ECM Charts

ENGINE SPEED - Engine speed is computed by the ECM from the fuel control reference input. It should remain close to desired idle under various engine loads with engine idling.

DESIRED IDLE - The idle speed that is commanded by the ECM. The ECM will compensate for various engine loads to keep the engine at or near the desired idle speed.

ENGINE COOLANT TEMP - The Engine Coolant Temperature (ECT) sensor is mounted in the intake manifold and sends engine temperature information to the ECM. The ECM supplies 5 volts to the coolant temperature sensor circuit. The sensor is a thermistor which changes internal resistance as temperature changes. When the sensor is cold (internal resistance high), the ECM monitors a high signal voltage which it interprets as a cold engine. As the sensor warms (internal resistance decreases), the voltage signal will decrease and the ECM will interpret the lower voltage as a warm engine.

INT AIR TEMP (IAT) - The ECM converts the resistance of the intake air temperature sensor to degrees. intake Air Temperature (IAT) is used by the ECM to adjust fuel delivery and spark timing according to oncoming air density.

MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR
The MAP sensor produces a low signal voltage when manifold pressure is low (high vacuum) and a high voltage when the pressure is high (low vacuum). With the ignition "ON," and the engine stopped, the manifold pressure is equal to atmospheric pressure and the signal voltage will be high. This information is used by the ECM as an indication of vehicle altitude and is referred to as BARO. Comparison of this BARO reading with a known good vehicle with the same sensor is a good way to check accuracy of a "suspect" sensor. Readings should be the same 1.4 volt.

BAROMETRIC PRESSURE (BARO) - This parameter represents a measurement of barometric air pressure and is used in conjunction with Manifold Air Pressure (MAP) to determine manifold vacuum. Since barometric air pressure depends on altitude, it may vary from 100 kPa (sea level) to 60 kPa (14,000 ft.). Both the BARO sensor voltage and the computed pressure are displayed for most vehicles.

THROT POSITION - Used by the ECM to determine the amount of throttle demanded by the driver. Should read .45-.65 volt at idle to above 4 volts at Wide Open Throttle (WOT).

THROTTLE ANGLE - Computed by the ECM from TP signal voltage, (Throt position) should read 0% at idle and 100% at Wide Open Throttle (WOT).

OXYGEN SENSOR (O2S) - Represents the exhaust oxygen sensor output voltage. Should fluctuate constantly within a range of between 100 mV (lean exhaust) and 1000 mV (rich exhaust) when operating in "Closed-Loop."

INJECTOR PULSE WIDTH - The reading given in milliseconds which is the "on-time" that the ECM is commanding the injector(s).

SPARK ADVANCE - This is a display of the spark advance IC calculation which the ECM is programming into the ignition system. It computes the desired spark advance using data such as engine temperature, RPM, load, vehicle speed and operating mode.

SHORT TERM FUEL TRIM (INTEGRATOR) - Represents a short term correction to fuel delivery by the ECM in response to the amount of time the oxygen sensor voltage spends above or below the 450 mV threshold. If the oxygen sensor voltage has mainly been below 450 mV, indicating a lean air/fuel mixture, fuel trim will increase to tell the ECM to add fuel. If the oxygen sensor voltage stays mainly above the threshold, the ECM will reduce fuel delivery to compensate for the indicated rich condition.

LONG TERM FUEL TRIM OR (BLOCK LEARN) CELL - Long term fuel trim is derived from the fuel short term fuel trim value and is used for long term correction of fuel delivery. A value of 128 counts indicates that fuel delivery requires no compensation to maintain a 14.7:1 air/fuel ratio. A value below 128 counts means that the fuel system is too rich and fuel delivery is being reduced (decreased injector pulse width). A value above 128 counts indicates that a lean condition exists and the ECM is compensating by adding fuel (increased injector pulse width).

FUEL TRIM CELL - Fuel trim cell is dependent upon engine speed and MAP sensor readings. A plot of RPM vs. MAP is broken into 16 cells. Fuel trim cell indicates which cell is currently active.

FUEL TRIM ENABLE - If air fuel system is learning (YES) then long term fuel trim is responding to short term fuel trim. If fuel trim enable reads (NO) then long term fuel trim will not respond to change in short term fuel trim normally, learning starts as soon as engine goes into "Closed Loop."

SPARK ADVANCE - This is a display of the spark advance IC calculation which the ECM is programming into the ignition system. It computes the desired spark advance using data such as engine temperature, RPM, load, vehicle speed and operating mode.

