All of the sensors and input switches can be diagnosed using a scan tool. Following is a short description of how the sensors and switches can be diagnosed by using a scan tool. The scan tool 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 engine coolant temperature sensor is a resistor which changes value based on temperature mounted in the engine coolant stream.

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 PCM supplies a 5.0 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. The engine coolant temperature affects most of the systems the PCM controls.

The scan tool displays 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 intake air temperature displays should be close to each other. A hard fault in the engine coolant sensor circuit should set DTC P0117 Engine Coolant Temperature (ECT) Sensor Circuit Low Voltage or DTC P0118 Engine Coolant Temperature (ECT) Sensor Circuit High Voltage . An intermittent fault should set DTC P1114 Engine Coolant Temperature (ECT) Sensor Circuit Intermittent Low Voltage or DTC P1115 Engine Coolant Temperature (ECT) Sensor Circuit Intermittent High Voltage . The DTC Diagnostic Aids also contains a chart to test for sensor resistance values relative to the temperature.

The ECT sensor (3) also contains another circuit which is used to operate the engine coolant temperature gauge located in the instrument panel.

Mass Air Flow (MAF) Sensor

The mass air flow (MAF) sensor measures the amount of air which passes through the sensor. The PCM uses this information to determine the operating condition of the engine in order to control fuel delivery.

A large quantity of air indicates acceleration, while a small quantity indicates deceleration or idle.

The scan tool reads the MAF value and displays the value in grams per second (g/s). At idle, the value should be between 4 gm/s and 6 gm/s on a fully warmed up engine. The 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 one of the following DTCs:

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. A low 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 PCM supplies a 5.0 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 the spark timing according to the incoming air density.

The scan tool displays the temperature of the air entering the engine, which should be close to the ambient air temperature when the engine is cold and rise as the underhood temperature increases.

If the engine has not been run for several hours the IAT and the engine coolant temperature should read close to each other. A failure in the IAT sensor circuit should set DTC P0112 Intake Air Temperature (IAT) Sensor Circuit Low Voltage or DTC P0113 Intake Air Temperature (IAT) Sensor Circuit High Voltage .

Manifold Absolute Pressure (MAP) Sensor

The manifold absolute pressure (MAP) sensor responds to changes in intake manifold pressure (vacuum). The MAP sensor signal voltage to the PCM varies from less than 2.0 volts at idle to more than 4.0 volts with the key ON and the engine OFF, or at wide open throttle (low vacuum).

The MAP sensor is used to determine engine vacuum level for other diagnostics, to determine the barometric pressure (BARO) and to determine any manifold pressure changes while the linear EGR flow test diagnostic is being run. Refer to DTC P0401 Exhaust Gas Recirculation (EGR) Flow Insufficient .

If the PCM detects a voltage that is less than the possible range of the MAP sensor, DTC P0107 Manifold Absolute Pressure (MAP) Sensor Circuit Low Voltage will be set. A signal voltage more than the possible range of the sensor will set DTC P0108 Manifold Absolute Pressure (MAP) Sensor Circuit High Voltage . An intermittent low or high voltage will set DTC P1107 Manifold Absolute Pressure (MAP) Sensor Circuit Intermittent Low Voltage or DTC P1106 Manifold Absolute Pressure (MAP) Sensor Circuit Intermittent High Voltage . The PCM can also detect a shifted MAP sensor. The PCM compares the MAP sensor signal to a calculated MAP based on the throttle position and various engine load factors

Fuel Control Heated Oxygen Sensor (HO2S 1)

The fuel control heated oxygen sensor (HO2S 1) is mounted in the exhaust manifold where the sensor 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 at a high oxygen content lean mixture to 900 mV at a 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 PCM calculates what fuel mixture command to give to the injectors .

An open HO2S 1 circuit should set DTC P0134 HO2S Circuit Insufficient Activity Sensor 1 and the scan tool will display a constant voltage between 400 - 500 mV. A constant voltage of less than 300 mV in a grounded sensor circuit should set DTC P0131 HO2S Circuit Low Voltage Sensor 1 , while a constant voltage of more than 800 mV in the circuit should set DTC P0132 HO2S Circuit High Voltage Sensor 1 . A fault in the HO2S 1 heater circuit should cause DTC P0135 to set. The PCM can also detect HO2S response problems. If the response time of an HO2S is determined to be too slow, the PCM will store a DTC that indicates degraded HO2S performance.

Catalyst Monitor Heated Oxygen Sensor (HO2S 2)

To control the emission of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), a 3-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 converts NOx to nitrogen. The PCM has the ability to monitor this process using the HO2S 1 and the HO2S 2 . The HO2S 1 produces an output signal which indicates the amount of oxygen present in the exhaust gas entering the 3-way catalytic converter. The HO2S 2 produces an output signal which indicates the oxygen storage capacity of the catalyst. This in turn indicates the catalyst's ability to convert the exhaust gases efficiently. If the catalyst is operating efficiently, the HO2S 1 signal will be far more active than that produced by the HO2S 2 .

