	Oxygen Sensor - Bank 1 Sensor 2
(9)	Catalytic Converter
(10)	Oxygen Sensor - Bank 1 Sensor 3

Four oxygen sensors are mounted in the exhaust system where they monitor the oxygen content in the exhaust stream. There is an oxygen sensor mounted in each exhaust manifold. The exhaust manifold are bolted on to the cylinder head, and one on each end of the catalytic converter. The oxygen sensor located in the front exhaust manifold is the Bank 2 Sensor 1 (Front) sensor. The oxygen sensor located in the rear exhaust manifold is the Bank 1 Sensor 1 (Rear) sensor. The oxygen sensor located in the Y-pipe ahead of the catalytic converter is the Bank 1 Sensor 2 (Pre-converter) sensor. The oxygen sensor located in the catalytic converter outlet is the Bank 1 Sensor 3 (Post-converter) sensor.

An oxygen sensor acts like a battery because it creates its own signal voltage once it reaches operating temperature. This voltage is produced when the oxygen content in the exhaust stream is different than the oxygen content in the atmosphere. A lean condition (high oxygen content in exhaust) will produce a low voltage (near 0 volts) and a rich condition (low oxygen content in exhaust) will produce a high voltage (near one volt). The PCM provides a reference signal voltage (0.45 volts) and a ground to the sensor. The PCM reference voltage is necessary because the oxygen sensors do not provide their own voltage until they reach operating temperature.

The oxygen sensors also incorporate a heating element inside of the sensor housing. This heating element is energized with the ignition On and allows the sensors to reach operating temperatures quickly. The PCM can then use oxygen sensor information sooner after engine startup.

The PCM uses oxygen sensor information during Closed Loop operation to constantly adjust fuel control to reduce exhaust emissions. Because the oxygen sensors provide information sooner after engine start-up, exhaust emissions are reduced. Faulty oxygen sensors will cause various driveability conditions and should set a DTC.

Power Steering Pressure Switch

The power steering pressure switch is located on the power steering pressure hose between the power steering pump and rack and pinion gear. The power steering pressure switch is normally closed and provides a battery voltage signal to the PCM with no power steering load. When power steering pressure increases to a calibrated level due to steering load (such as full steering lock), the switch opens and causes the signal voltage to the PCM to drop to 0 volt.

The PCM uses the power steering pressure switch signal for idle speed control during high steering loads to maintain a stable idle. A shorted power steering pressure switch may cause a stumble or stall during steering loads and an open switch should set a DTC.

Transaxle Pressure Switch

The transaxle pressure switch is located inside of the transaxle and it composed of five discrete pressure switches that are combined to provide three status indications to the PCM. The PCM then uses these three inputs in a digital format (001, 010, 011, etc.) to determine the six possible transaxle ranges.

The PCM uses the transaxle pressure switch status for idle speed control, transaxle shift control and ignition control. A faulty transaxle pressure switch will cause various transaxle and engine driveability conditions and should set a DTC.

Transaxle Range (TR) Switch

The TR switch (formerly called a PRNDL, P/N or NSBU switch) is mounted externally on the transaxle at the manual shaft. The TR switch provides information to the PCM about the selected gear range of the transaxle. The TR switch identifies whether the transaxle is in Park/Neutral (P/N) or any of the five drive ranges (R/D4/D3/D2/D1). The TR switch is also used to illuminate the backup lamps when the transaxle is in Reverse (R).

The PCM uses TR switch status for idle speed control, and also as a backup to the transaxle pressure switch. If the transaxle pressure switch is faulty, the PCM will select a transaxle gear based on TR switch status. A faulty TR switch should set a DTC.

Vehicle Speed Sensor (VSS)

The vehicle speed sensor (VSS) is a pulse generator mounted in the transaxle. A toothed reluctor ring rotates near a magnetic pickup which produces a varying voltage signal in the pickup coil. This voltage signal varies in proportion to vehicle speed and is sent to the PCM, where it is converted to MPH.

The PCM uses VSS information for fuel economy calculations, idle speed control, transaxle shifts, and to control fuel under certain conditions. A faulty VSS may cause various transaxle and engine driveability conditions and should set a DTC.

Accelerator Controls Cable - Engine Compartment

The accelerator control system is cable type. There are no linkage adjustments. Therefore, the specific cable for each application must be used. The accelerator cable is routed through the groove in the throttle body lever.

Accelerator Controls Pedal