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W-Body Performance - Pushrod / DOHC
TTs - Driveability & Emission Controls

"...Um, you're right.  I don't have a clue what that sensor does..."

Backed by popular demand and many requested e-mails, here's the place to figure out what that little piece of plastic of a sensor really does for you car.  Many of these are exact descriptions cited from various service manuals and technicians.

Air Injection Reaction (AIR) System -
This system is used to reduce carbon monoxide (CO), hydrocarbons (HC's) and oxides of nitrogen (NOx) emissions.  The system also heats up the catalytic convertor on engine start-up so the exhaust gases will be converted more quickly.  The AIR system is only utilized on six cylinder engines with manual transmissions.

Camshaft Position (CMP) Sensor -
The Electronic Control Module (ECM) uses the camshaft signal to determine the position of the number 1 cylinder piston during it's power stroke.  The signal is used by the ECM to calibrate fuel injection mode of operation.  If the camshaft signal is lost while the engine is running, the fuel injection will shift to a calcuated fuel injected mode based on the last fuel injection pulse, and the engine will continue to run.

Coolant Tempreture Sensor (CTS) -
Most engine functions are affected by the coolant tempreture.  Determining whether the engine is hot or cold is larely dependent on the tempreture.  An accurate tempreture signal to the Electronic Control Module (ECM) is recieved from a thermistor mounted in the coolant stream.  That thermistor changes electrical resistance dependent on the changing tempreture; low tempretures produce high resistances (100,000 ohms @ 40 degrees F) and high tempretures produce low resistances (70 ohms @ 266 degrees F).  The ECM supplies 5 volts to the CTS through a resistor in the ECM measures the returning voltage.  The CTS is mainly located on the intake manifold water jacket or near the thermostat housing.

Crankshaft Position (CKP) Sensor -
The CKP sensor provides a very valuable signal through the ignition module which the Electronic Control Module (ECM) uses as a reference to calculate RPM's and crankshaft position.  If the signal of the CKP sensor is lost to the ECM, the car will quit and not re-fire untill accurate signal is regained.

Electronic Control Module (ECM) -
The heart of the electronic control system is a computer control module.  The module gathers information from various sensors, then controls fuel supply and engine emission systems.  Most early models are equipped with an ECM which controls the engine and related emissions systems.  All computer control module are serviced in a similar manner; care must be taken when handling these expensive components in order to protect them from damage.

Exhuast Gas Recirculation (EGR) System -
The EGR system is used to reduce oxides of nitrogen (NOx) emission levels caused by hugh combustion chamber tempretures.  Monuted to the intake manifold with a opening to the exhaust manifold, the EGR valves opens, under specific engine operating conditions, to admit a small amount of exhaust gas into the intake manifold, below the throttle body.  The exhaust and intake gases mixes and displaces a portion of the oxygen in the air / fuel mixture entering the combustion chamber.  The net effect of mixing and displacing lowers the tempreture of combustion; lowered tempreture also helps controls auto-ignition.  The EGR system is activated once the engine reaches normal operating tempretures and when the throttle is between idle and Wide Open Throttle (WOT).

Idle Air Control (IAC) Valve -
Engine idle speeds are controlled by the Electronic Control Module (ECM) through the IAC valve mounted on the throttle body.  The ECM sends voltage pulses to IAC motor windings causing the IAC motor shaft and pintle to move in or out a given distance (number of steps) for each pulse, also knows as a count.  The movement of the pintle controls the airflow around the throttle plate, which in turn, controls engine idle speed.  Minimum idle air speed is set at the factory with a screw stop; this setting allows enough airflow by the throttle valves to cause the IAC valve pintle to be positioned a calibrated number of steps (counts) from the seat during normal idle operation.  Tampering with the minimum idle speed screw is highly discouraged since it may result in premature failure of the IAC valve.

