Information Sensors/Switches Description. GMC Truck GMC P42 Commercial Chassis 1998

For 1990-2009 cars only

Makes 🢒 GMC_Truck 🢒 1998 🢒 GMC P42 Commercial Chassis 🢒 Engine 🢒 Engine Controls - 4.3L 🢒 Information Sensors/Switches Description

Engine Coolant Temperature (ECT) Sensor


(1)	ECT Electrical Connector
(2)	Connector Tab
(3)	Engine Coolant Temperature (ECT) Sensor

The engine coolant temperature sensor is a thermistor (a resistor which changes value based on the temperature) mounted in the engine coolant passage. A low coolant temperature produces a high resistance (100,000 ohms at -40°C/-40°F) while a high temperature causes a low resistance (70 ohms at 130°C/266°F).

The VCM supplies a 5 volt signal to the engine coolant temperature sensor through a resistor in the VCM and then measures the voltage. The voltage will be high when the engine is cold. The voltage will be low when the engine is hot. By measuring the voltage, the VCM calculates the engine coolant temperature. The engine coolant temperature affects most systems the VCM controls.

The scan tool displays the engine coolant temperature in degrees. After engine startup, the temperature should rise steadily to about 90°C (194°F), then stabilize when thermostat opens. If the engine has not been run for several hours (overnight), the engine coolant temperature and the intake air temperature displays should be close to each other. When the VCM detects a malfunction in the ECT sensor circuit, the following DTCs will set:

•	DTC P0117 circuit low

•	DTC P0118 circuit high

•	DTC P0125 excessive time to Closed Loop

•	DTC P1114 circuit intermittent low

•	DTC P1115 circuit intermittent high

The above DTCs contain a table in order to check for sensor resistance values that are relative to temperature.

Mass Air Flow (MAF) Sensor

The mass air flow (MAF) sensor measures the amount of air entering the engine. The VCM uses this information to determine the operating condition of the engine in order to control the fuel delivery. A large quantity of air indicates an acceleration. A small quantity of air indicates a deceleration or an idle.

The scan tool reads the MAF value and displays the MAF value in grams per second (gm/Sec). At idle, the MAF value should read between 5 gm/sec - 7 gm/sec on a fully warmed up engine. The values should change rather quickly on acceleration, but values should remain fairly stable at any given RPM. When the VCM detects a malfunction in the MAF sensor circuit, the following DTCs will set:

•	DTC P0101 System performance

•	DTC P0102 Frequency low

•	DTC P0103 Frequency high

Intake Air Temperature (IAT) Sensor


(1)	Intake Air Temperature (IAT) Sensor
(2)	Electrical Harness Connector

The intake air temperature (IAT) sensor is a thermistor which changes the value based on the temperature of the air entering the engine. A low temperature produces a high resistance (100,000 ohms at -40°C/-40°F). A high temperature causes a low resistance (70 ohms at 130°C/266°F). The VCM supplies a 5.0 volt signal to the sensor through a resistor in the VCM and measures the voltage. The voltage will be high when the incoming air is cold, and low when the air is hot. By measuring the voltage, the VCM calculates the incoming air temperature. The IAT sensor signal is used in order to adjust the spark timing according to the incoming air density.

The scan tool displays the temperature of the air entering the engine, which should read close to the ambient air temperature when the engine is cold. The temperature should rise as the underhood temperature increases. If the engine has not been run for several hours (overnight), the IAT sensor temperature and the engine coolant temperature should read close to each other. If the VCM detects a malfunction in the IAT sensor circuit, the following DTCs will set:

•	DTC P0112 circuit low

•	DTC P0113 circuit high

•	DTC P1111 circuit intermittent high

•	DTC P1112 circuit intermittent low

Manifold Absolute Pressure (MAP) Sensor

The manifold absolute pressure (MAP) sensor responds to any changes in the intake manifold pressure. The pressure any changes as a result of the engine load and speed. The MAP sensor converts this to a voltage output.

