With Vehicle Control Module (VCM)

A list of each data message displayed on the Tech 1 scan tool will be explained in two groups; "Engine" or "Transmission."

TYPICAL TECH 1 DATA DEFINITIONS
This information will assist in emission or driveability problems. The displays can be viewed while the vehicle is being driven. Always perform the "On-Board Diagnostic (OBD) System Check" first. Diagnostic System Check - With VCM The "OBD System Check" will confirm proper system operation. The following positions may not be applicable to all engines:

ENGINE SPEED - Engine RPM is determined from distributor reference signal by the Vehicle Control Module (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 ±.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 0.45-0.65 volt at idle to above 4 volts at Wide Open Throttle (WOT).

THROTTLE ANGLE - Computed by the VCM from Throttle Position (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 Engine Control Module (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.