Ignition Control Module (ICM) -
The ICM controls all timing and which cylinder fires.  Under 400 RPM's the ICM also controls the amount of spark advance / retard. Above 400 RPM's the Electronic Control Module (ECM) calculates advance / retard, disables the auto calculation for advance on the ICM, and then procedes to transmit advance / retard rates to the ICM.  The ICM still controls which coil fires when no matter which mode it is run in, but uses the ECM input when its commanded by electrical pulses.

Intake Air Tempreture (IAT) Sensor -
The IAT sensor is a thermistor which supplies manifold air tempreture information to the Electronic Control Module (ECM).  Like the CTS above it has a high resistance at cold tempretures and low resistance at hot tempretures.  The ECM supplies a 5 volt signal to the IAT sensor, and measures the IAT output voltage; the ECM calculates the intake air tempreture from that return voltage.  The IAT is used to adjust spark timing according to incoming air density.  Usually it's mounted in the air filter housing or the in the air intake duct / tubing.

Knock Sensor (KS) -
The KS is mounted to the engine block.  When spark knock, or pinging is present, the sensor produces a voltage signal which is sent to the Electronic Control Module (ECM).  The ECM will then retard the ignition timing based on these signal.  When a KS problem is suspected, first preform a visual inspection; most problems can be found in wiring harnesses and connectors.

Malfunction Indication Lamp (MIL) -
Please refer to Service Engine Soon (SES) Light.

Manifold Absolute Pressure (MAP) Sensor -
The MAP sensor measures the changes in intake manifold pressure, which result from the engine load and speed changes, and converts this to voltage output.  The MAP sonser is used to measure barometric pressure under certian conditions, which allows the Electronic Control Module (ECM) to automatically adjust for different altitudes.  The ECM uses the MAP sensor to control fuel delivery and ignition timing.  A closed throttle on engine coastdown will produce a low MAP output, while a wide open throttle (WOT) will produe a high output.  This high output is produced since the pressure inside the manifold is the same as the outside so 100% of the outside air pressure is measured.  The MAP sensor reading is the opposite of what you would measure on a vacuum gauge; when manifold pressure is high, vacuum is low and vise versa.  When a higher pressure, low vacuum (high voltage) requires more fuel; while a lower pressure, higher vacuum (low voltage) requires less fuel.

Manifold Air Tempreture (MAT) Sensor -
Please refer to Intake Air Tempreture (IAT) Sensor.

Mass Air Flow (MAF) Sensor -
The MAF sensor measures the amount of air entering the engine during a given time interval.  The Electronic Control Module (ECM) uses the voltage input / information from the MAF for fuel delivery calculations.  A large quantity of air passing by the MAF indicates an accleration or high load situtation, while a small quantity of air indicates deceleration or idle.

On-Board Diagnostic I (OBDI) System -
On-Board Diagnostic systems are in most cars and light trucks on the road today. During the 1970's and early 1980's manufacturers started using electronic means to control engine functions and diagnose engine problems. This was primarily to meet EPA emission standards.  Generation I OBD systems run up to the 1995+ model with having minor variations and hybrid systems before OBDII system was standardized in the 1996 model year.  (* From OBDII.com)

On-Board Dignostics II (OBDII) System -
The predicessor to OBDI, all cars built since January 1, 1996 have OBDII systems. Manufacturers started incorporating OBD-II in various models as early as 1994.  Some early OBD-II cars were not 100% compliant.  Through the years on-board diagnostic systems have become more sophisticated. OBD-II, a new standard introduced in the mid-'90s, provides almost complete engine control and also monitors parts of the chassis, body and accessory devices, as well as the diagnostic control network of the car. (* From OBDII.com)

Oxygen Sensor (O2S) -
The oxygen sensor is a spark-plug shaped device that is screwed into the exhaust manifold to monitor the oxygen content of the exhaust gases and sends a voltage signal to the Electronic Control Module (ECM).  The ECM monitors this voltage and depeding on the value of the recieved signal, issues a command to the mixture control solenoid to adjust for rich or lean air / fuel mixture conditions.