A closed throttle on the engine coast down would produce a relatively low MAP output voltage. A wide open throttle would produce a high MAP output voltage. This high output voltage is produced because the pressure inside of the manifold is the same as outside the manifold. The MAP is inversely proportional to what is measured on a vacuum gauge. The MAP sensor is used for the following:

•	Altitude determination

•	Ignition timing control

•	EGR diagnostic

•	Speed density fuel management default

When the VCM detects a malfunction in the MAP sensor circuit, the following DTCs will set:

•	DTC P0106 circuit performance malfunction

•	DTC P0107 circuit low

•	DTC P0108 circuit high

•	DTC P1106 intermittent circuit high

•	DTC P1107 intermittent circuit low

Heated Oxygen Sensor (Cutaway View)


(1)	Four Wire In-Line Connector
(2)	Heater Termination
(3)	Water Shield Assembly
(4)	Sensor Lead
(5)	Flat Seat Shell
(6)	Seat Gasket
(7)	Outer Electrode and Protective Coating
(8)	Rod Heater
(9)	Inner Electrode
(10)	Zirconia Element
(11)	Insulator
(12)	Clip Ring
(13)	Gripper

Heated Oxygen Sensors (HO2S)

The Heated Oxygen Sensors are mounted in the exhaust system where they can monitor the oxygen content of the exhaust gas stream. The oxygen present in the exhaust gas reacts with the sensor to produce a voltage output. This voltage should constantly fluctuate from approximately 100 mV (high oxygen content -- lean mixture) to 900 mV (low oxygen content -- rich mixture). The heated oxygen sensor voltage can be monitored with a scan tool. By monitoring the voltage output of the oxygen sensor, the VCM calculates what fuel mixture command to give to the injectors (lean mixture -- low HO2S voltage = rich command, rich mixture -- high HO2S voltage = lean command).

When the VCM detects an HO2S signal circuit that is low, the VCM will set the following DTCs:

•	DTC P0131 Bank 1 Sensor 1 HO2S

•	DTC P0151 Bank 2 Sensor 1 HO2S

•	DTC P0143 Bank 1 Sensor 3 HO2S

When the VCM detects an HO2S signal circuit that is high, the VCM will set the following DTCs:

•	DTC P0132 Bank 1 Sensor 1 HO2S

•	DTC P0152 Bank 2 Sensor 1 HO2S

•	DTC P0144 Bank 1 Sensor 3 HO2S

When the VCM detects no HO2S activity, the VCM will set the following DTCs:

•	DTC P0134 Bank 1 Sensor 1 HO2S

•	DTC P0154 Bank 2 Sensor 1 HO2S

•	DTC P0146 Bank 1 Sensor 3 HO2S

A fault in the heated oxygen sensor heater element or its ignition feed or ground will result in an increase in time to Closed Loop fuel control. This may cause increased emissions, especially during start-up. When the VCM detects a malfunction in the HO2S heater circuits, the following DTCs will set:

•	DTC P0135 Bank 1 Sensor 1 HO2S heater

•	DTC P0155 Bank 2 Sensor 1 HO2S heater

•	DTC P0147 Bank 1 Sensor 3 HO2S heater

The VCM also has the ability to detect HO2S response, switching, transition time, and incorrect ratio voltage problems. If an HO2S response switching, transition time, or ratio problem is detected, the VCM will store a DTC which indicates degraded an HO2S performance.

Heated Oxygen Sensors (HO2S)

In order to control the emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NOx), a three-way catalytic converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it to nitrogen. The VCM has the ability to monitor this process by using the Bank 1 Sensor 1 and the Bank 1 Sensor 3 heated oxygen sensors. The Bank 1 Sensor 1 sensor produces an output signal which indicates the amount of oxygen present in the exhaust gas entering the three-way catalytic converter. The Bank 1 Sensor 3 sensor produces an output signal which indicates the oxygen storage capacity of the catalyst; this in turn indicates the ability of the catalyst to convert the exhaust gases efficiently. If the catalyst is operating efficiently, the Bank 1 Sensor 1 sensor will produce a far more active signal than that produced by the Bank 1 Sensor 3 sensor.

The HO2S sensors which are used in order to monitor the catalyst function are the same as the HO2S sensors which are used in order to control the fuel. Bank 1 Sensor 1 and Bank 1 Sensor 3 are used primarily for catalyst monitoring. These sensors also play a limited role in fuel control. If a sensor output indicates a voltage either above or below the 450 millivolt bias voltage for an extended period of time, the VCM will make a slight adjustment to the fuel trim in order to ensure that the fuel delivery is correct for catalyst monitoring.

