PCM Inputs
All of the sensors and input switches can be diagnosed by the use of a Tech 1. Following is a short description of how the sensors and switches can be diagnosed by using a Tech 1. The Tech 1 can also be used to compare the values for a normal running engine with the engine you are diagnosing.Engine Coolant Temperature Sensor:
ENGINE COOLANT TEMPERATURE (ECT) SENSOR
The Engine Coolant Temperature (ECT) sensor is a thermistor (a resistor which changes value based on temperature) mounted in the engine coolant stream. Low coolant 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 Powertrain Control Module (PCM) supplies a 5 volt signal to the engine coolant temperature sensor through a resistor in the PCM and measures the voltage. The voltage will be high when the engine is cold, and low when the engine is hot. By measuring the voltage, the PCM calculates the engine coolant temperature. Engine coolant temperature affects most systems the PCM controls.
The Tech 1 displays engine coolant temperature in degrees. After engine start-up, 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 intake air temperature displays should be close to each other. A fault in the engine coolant sensor circuit should set DTC P0117 or DTC P0118. The DTC charts also contain a table to check for sensor resistance values relative to temperature.
MASS AIR FLOW (MAF) SENSOR
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 fuel delivery. A large quantity of air indicates acceleration, while a small quantity indicates deceleration or idle.
The Tech 1 reads the MAF value and displays it in grams per second. It should read between 4-7 on a fully warmed up idling engine. Values should change rather quickly on acceleration, but values should remain fairly stable at any given RPM. A failure in the MAF sensor or circuit should set DTC P0101.
INTAKE AIR TEMPERATURE (IAT) SENSOR
The Intake Air Temperature (IAT) sensor is a thermistor which changes value based on the temperature of air entering the engine. 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 to the sensor through a resistor in the PCM 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 PCM calculates the incoming air temperature.
The IAT sensor signal is used to adjust spark timing according to incoming air density.
The Tech 1 displays temperature of the air entering the engine, which should read close to ambient air temperature when engine is cold, and rise as underhood temperature increases. If the engine has not been run for several hours (overnight) the IAT sensor temperature and engine coolant temperature should read close to each other. A failure in the IAT sensor circuit should set DTC PO112 or DTC PO113.
TRANSAXLE FLUID TEMPERATURE (TFT) SENSOR
The Transaxle Fluid Temperature (TFT) sensor is a thermistor which changes value based on the temperature of the transaxle fluid. A high transaxle fluid temperature may cause the vehicle to operate in "Hot Mode." While in "Hot Mode," shift points may be altered, 4th gear disabled, and [1][2]Torque Converter Clutch (TCC) forced ON in 2nd gear.
A failure in the TFT sensor or associated wiring should cause DTC P0712 or P0713 to set. In this case, engine coolant temperature will be substituted for the TFT sensor value, and the transaxle will operate normally.
HEATED OXYGEN SENSOR 1 (H02S 1)
The exhaust Heated Oxygen Sensor 1 (HO2S 1) is mounted in the exhaust manifold where it can monitor the oxygen content of the exhaust gas stream.
The oxygen content in the exhaust reacts with the sensor to produce 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 Tech I.
By monitoring the voltage output of the oxygen sensor, the PCM calculates what fuel mixture command to give to the injector (lean mixture-low HO2S 1 voltage=rich command, rich mixture-high HO2S 1 voltage=lean command).
The heated oxygen sensor circuit, if open, should set a DTC P0134 and the Tech 1 will display a constant voltage between 350 - 550 mV. A constant voltage below 250 mV in the sensor circuit should set DTC P0131, while a constant voltage above 750 mV in the circuit should set DTC P0132. DTC P0131 and DTC P0132 could also be set as a result of fuel system problems. Refer to DTC charts for conditions that can cause a lean or rich system.
A fault in the heated oxygen sensor heater element or its ignition feed or ground will result in longer times to "Closed Loop" fuel control and slower oxygen sensor response. This may cause increased emissions, especially at start-up.
HEATED OXYGEN SENSOR 2 (H02S 2)(Catalyst Monitor)
In order to control 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 PCM has the capability to monitor this process using HO2S 2. HO2S 2, located in the exhaust stream past the three-way catalytic converter, produces an output signal which indicates the oxygen storage capacity of the catalyst; this in turn indicates the catalyst's ability to convert exhaust emissions effectively.
A problem with the HO2S 2 electrical circuits should set DTC P0137, P0138 or P0140, depending on the specific condition. If the catalyst is functioning correctly, the HO2S 2 signal will be far less active than that produced by HO2S 1. If a problem exists which causes the PCM to detect excessive HO2S 2 activity outside of an acceptable range for an extended period of time, the PCM will set DTC P0420, indicating that the three-way catalytic converter's oxygen storage capacity is below a threshold considered acceptable.
