Data Sensors

Engine Coolant Temperature Sensor
The engine temperature sensor is located in the flow of the engine coolant and sends information concerning temperature to the ECM.

Mass Air Flow (MAF) Sensor (Exc. 4-110, 1.8L Turbo)
The mass air flow (MAF) sensor measures the amount of air being consumed by the engine and sends the information to the ECM. The ECM uses this information to determine the operating condition of the engine and to meter the fuel accordingly. If the MAF sensor indicates a large quantity of air is being consumed, the ECM will recognize the engine is accelerating. If the MAF sensor indicates a small quantity of air is being consumed, the ECM will recognize this as deceleration or idle.

V6-173 Engine w/AC Delco MAF Sensor (Exc. Fiero)
The MAF sensor used on this engine produces a frequency output which is directly proportional to the air entering the engine. The output will vary from approximately 32 Hertz at idle to 150 Hertz during wide open throttle applications. If the sensor fails at a low frequency, a code 34 will be set and stored. A code 44 or 45 may also be caused by a faulty MAF sensor.

V8-305 Engine w/Bosch MAF Sensor
The Bosch MAF Sensor is of the hot wire type. Current is supplied to the sensing wire (hot wire) to maintain a calibrated temperature, and as air flow increases or decreases the current will vary. This varying of current causes a voltage drop within the meter circuitry which is directly proportional to the incoming air mass. The ECM supplies a current limiting 5 volt source on the signal line. The MAF sensor draws approximately .4 volts with a low air flow and up to 5 volts with a high air flow (during wide open throttle applications).

Manifold Air Temperature (MAT) Sensor
The manifold air temperature (MAT) sensor is a thermistor mounted in the intake manifold. Low intake air temperature produces a high resistance, while high intake air resistance produces low resistance. The ECM supplies a 5 volt signal to the sensor through a resistor and measures the voltage. By measuring the voltage, the ECM can determine MAT.

Manifold Absolute Pressure (MAP) Sensor
The manifold absolute pressure (MAP) sensor measures changes in the intake manifold pressure resulting from engine load and speed changes and converts this to a voltage output. By monitoring the sensor output voltage, the ECM can determine the MAP. The higher the MAP, the lower the engine vacuum and the higher the sensor output, which requires more fuel. The lower the MAP, the higher the engine vacuum and the lower the sensor output, which requires less fuel.

Vehicle Speed Sensor (VSS)
The vehicle speed sensor (VSS) sends a pulsating voltage signal to the ECM, which the ECM converts to MPH. This sensor mainly controls the operation of the Transmission Controlled Clutch (TCC) system.

Oxygen Sensor
The exhaust oxygen sensor is located in the exhaust system and monitors oxygen content in the exhaust gas stream. The oxygen content of the exhaust gas reacts with the oxygen sensor to produce a voltage output. By monitoring the oxygen sensor output voltage, the ECM can determine the amount of oxygen in the exhaust gas and adjust the air/fuel mixture accordingly.

Throttle Position Sensor (TPS)
The throttle position sensor is connected to the throttle valve shaft in the throttle body unit. The TPS is a potentiometer with one connection to ground and the other to a ECM 5 volt voltage source. A third wire from the ECM is used to measure the voltage from the TPS. As the throttle angle changes, so does the TPS voltage output. By monitoring TPS voltage output, the ECM can determine fuel delivery rate based on throttle valve angle.

Park/Neutral Switch
The park/neutral switch indicates to the ECM when the transmission is in park or neutral. This information is used by the ECM for ignition timing, IAC operation and Transmission Controlled Clutch (TCC) operation. Do not drive vehicle with park/neutral switch disconnected, as idle quality may be affected.

Crank Signal
This signal is used to inform the ECM that the engine is cranking. The ECM uses this information to tell when the engine is in the starting mode.

Distributor (EST) Reference Signal
This signal is used to inform the ECM of engine RPM and crankshaft position.

Cam Sensor (V6-231, 3.8L & 3800)
The cam sensor transmits a ``sync pulse'' signal to the ECM to trigger the injectors in proper sequence.

Power Steering Pressure Switch
The power steering pressure switch opens during high pressure power steering situations. When the power steering switch is open, power to the air conditioning relay is shut off, resulting in compressor clutch disengagement. A signal is also sent to the ECM. The ECM uses this signal for idle control.

Air Conditioning ``ON'' Signal
The air conditioning ``On'' signal informs the ECM the A/C selector switch is turned on, and the A/C pressure cycling switch is closed. Using this information, the ECM adjusts engine idle speed as necessary to ensure a smooth idle.

``Check Engine'' Or ``Service Engine Soon'' Lamp (CEL)
This warning lamp, located on the instrument panel, serves two purposes: to alert the vehicle operator that a fault has been detected in the system and should be serviced as soon as possible and as a diagnostic aid to display trouble codes. The ``Check Engine'' Lamp (CEL) and ``Service Engine Soon'' lamp perform the same function and which light is used depends on particular vehicle application.
This lamp will illuminate with the ignition key on and the engine not running. When the engine is started, the lamp should go out. If the lamp remains lit, the self-diagnostic system has detected a malfunction. If the malfunction clears, the lamp will go out after ten seconds and a trouble code will be stored in the ECM.

ALDL connector:

Assembly Line Diagnostic Link (ALDL) Connector
To extract a trouble code from the ECM for diagnostic purposes, the Assembly Line Diagnostic Link (ALDL) connector, Fig. 6, is used. The ALDL connector is located in the passenger compartment. Terminal B of the connector is the test terminal and terminal A is the ground used for diagnostic display.
If the test terminal is grounded with ignition on and engine not running, the system will enter the diagnostic mode. In this mode, any stored trouble codes will be displayed by flashing the CEL. In the diagnostic mode, the ECM will also energize all ECM controlled relays and solenoids. If the test terminal is grounded with the engine running, the system will enter the field service mode. In this mode, the CEL will indicate whether the systems in open or closed loop operation. If the system is in open loop operation, the lamp will flash approximately two and one-half times per second. Closed loop operation is indicated by the lamp flashing approximately once per second. In closed loop operation, the lamp will stay out most of the time if system is too lean and remain on most of the time if system is too rich.
The remaining terminals of the ALDL connector allow access to specific system circuit information. Terminals ``E'' or ~``M,'' depending upon the application, allow for the connection of SCAN type testers in order to read serial data. Serial data is a constant stream of information which relates all current ECM sensor inputs and ECM commands to controlled devices. Reading this data allows the system to be monitored under actual operating conditions, and aids in diagnosing system malfunctions.



Trouble Codes
The ECM uses sensors to monitor various engine operating conditions and compares present values with preset values stored in its memory. When the ECM detects a malfunction in any of the monitored systems, it will turn on the CEL and store a corresponding trouble code in the memory. Each trouble code indicates a malfunction in a specific circuit.