The catalyst monitor sensor operate the same as the fuel control sensor. Although the main function of the HO2S 2 is catalyst monitoring, the HO2S 2 also plays a limited role in fuel control. If the sensor output indicates a voltage either above or below the 450 millivolt bias voltage for an extended period of time, the PCM will make a slight adjustment to the fuel trim to ensure that fuel delivery is correct for catalyst monitoring.

A problem with the HO2S 2 signal circuit should set DTC P0137 HO2S Circuit Low Voltage Sensor 2 , DTC P0138 HO2S Circuit High Voltage Sensor 2 , or DTC P0140 HO2S Circuit Insufficient Activity Sensor 2 , depending on the specific condition. A fault in the heated oxygen sensor heater element or its ignition feed or ground will result in slower oxygen sensor response. This may cause erroneous catalyst monitor diagnostic results. A fault in the HO2S 2 heater circuit should cause DTC P0141 HO2S Heater Performance Sensor 2 to set.

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 when 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 more than 4.0 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 hard failure in the TP sensor 5.0 volt reference or signal circuits should set either DTC P0122 Throttle Position (TP) Sensor Circuit Low Voltage or DTC P0123 Throttle Position (TP) Sensor Circuit High Voltage . A hard failure with the TP sensor ground circuit may set one of the following codes:

Once a DTC is set, the PCM will use an artificial default value based on the engine RPM, the engine load, and the mass air flow for the throttle position and some vehicle performance will return. A high idle may result when either DTC P0122 Throttle Position (TP) Sensor Circuit Low Voltage or DTC P0123 Throttle Position (TP) Sensor Circuit High Voltage is set.

The PCM can detect intermittent TP sensor faults. DTC P1121 Throttle Position (TP) Sensor Circuit Intermittent High Voltage or DTC P1122 Throttle Position (TP) Sensor Circuit Intermittent Low Voltage will set if an intermittent high or low circuit failure is being detected.

The PCM can also detect a shifted TP sensor (2). The PCM monitors the throttle position and compares the actual TP sensor value to a predicted TP value calculated from the engine speed. If the PCM detects an out of range condition, DTC P0121 Throttle Position (TP) Sensor Performance will be set.

EGR Pintle Position Sensor

The EGR pintle position sensor is an integral part of the EGR valve assembly. This sensor can not be serviced separately from the EGR valve assembly.

The PCM monitors the EGR valve pintle position input to ensure that the valve responds properly to commands from the PCM and to detect a fault if the pintle position sensor and control circuits are open or shorted.

If the PCM detects a pintle position signal voltage outside the normal range of the pintle position sensor, or a signal voltage that is not within a tolerance considered acceptable for proper EGR system operation, the PCM will set DTC P1404.

Knock Sensor (KS)

The knock sensor detects abnormal vibration or spark knocking in the engine. The sensor is located on the engine block near the cylinders. The sensor produces an AC output voltage which increases with the severity of the knock. This signal voltage is sent to the PCM. The PCM then adjusts the ignition control (IC) timing in order to reduce the spark knock. DTC P0325 Knock Sensor (KS) Circuit and DTC P0327 Knock Sensor (KS) Circuit are designed to diagnose the PCM, the knock sensor, and the related wiring, so problems encountered with the KS system should set a DTC.

Refer to Knock Sensor (KS) System Description .

PCM Controlled A/C

This vehicle may be equipped with a dual zone A/C (CJ3) system or the standard A/C (C60). The both systems are designed to provide a comfortable environment inside the passenger compartment. The PCM controlled A/C operation is the same for either system. Refer to Powertrain Control Module Controlled Air Conditioning Description .

Transaxle Controls

The electrical components of this unit are as follows:

For more information, refer to Electronic Component Description .

7X Crankshaft Position (CKP) Sensor

The 7X crankshaft position sensor provides a signal used by the ignition control module.

The ignition control module also uses the 7X crankshaft position sensor to generate 3X reference pulses which the PCM uses to calculate RPM and crankshaft position.

24X Crankshaft Position (CKP) Sensor

The 24X crankshaft position (CKP) sensor (1) is used to improve the idle spark control at engine speeds up to approximately 1600 RPM.

3X Reference

The PCM uses this signal, from the ignition control module to calculate engine speed and crankshaft position over 1600 RPM.

The PCM also uses the pulses on this circuit to initiate injector pulses. If the PCM receives no pulses on this circuit, DTC P1374 Crankshaft Position (CKP) High to Low Resolution Frequency Correlation will set and the PCM will use the 24X reference signal circuit for fuel and ignition control.

