PCM Inputs

Following is a short description of how the sensors and switches can be diagnosed by the use of a Tech 1 Scan Tool (or equivalent). 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 (ECT) SENSOR
The Tech 1 Scan Tool displays engine coolant temperature in degrees Celsius. After engine is started, the temperature should rise steadily to about 90°C (194°F) then stabilize when the thermostat opens. If the engine has not been run for several hours (overnight), the ECT and Intake Air Temperature (IAT) should be close to each other. A fault in the Engine Coolant Temperature (ECT) sensor circuit should set a Diagnostic Trouble Code (DTC) 14 or 15. The DTC charts also contain a chart to check for Engine Coolant Temperature (ECT) sensor resistance values relative to temperature.

INTAKE AIR TEMPERATURE (IAT) SENSOR
The Tech 1 Scan Tool displays temperature of the air entering the engine and should be 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 ECT sensors should display temperature readings close to each other. A failure in the IAT sensor circuit should set a DTC 23 or 25. The DTC charts also contain a chart to check for sensor resistance values relative to temperature.

MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR
A Tech 1 scan tool displays manifold pressure in volts. Low pressure (high vacuum) displays a low voltage while a high pressure (low vacuum) displays a high voltage. A failure in the MAP sensor circuit should set a DTC 33 or 34 and using the chart will find the cause of the problem. A DTC 33 may be set if a rough or unstable idle exists. CHART C-1D can also be used to check MAP sensor output.

HEATED OXYGEN SENSOR (HO2S)
The Tech 1 Scan Tool has several positions that will indicate the state of the exhaust gases. HO2S voltage, short term fuel trim. and long term fuel trim.
A problem in the HO2S circuit , or fuel system, should set a DTC 13 (open circuit), DTC 44 (lean indication), DTC 45 (rich indication). Normal Tech 1 Scan Tool voltage varies between 10 mV to 1000 mV (0.01 and 1.0 volt) while in "Closed Loop." DTC 13 and 63 will in one minute if voltage remains between 350 mV and 550 mV, but the system will go "open Loop" in about 15 seconds.

THROTTLE POSITION (TP) SENSOR
The Tech 1 displays throttle position in volts. Voltage should increase at a steady rate as throttle is moved toward Wide Open Throttle (WOT).
The PCM has the ability to auto-zero the TP sensor voltage if it is between about 0.29 volt (290 mV) and 0.98 volt(980 mV). This means that any voltage in this window will be determined by the PCM to be 0% throttle. Tech 1 scan tools have the ability to display the throttle angle and should be 0% when the throttle is closed.

If the throttle angle at idle displayed on the Tech 1 at a value greater than 1%, check for cable binding or a stuck throttle valve. If there is no mechanical fault, remove power from the PCM for 20 seconds. This will clear throttle angle from memory. Restore PCM power and then monitor throttle angle with the Tech 1 at idle. If the value indicated is still greater than 1% at idle, the TP sensor or PCM may be at fault.
A failure in the TP sensor or circuit should set a DTC 21 or 22.

VEHICLE SPEED SENSOR (VSS)
The Tech 1 reading should closely match the speedometer reading with drive wheels turning. A failure in the VSS circuit should set a DTC 24.

TRANSAXLE RANGE SWITCH
The input from the range switch indicates to the PCM which gear the transaxle selector lever has selected. This information is used for ignition timing, evaporative emissions (EVAP) canister purge and IAC valve operation.

NOTICE: Vehicle should not be driven with PRNDL transmission range switch disconnected as idle quality will be affected.

3X CRANKSHAFT POSITION SENSOR
The Tech 1 scan tool displays crankshaft position sensor data in RPM. An error in the crankshaft position sensor circuit should set a DTC 82.

CAMSHAFT POSITION SENSOR
The Tech 1 scan tool will display camshaft position sensor data as a 0 and 1. As the sensor pulses, the scan data will switch from 0 to 1. All camshaft position sensor data should be checked at idle. An error in the camshaft position sensor circuit should set a DTC 17.

24X CRANKSHAFT POSITION SENSOR
The Tech 1 scan tool displays crankshaft position sensor data in RPM. An error in the 24X crankshaft position sensor circuit should set a DTC 82.

3X REFERENCE HIGH SIGNAL
The ignition control module receives signals from the 3X crankshaft position sensor located in the engine block. This signal is then conditioned and sent to the PCM as a 5 volt digital signal. The PCM uses this input as part of it's final calculation for Ignition Control (IC) timing.