INTAKE TUNING VALVE - The tuning valve is electrically controlled by the ECM based on RPM and TP sensor inputs, for accurate switching capability at optimum switching points to enable either a "split" or "single" plenum condition.

KNOCK SIGNAL - Tech 1 Displays "YES" OR "NO" - Indicates whether or not a knock signal is being detected by the ECM.

KNOCK RETARD - Indicates amount of spark advance the ECM is removing from IC in response to the Knock Sensor (KS) signal.

LOOP STATUS - This position will indicate whether the engine control system is operating in "Open" or "Closed Loop." Most systems go "Closed Loop" after a certain amount of run time occurs, and when coolant temperature is high enough, and when the oxygen sensor becomes active.

CONVERTER HIGH TEMP - Air fuel ratio enriches from 14.7 to 1 to 12 to 1 to help cool catalytic converter.

EGR DESIRED POSITION - The ECM command for EGR valve position that is desired.

EGR ACTUAL POSITION - Current actual EGR position of the EGR valve.

EGR PINTLE POSITION - ECM command for linear EGR valve pintle position that is desired.

EGR DUTY CYCLE - On some systems, a more precise control of Exhaust Gas Recirculation (EGR) is used. This is done by cycling the EGR solenoid "ON" and "OFF."

MPH/km/h - Vehicle speed is an ECM internal parameter. It is computed by timing pulses coming from the Vehicle Speed Sensor (VSS). Vehicle speed is used in checking TCC lock-up speed or speedometer accuracy. Speed is displayed in both Miles Per Hour (MPH) and Kilometer Per Hour (km/h).

ENGINE SPEED - Engine speed is computed by the ECM from the fuel control reference input. It should remain close to desired idle under various engine loads with engine idling.

A/C REQUEST - Displays the state of the A/C signal line to the ECM. Should read "YES" whenever the A/C is requested.

IDLE AIR CONTROL (IAC) - This system is used to control engine idle speed to the desired RPM for different operating conditions. In this mode, the numbers will indicate what position the ECM thinks the pintle valve is in. The ECM moves the IAC in counts and these counts are displayed on a Tech 1 scan tool.

FUEL EVAP PURGE - Systems use a proportional signal to control the canister purge function. 0% means the CCP valve is completely closed while 100% implies a fully open canister purge valve.

SYSTEM VOLTAGE - Battery/ignition voltage is an analog input signal read by the ECM. It is the ignition switched battery voltage which is mainly used for diagnostics. Certain ECM functions will be modified if the battery voltage falls below or rises above programmed thresholds.

FUEL PUMP VOLTS - This parameter is a reading of the voltage going to the fuel pump. It is used by the ECM as the system voltage. The ECM uses fuel pump volts as a reference.

CALIBRATION ID - The calibration identification parameter describes the particular PROM used in the ECM being tested. The PROM contains the ECM program. Calibration ID is used when it is necessary to replace the PROM. Calibration ID must be specified when ordering new PROMs. Calibration ID should not be confused with "Part Number."

TIME FROM START - Time from start is a measure of how long the engine has been running. If the engine stops, time from start will reset to 0:00:00.

ENGINE CONTROL MODULE (ECM)
The diagnosis of the Engine Control Module (ECM) starts with the "On-Board Diagnostic (OBD) System Check." The Diagnostic Trouble Code (DTC) system indicates a failure of a specific circuit and diagnosis may indicate replacement of the ECM. DTC 55 indicates that the ECM has failed and must be replaced.

If the ECM has been replaced and the condition was not corrected, the following information may be the cause:

^ An incorrect ECM or PROM (MEM-CAL) application may cause a malfunction and may or may not set a DTC.
^ If the connector at the ECM is the possible problem, the terminal may have to be removed from the connectors in order to properly check it.
^ Although the PROM (MEM-CAL) rarely fails, it operates as part of the ECM; therefore, it could be the cause of the problem.
^ Although a rare condition, the replacement ECM may be faulty.
^ In the case of an intermittent problem, refer to "Intermittents" and make a careful physical inspection of the system involved. - Intermittent Malfunctions
^ A shorted solenoid, relay coil or harness may cause an ECM to fail and a replacement ECM to fail when it is installed. Use a short tester J 34636 or BT-8405 as a fast, accurate means of checking for a short circuit.

ECM Quad-Driver (QDR) Check
The ECM uses an Integrated Circuit (IC) called a Quad-Driver (QDR) in place of separate transistors to turn "ON" or "OFF" different circuits controlled by the ECM. Each QDR has four separate outputs that can independently turn "ON" or "OFF" four different circuits.