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, the circuit is not connected to ground at the PCM.

The PCM compares voltage pulses on the reference input circuits to pulses on this circuit, ignoring any pulses that appear on both.

Camshaft Position (CMP) Sensor and CAM Signal

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 PCM uses the CAM signal to indicate the position of the #1 piston during the intake stroke. This allows the PCM to calculate true sequential fuel injection (SFI) mode of operation. If the PCM detects an incorrect CAM signal while the engine is running, DTC P0341 Camshaft Position (CMP) Sensor Performance will set.

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 the injector sequence being correct.

Fuel Level Sensor

The fuel level sensor (4) is mounted on the modular fuel sender assembly (5). The PCM uses the fuel level input for various diagnoses including the EVAP system. In addition the PCM transmits the fuel level over the Class II communication circuit to the IPC. The low fuel level message may not appear if other messages are being commanded, such as the rear deck lid, driver, or passenger doors ajar. Ensure that all doors and compartment lids are completely closed. For further information regarding the fuel level sensor refer to Fuel Metering Modes of Operation .

Fuel Tank Pressure Sensor

The fuel tank pressure (FTP) sensor (6) is mounted on top the modular fuel sender assembly (5). The PCM uses the fuel tank pressure input for the EVAP system. The PCM supplies a 5 volt reference to the sensor and a sensor return ground. The PCM monitors the signal circuit from the sensor within a voltage range of 0.1 volts to 4.9 volts. When the pressure inside the fuel tank is totally vented the pressure is equal to the atmospheric pressure or approximately 1.3-1.7 volts. When the tank is pressurized the voltage can reach more than 4.5 volts. For further information regarding the FTP sensor, refer to Fuel Metering Modes of Operation and to Evaporative Emission Control System Operation Description .

Idle Air Control (IAC) Valve

The IAC valve is used to control the engine idle speed while preventing stalls due changes in the engine load. For further information regarding the IAC valve refer to Fuel Metering Modes of Operation .

Cruise Control

The cruise control module sends the cruise status input to the PCM to indicate when the cruise control is engaged.

The PCM monitors the cruise status signal while commanding the cruise be disengaged via the cruise inhibit circuit.

The cruise control module terminal K is the vehicle speed signal input terminal through circuit 817. In operation, the voltage varies between 0.0-5.0 V. The cruise control module terminal J is used to signal the PCM when the cruise control is engaged through circuit 85. The PCM will then determine the correct shift pattern for the transmission. The cruise control module terminal H is used by the PCM through circuit 83 to inhibit cruise control when conditions inconsistent with cruise operation are present.

The PCM will inhibit cruise control under the following conditions:

For more information regarding the cruise control system refer to the following descriptions in Cruise Control:

Engine Oil Level Switch

The PCM monitors the engine oil level switch signal at start-up to determine if the engine oil level is OK. If the PCM determines that a low oil level condition exists, the PCM will communicate the information over the Class II circuit to the IPC and illuminate the indicator lamp or initiate a message.

The low oil level message may not appear if other messages are being commanded, such as the rear deck lid, driver, or passenger doors ajar. Ensure that all doors and compartment lids are completely closed.

Engine Oil Pressure Switch

The PCM monitors the engine oil pressure switch (1) signal to determine if the engine oil pressure is OK. If the PCM determines that a low oil pressure condition exists, the PCM will communicate the information over the Class II circuit to the IPC and illuminate the indicator lamp or initiate a message.

The low oil pressure message may not appear if other messages are being commanded, such as the rear deck lid, driver, or passenger doors ajar. Ensure that all doors and compartment lids are completely closed.

PCM Controlled Warning Lamps

The PCM controlled warning lamps are intended to alert the driver about an operating condition which may require immediate attention. The IP Cluster has a hierarchy for displaying messages. If the Rear Compartment Lid, Driver, or Passenger Doors are open this will take precedent over other messages such as low oil level or low oil pressure. It is imperative that all passenger doors and rear compartment lid be completely closed while diagnosing the warning lamps, or messages. For further information regarding warning lamps not controlled by the PCM refer to the following in Instrument Panel, Gauges, and Console:

MIL Operation

The malfunction indicator lamp (MIL) is located on the instrument panel and is displayed as CHECK ENGINE lamp.

MIL Function

MIL Illumination

1. The MIL will illuminate with ignition ON and the engine OFF
2. The MIL will turn OFF when the engine is started
3. The MIL will remain ON if the self-diagnostic system has detected a malfunction
4. The MIL may turn OFF if the malfunction is not present
5. If the MIL is illuminated and the engine stalls, the MIL will remain illuminated while the ignition is ON.
6. If the MIL is not illuminated and the engine stalls, the MIL will not illuminate until the ignition is cycled OFF, then ON.