Throttle Position (TP) Sensor


(1)	Vehicle Control Module (VCM)
(2)	Throttle Position (TP) Sensor
(3)	Throttle Valve

The Throttle Position (TP) sensor is a potentiometer. The TP sensor is connected to the throttle shaft on the throttle body. By monitoring the voltage on the signal line, the VCM calculates the throttle position. As the throttle valve angle is changed (the accelerator pedal is moved), the TP sensor signal also changes. At a closed throttle position, the output of the TP sensor is low. As the throttle valve opens, the output increases so that at Wide Open Throttle (WOT), the output voltage should be above 4.0 volts.

The VCM calculates the fuel delivery based on the throttle valve angle (driver demand). A broken or loose TP sensor may cause intermittent bursts of fuel from an injector. This may cause an unstable idle because the VCM detects the throttle is moving.

When the VCM detects a malfunction with the TP sensor circuits, the following DTCs will set:

•	DTC P0121 circuit performance malfunction

•	DTC P0122 circuit low

•	DTC P0123 circuit high

•	DTC P1121 intermittent circuit high

•	DTC P1122 intermittent circuit low

Evaporative Canister Purge Valve

The Evaporative Canister Purge Valve allows the fuel vapor to flow from the EVAP canister to the engine when active. The normally closed valve is pulse width modulated by the VCM in order to control vapor flow.

Knock Sensor (KS) Sensor

The knock sensor (KS) system is used in order to detect the engine detonation. The VCM will retard the spark timing based on the signals from the KS module. The knock sensors produce an AC voltage that is sent to the KS module. The amount of the AC voltage produced is proportional to the amount of knock.

An operating engine produces a normal amount of engine mechanical vibration (Noise). The knock sensors will produce an AC voltage signal from this Noise. When an engine is operating, the VCM will learn the minimum and maximum frequency of the noise that the engine produces. When the VCM determines that this frequency is less than or greater than the expected amount, a knock sensor DTC will set.

A/C Request Signal

The A/C request circuit signals the VCM when an A/C mode is selected at the A/C control head. The VCM uses this information in order to enable the A/C compressor clutch and to adjust the idle speed before turning ON the A/C clutch. If this signal is not available to the VCM, the A/C compressor will be inoperative.

Refer to A/C Control Circuit Diagnostics for A/C wiring diagrams and diagnosis of the A/C electrical system.

Vehicle Speed Sensor (VSS)

The vehicle speed sensor (VSS) is a pulse counter type input that informs the VCM how fast the vehicle is being driven. The VSS system uses an inductive sensor mounted in the tail housing of the transmission and a toothed reluctor wheel on the tail shaft. As the reluctor rotates, the teeth alternately interfere with the magnetic field of the sensor creating an induced voltage pulse.

The VSS produces an AC voltage signal that increases with the vehicle speed. The VCM processes this signal and sends it to the following components:

•	The instrument panel

•	The cruise control module

Crankshaft Position (CKP) Sensor

The crankshaft position sensor provides the VCM with the crankshaft speed and the crankshaft position. The VCM utilizes this information in order to determine if an engine Misfire is present. The VCM monitors the CKP sensor for a momentary drop in the crankshaft speed in order to determine if a misfire is occurring. When the VCM detects a misfire, a DTC P0300 will set.

The VCM also monitors the CKP sensor signal circuit for malfunctions. The VCM monitors CKP signal and the High and Low resolution signals. The VCM calculates these signals in order to determine a ratio. When the VCM detects that the ratio is out of normal operating range, the VCM will set a DTC P0337 or a DTC P0338.

Camshaft Position (CMP) Sensor

The Camshaft Position (CMP) sensor is located within the distributor. The operation of the CMP sensor is very similar to the Crankshaft Position (CKP) sensor. The CMP sensor will provide one pulse per camshaft revolution (1x signal). This signal will not affect the driveability of the vehicle. The VCM utilizes this signal in conjunction with the crankshaft position in order to determine which cylinder(s) are misfiring.