ECM on all engines are fault protected, therefore, a single faulty circuit may cause all four QDR outputs to be inoperative or "ON" all the time. A failed QDR usually results in either a shorted or open ECM output.

The ECM has integrated circuits that are fault protected, therefore, if a circuit has failed, the IC may not be damaged and will keep the circuit open until the fault in the circuit has been corrected. When the fault has been corrected, reinstall ECM and check circuit. Replace ECM only if the circuit is still inoperative.

PROM (MEM-CAL)
A PROM (MEM-CAL) which has failed or was installed improperly will set a DTC 51.

FUEL CONTROL
Fuel delivery is controlled by the engine control module system.

The diagnosis of fuel control starts with "Engine Cranks But Will Not Run" CHART A-3. Related Tests, Information and Procedures This chart will test the fuel system and if there is a problem, it will lead you to diagnose the fuel pump relay circuit, the injector circuit or the fuel system.

Fuel Injector
Testing the fuel injector circuit is in CHART A-3 with additional diagnosis in CHART A-4. Related Tests, Information and Procedures

A fuel injector poppet nozzle which does not open may cause a "no start" condition. An injector poppet nozzle which is stuck partly open, could cause loss of pressure after sitting, long crank times would be noticed. Also, dieseling could occur because some fuel could be delivered to the engine after the key is turned "OFF." Refer to CHART A-8 for diagnosis. Related Tests, Information and Procedures

Pressure Regulator
Testing the pressure regulator circuit is in CHART A-6. Related Tests, Information and Procedures

If the pressure regulator to the CMFI supplies pressure which is too low, poor performance could result. If the pressure is too high, unpleasant exhaust odor may result.

Idle Air Control (IAC)
The diagnosis of Idle Air Control (IAC) can be found in "Idle Air Control (IAC) Valve".

If the IAC valve is disconnected or connected while the engine is operating, the idle RPM may be wrong. The IAC valve may be reset disconnecting the negative battery cable for 10 seconds, then reconnect battery cable, ignition "ON" engine "OFF" for 5 seconds then ignition "OFF" for 10 seconds.

The IAC valve affects the idle characteristics of the engine. If it is fully open, too much air will be allowed in the manifold and idle speed will be high. If it is stuck closed, too little air will be allowed in the manifold, and idle speed will be too low. If it is stuck part way open, the idle may be rough, and will not respond to engine load changes.

Fuel Pump Circuit
The fuel pump relay has a terminal to test the fuel pump operation which is a separate terminal located near the fuel pump relay. By applying voltage at this terminal, it can be determined if the fuel pump will operate. This terminal will also prime the fuel line to CMFI.

Refer to CHART A-5 for diagnosis of the fuel pump relay circuit. Chart A-5 Fuel Pump Relay Circuit Diagnosis

An inoperative fuel pump will cause a "no start" condition. A fuel pump which does not provide enough pressure can result in poor performance.

An inoperative fuel pump relay can result in long cranking times, particularly if the engine is cold. The oil pressure switch will turn "ON" the fuel pump, as soon as oil pressure reaches about 28 kPa (4 psi).

ENGINE COOLANT TEMPERATURE (ECT) SENSOR
DTC 14 or DTC 15 indicates a failure in the engine coolant temperature sensor circuit. Testing and Inspection

Most scan tools display engine coolant temperature in degrees centigrade. After the engine is started, the temperature should rise steadily to about 90°C then stabilize when the thermostat opens.

MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR
DTC 33 or DTC 34 indicates a failure in the MAP sensor circuit. Testing and Inspection Also refer to "MAP Output Check Diagnosis" to check the MAP sensor if there is no DTC. Testing and Inspection

OXYGEN SENSOR (O2S)
DTC 13 indicates an open in the Oxygen Sensor (O2S) circuit. DTC 44 indicates a shorted oxygen sensor circuit. DTC 45 indicates a high voltage in the oxygen sensor circuit. If a DTC is set, the engine will always run in the "Open Loop" mode. The oxygen sensor voltage output can be measured with a digital voltmeter having at least a 10 megohm input impedance or J 39200. Use of a standard shop type voltmeter will result in an inaccurate reading.

Normal scan voltage varies between 100 mV to 999 mV (.1 and 1.0 volt) while in "Closed Loop." DTC 13 sets in one minute if the voltage remains between .35 and .55 volt, causing the system to enter "Open Loop" in about 15 seconds.