Throttle Position (TP) Sensor (Typical):
THROTTLE POSITION (TP) SENSOR
The Throttle Position (TP) sensor is a potentiometer connected to the throttle shaft on the throttle body. By monitoring the voltage on the signal line, the PCM calculates throttle position. As the throttle valve angle is changed (accelerator pedal 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 volts.
The PCM calculates fuel delivery based on throttle valve angle (driver demand). A broken or loose TP sensor may cause intermittent bursts of fuel from an injector and unstable idle because the PCM thinks the throttle is moving. A problem in the TP sensor 5 volts reference or signal circuits should set either a DTC P0122 or DTC P0123. A problem with the TP sensor ground circuit may set DTCs P0123 and PO117. Once a DTC is set, the PCM will use an artificial default value based on mass air flow for TP sensor and some vehicle performance will return. A high idle may result when either DTC P0122 or DTC P0123 is set.
VEHICLE SPEED SENSOR (VSS)
The Vehicle Speed Sensor (VSS) sends a pulsing voltage signal to the PCM which the PCM converts to miles per hour. This sensor mainly controls the operation of the TCC, shift solenoids, and cruise control systems. There are several different types of vehicle speed sensors. A scan tool display should closely match with speedometer reading with drive wheels turning.
KNOCK SENSOR
To control spark knock, a Knock Sensor (KS) system is used. This system is designed to retard spark timing up to 10° to reduce spark knock in the engine. This allows the engine to use maximum spark advance to improve driveability and fuel economy.
The knock sensor detects abnormal vibration (spark knocking) in the engine. The sensor is mounted in the engine block near the cylinders. The sensor produces an AC output voltage which increases with the severity of the knock. This signal voltage inputs to the PCM. The PCM then adjusts the Ignition Control (IC) timing to reduce spark knock.
A/C REQUEST SIGNAL
This signal tells the PCM when an A/C mode is selected, and the A/C pressure cycling switch is closed. The PCM uses this to adjust the idle speed before turning ON, the A/C clutch. If this signal is not available to the PCM, the A/C compressor will be inoperative.
GENERATOR CONTROL (CKT 225)
The PCM controls the vehicle charging system by applying B+ to the generator "L" terminal while the engine is running.
TMNSS/Transmission Range Switch:
PRNDL INPUTS/TRANSAXLE RANGE SWITCH
The PRNDL inputs from the transaxle range switch indicate to the PCM which position is selected by the transaxle selector lever. This information is used for ignition timing, EVAP canister purge and Idle Air Control (IAC) valve operation.
CAUTION: Vehicle should not be driven with transaxle range switch disconnected, as idle quality will be affected.
CRANKSHAFT POSITION SENSOR
The crankshaft position sensor provides a signal used by the ignition control module to calculate ignition sequence and provide the fuel control reference signal, which the PCM uses as reference to calculate RPM and crankshaft position.
FUEL CONTROL REFERENCE PCM INPUT (CKT 430)
From the ignition control module, the PCM uses this signal to calculate engine RPM and crankshaft position. The PCM compares pulses on this circuit to reference ground CKT 453. The PCM also uses the pulses on this circuit to initiate injector pulses. If the PCM receives no pulses on this circuit, no fuel injection pulses will occur and the engine will not run.
CRANKSHAFT REFERENCE GROUND (CKT 453)
This is a ground circuit for the digital RPM counter inside the PCM, but the wire is connected to engine ground only through the ignition control module. Although this circuit is electrically connected to the PCM, it is not connected to ground at the PCM. The PCM compares voltage pulses on the reference input CKT 430 to any on this circuit, ignoring pulses that ground on both. If the circuit is open, or connected to ground at the PCM, it may cause poor engine performance and possibly a MIL (Service Engine Soon) with no DTC.
CAMSHAFT POSITION SENSOR AND CAM SIGNAL (CKT 630)
The camshaft position sensor sends a cam signal to the PCM which uses it as a "sync pulse" to trigger the injectors in proper sequence. The cam signal is passed through the ignition control module. It is not processed in any way.
The PCM uses the cam signal to determine the position of the #1 piston during its power stroke. This allows the PCM to calculate true Sequential Fuel Injection (SFI) mode of operation. A loss of this signal will set DTC P0342.
If the cam signal is lost while the engine is running, the fuel injection system will shift to a calculated sequential fuel injection mode based on the last fuel injection pulse, and the engine will continue to run. The engine can be restarted and will run in the calculated sequential mode as long as the fault is present with a 1 in 6 chance of injector sequence being correct.