VCM Charts

SCAN TOOL DISPLAY
ENGINE SPEED - Engine RPM is determined from distributor reference signal by the VCM from the fuel control reference input. It should remain close to desired idle under various engine loads at idle.

DESIRED IDLE - The idle speed that is commanded by the VCM. The VCM will compensate for various engine loads to maintain 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 VCM. The VCM 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 VCM 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 VCM will interpret the lower voltage as a warm engine.

START-UP COOLANT TEMPERATURE - The engine coolant temperature when the engine is first started is displayed. This value can be used to determine if VCM or IGN has a loose terminal or if there is a voltage loss.

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 causing the signal voltage to be high. This information is used by the VCM 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) - The BARO represents a measurement of barometric air pressure that is determined by using the Manifold Air Pressure (MAP) sensor 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.).

THROT POSITION - Used by the VCM 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 VCM from TP signal voltage (throt position) should read 0% at idle, 100% at Wide Open Throttle (WOT).

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

INJECTOR PULSE WIDTH - In this position, the reading is given in milliseconds which is the "on-time" that the VCM is commanding the injector(s).

SHORT TERM FUEL TRIM - Short term fuel trim (formerly fuel integrator) represents a short-term correction to fuel delivery by the VCM 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, short term fuel trim will increase to tell the VCM to add fuel. If the oxygen sensor voltage stays mainly above the threshold, the VCM will reduce fuel delivery to compensate for the indicated rich condition.

LONG TERM FUEL TRIM - Long term fuel trim (formerly block learn) is derived from the 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 VCM is compensating by adding fuel (increased injector pulse width).

FUEL TRIM CELL - Engine operation is divided into 16 cells. A particular cell is selected when the engine operating point on the RPM/MAP map lies within the corresponding cell.

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 calculation which the VCM 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.

KS ACTIVITY Tech 1 Displays "YES" OR "NO"
Indicates whether or not a knock signal is being detected by the VCM.

KS RETARD - This indicates the amount of spark advance being removed to diminish spark knock.

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, when coolant temperature is high enough, and the oxygen sensor becomes active.

RICH/LEAN STATUS - Indicates whether exhaust oxygen sensor voltage is above (rich) or below (lean) the 450 mV oxygen sensor threshold voltage. Should change constantly indicating that the VCM is controlling the air/fuel mixture properly.

AIR FUEL RATIO - The reading reflects the commanded value. This should be at or near 14.7. A lower number indicates a richer air fuel mixture while a higher number indicates a leaner mixture.

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." The percent value refers to the amount of time that the solenoid is "ON."

MPH km/h - Vehicle speed is a VCM internal parameter. It is computed by timing pulses coming from the Vehicle Speed Sensor (VSS). Vehicle speed is used in checking speedometer accuracy. Speed is displayed in both miles per hour (MPH) and kilometer per hour (km/h).

BRAKE SWITCH - The brake switch is operated by brake pedal travel. The switch is normally closed when the brake is released. This applies battery voltage to the VCM signal line. When brakes are applied, the VCM receives a zero (0) volt signal on the signal line.

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 VCM thinks the valve is in. The VCM moves the IAC in counts and these counts are what is displayed on a Tech 1 scan tool.

DESIRED IDLE AIR - This indicates the number of counts (steps) that the VCM is commanding.

A/C REQUEST - This parameter will indicate if A/C is being requested (control switch "ON").

IGNITION 1 - This parameter will indicate if ignition voltage is present at the VCM.

SYSTEM VOLTAGE - Battery/ignition voltage is an analog input signal read by the ECM. It is the ignition switched battery voltage and 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.

LOW SYSTEM VOLTS - The Tech 1 will display "YES" if system voltage drops below 12 volts.

HIGH SYSTEM VOLTS - The Tech 1 will display "YES" if system voltages are over 15.5 volts.

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

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.

FUEL CONTROL
Fuel delivery is controlled by the 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 to determine if there is a problem.