Using the scan tool, observe the fuel trim values at different RPM and air flow conditions to determine when DTC 44 or DTC 45 may have been set. If the condition for DTC 44 exists, the long term fuel trim values will be around 150. If the condition for DTC 45 exists, the long term fuel trim values will be around 115.

THROTTLE POSITION (TP) SENSOR
When a DTC 21 or DTC 22 is set, the ECM will use an artificial value for throttle position allowing some engine performance to return.

A broken TP sensor can cause intermittent bursts of fuel from the injector and an unstable idle because the ECM thinks the throttle is moving.

A scan tool reads throttle position in volts and should read .45 - .85 with the throttle closed, ignition "ON" or at idle. Voltage should increase at a steady rate as throttle is moved toward Wide Open Throttle (WOT).

Scan TP signal while depressing accelerator pedal with engine stopped and ignition "ON." Display should vary from below .85 volts (1250 mV) when throttle is closed to over 4.5 volts (4500 mV) when throttle is held at wide open throttle position.

TP Sensor Output
This check should be performed when TP sensor attaching parts have been replaced. A Tech 1 scan tool can be used to read the TP signal output voltage, or:

1. Connect digital voltmeter J 39200 from TP sensor connector terminal "B" (BLK wire) to terminal "C" (DK BLU wire). Jumpers for terminal access can be made using terminals "1214836" and "12014837".

2. With ignition "ON," engine stopped, the TP signal voltage should be less than .85 volts. If more than .85 volts, verify free throttle movement. If still more than .85 volts, replace TP sensor.

3. Remove the voltmeter and jumpers, reconnect the TP sensor connector to the sensor.

VEHICLE SPEED SENSOR (VSS)
The vehicle speed sensor circuit diagnosis is in DTC 24 chart. Testing and Inspection

Scan reading should closely match speedometer readings with the drive wheels turning.

INTAKE AIR TEMPERATURE (IAT) SENSOR
DTC 23 indicates that there is an open in the IAT circuit. DTC 25 indicates that there is a short to ground in the IAT circuit.

The scan tool reads the temperature of the air entering the engine and should read close to ambient air temperature when the engine is cold, and rise as underhood temperatures increase.

EGR SYSTEM
DTC 32 indicates that there is a failure in the EGR system circuit.

IDLE SPEED
Refer to "Idle Air Control (IAC) Valve" in for Idle Air Control (IAC) check.

^ System too lean (High air/fuel ratio) - Idle speed may be too high or too low. Engine speed may vary up and down, disconnecting IAC does not help. May set DTC 44.

Scan tool and/or voltmeter will read an oxygen sensor output less than 300 mV (.3 volt). Check for low regulated fuel pressure or water in fuel. A lean exhaust with an Oxygen Sensor (02S) output fixed above 800 mV (.8 volt) will indicate a contaminated sensor, usually silicone This may also set a DTC 45.

^ System too rich (Low air/fuel ratio) - Idle speed too low. Scan counts usually above 80. System obviously rich and may exhibit black exhaust smoke. Scan tool and/or voltmeter will read an Oxygen Sensor (02S) signal fixed above 800 mV (.8 volt).

IGNITION CONTROL (IC)
When the system is running off the module, that is, no voltage on the bypass line, the ignition module grounds the Ignition Control (IC) signal The ECM expects to see no voltage on the IC line during this condition. If it sees a voltage, it sets DTC 42 and will not go into the IC mode.

When the RPM for IC has reached (about 400 RPM), and bypass voltage is applied, the IC should no longer be grounded in the ignition module so the IC voltage should vary.

If the bypass line is open or grounded, the ignition module will not switch to IC mode so the IC voltage will be low and DTC 42 will be set.

If the IC line is grounded, the ignition module will switch to IC, but because the line is grounded there will be no IC signal. DTC 42 will be set.

DTC 42 sets if there is an open or a short to ground in the IC or bypass circuit.

SYSTEM OVER VOLTAGE
DTC 53 sets if the voltage is greater than 17.1 volts for two seconds at ECM terminal "B 1". This indicates that there is a basic generator problem.

INTAKE MANIFOLD TUNING VALVE ASSEMBLY
Refer to "Intake Manifold Tuning Valve" Diagnosis.

DISTRIBUTOR REFERENCE SIGNAL
The distributor reference signal is covered in the "Ignition System".

A/C CLUTCH CONTROL
Refer to "A/C Clutch Control." A/C Clutch Control

EXHAUST SYSTEM
Refer to "Restricted Exhaust System Check." System Check - Restricted Exhaust