Postive Crakcase Ventilation (PCV) System -
this system is used on all vehicles to evacuate the crankcase vapors.  Fresh air from the air filter / intake duct is supplied to the crankcase, mixed with blow-by gasses and then passed through the PCV valve into the intake manifold / plenum.  When manifold vacuum is high, such as at idle, the valve restricts the flow of blow-off gases allowed into the manifold.  If abnormal operating conditions occur, the system will aloow excessive blow-by to back flow throgh the hose into the air filter.  A plugged PCV valve may cause rough idle, stalling, slow idle speed, oil leaks, oil in the air filter and carbon / sludge build-up in the intake system.  Clean / replace dirty and clogged PCV valve along with inspecting the throttle body and air intake tube for oily deposits.  during replacement, check any hoses for deterioration due to heat and age.

Powertrain Control Module (PCM) -
Similar to the Electronic Control Module (ECM), the PCM is it's predicessor and it handles addition systems as well.  The PCM may control the manual transmission shift lamp or the shift functions of the electronic automatic transmission.

Service Engine Soon (SES) Light -
Since the computer controled module is porgrammed to recognize the presence and value of electrical inputs, it will also note the lack of signal or a radical change in values.  Once a fault in the vehicle is recognized, a numeric code is assigned and held in memory.  Also the dashboard warning lamp: CHECK ENGINE or SERVICE ENGINE SOON (SES) LIGHT will illuminate to advise the operator that the system has detected a error or fault.  More than one code may be stored, and not all engines uses every GM specified trouble code.  In the event of an computer control module failure, the system will default to a pre-programmed set of values.  These are comprmised values which allow the engine to operate, although at reduced efficiency.  This is variously known as the Default, Limp - in, or Back - Up mode.  Driveability is alomst always affected when the Electronic Control Module (ECM) enters this mode.

Throttle Position Sensor (TPS) -
The TPS is connected to the throttle shaft on the throttle body; it's a pontentimeter with one end connected to 5 volts from teh Electronic Control Module (ECM) and the other to an electrical ground.  A third wire is connected to ECM to read the voltage from the TPS.  As the throttle vavle angle is changed (accelerator peal moved), the output voltage of the TPS changed respectively.  At a closed throttle the output of the TPS is low (approximately 0.5 volts).  As the throttle valve opens, the output increases all to 4.5 volts at wide open throttle (WOT).  By monitoring the output signal, the ECM can determine fuel delivery based on the throttle valve angle (driver demand).

Torque Convertor Clutch (TCC) Solenoid -
Most late model vehicles with an automatic transmission usa a TCC system.  The Electronic Control Module (ECM) controls the torque converter by means of a solenoid mounted in the output drive housing of the transmission.  When the vehicle reaches a cetain speed, the ECM sends a signal to the TCC solenoid, and allows the torque converter to mechanically couple the transaxle to the engine.  When operating conditions (according to various speed sensors) indicate that the transaxle should operate as a normal fluid coupled transaxle, then the ECM will de-engergize the soleniod.  Depressing the brake pedal will return to normal automatic operation.

Vehicle Speed Sensor (VSS) -
The VSS is located on the transmission and sends a pulsing voltage signal to the Electronic Control Module (ECM) which is converted to vehicle speed.  The sensor mainly controls the operation of the Torque Converter Clutch (TCC) system, shift light (if manual transmission), cruise control, and the activation of the Exhaust Gas Recirculation (EGR) system.

Wide Open Throttle (WOT) -
WOT is when the operator of the vehicle has the gas pedal depressed down to the floorboard, cauing the throttle valve to be parallel with air-flow.

Not fully in-depth for your tastes?  Need explaination of a certian trouble code?  Want to add another automotive system or acronym?  Easy!  Head to the Contact Us page to grab the e-mail to get a hold of W-Body Performance.