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

A fuel injector which does not open may cause a no start" condition. An injector which is stuck partially open could cause loss of pressure after sitting, resulting in extended crank times on some engines. Also, dieseling could occur because some fuel could be delivered to the engine after the key is turned "OFF."

Pressure Regulator
Testing the pressure regulator circuit is in CHART A-3 and A-6. Related Tests, Information and Procedures

If the pressure regulator in the TBI 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 running, the idle RPM may be wrong. The IAC valve may be reset by turning the ignition switch "ON" for 10 seconds, "OFF" for 5 seconds.

The IAC valve affects the idle characteristics of the engine as well as throttle follow-up to compensation for sudden throttle closing. If it is open fully 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
DTC P1222 indicates a failure in the fuel pump circuit.

The relay has a terminal to test the fuel pump operation which is a separate terminal located near the fusible link cluster. 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 the TBI unit.

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
Diagnostic Trouble Code (DTC) P0118 or DTC P0117 indicates a failure in the engine coolant temperature sensor circuit.

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

MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR
Diagnostic Trouble Code (DTC) P0108 or DTC P0107 indicates a failure in the MAP sensor circuit. Also refer to "MAP Output Check Diagnosis" to check the MAP sensor if there is no DTC. MAP Sensor Output Diagnosis

OXYGEN SENSOR (O2S)
Diagnostic Trouble Code (DTC) P0139 indicates an open in the Oxygen Sensor (O2S) circuit. DTC P0171 indicates a low voltage (lean) in the Oxygen Sensor (O2S) circuit DTC P0172 indicates a high voltage (rich) in the Oxygen Sensor (025) circuit. If a DTC is set, the engine will always run in the "Open Loop" mode. The Oxygen Sensor (02S) voltage output can be measured with a digital voltmeter (J 39200) having at least a 10 megohm input impedance 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 P0139 sets in one minute if voltage remains between .35 and .55 volt.

Using the scan, observe the long term fuel trim values at different RPM and air flow conditions to determine when DTC P0171 or DTC P0i?2 may have been set. If the condition for DTC P0171 exists, the long term fuel trim values will be around 150. if the condition for DTC P0172 exists, the long term fuel trim values will be around 115.

THROTTLE POSITION (TP) SENSOR
When Diagnostic Trouble Code (DTC) P0123 or P0122 is set, the VCM will use a default value for throttle position and some engine performance will return.

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

A scan tool reads throttle position in volts and should read about .45 to .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 volt (1250 mV) when throttle was 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 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 1.25 volts if more than 1.25 volts verify free throttle movement. If still more than 1.25 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 Diagnostic Trouble Code (DTC) P0500 chart.

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

EXHAUST GAS RECIRCULATION (EGR) SYSTEM
Diagnostic Trouble Code (DTC) P0400 indicates that there is a failure in the EGR system circuit.

IDLE SPEED
When there is a problem with idle air control system or refer to "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 P0171.

Scan tool and/or voltmeter will indicate 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 (O2S) output fixed above 800 mV (.8 volt) may be the result of a contaminated sensor, usually silicone. This may also set a DTC P0172.

^ 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 (O2S) signal fixed above 800 mV (.8 volt).

DISTRIBUTOR IGNITION (DI) SYSTEM
When the system is running on the ignition module, that is, no voltage on the bypass line, the ignition module grounds the Ignition Control (IC) signal. The VCM expects to see no voltage on the IC line during this condition. If it sees a voltage, it sets DTC P0320 and will not go into the IC mode.

When the RPM for IC is 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 be varying.

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 P0320 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. A DTC P0320 will be set.

DTC P0320 sets if there is an open or a short to ground in the IC or bypass circuit

KNOCK SENSOR (KS) SYSTEM
DTC P0328 will set when KS line is open.
DTC P0327 will set when KS line is shorted to ground.

SYSTEM OVER VOLTAGE
DTC P1632 sets if there is voltage greater than 17.1 volts for two seconds at VCM terminal "BL32". This indicates that there is a basic generator problem.

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