With Vehicle Control Module (VCM-A)

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. The "OBD System Check" will confirm proper system operation. The following positions may not be applicable to all engines:

Engine Speed - Range (0-9999) RPM - Engine RPM is determined from Camshaft Position (CMP) Sensor signal by the Vehicle Control Module (VCM-A) from the fuel control reference input. It should remain close to desired idle under various engine loads at idle.

Desired Speed - Range (0-9999) RPM - The speed that is commanded by the VCM-A. The VCM-A will compensate for various engine loads to maintain desired idle speed.

Engine Coolant Temp - Range (-40°C - 151°C), -40°F to 304°F - The Engine Coolant Temperature (ECT) sensor is mounted in the intake manifold and sends engine temperature information to the VCM-A. The VCM-A 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-A monitors a high signal voltage which it interrets as a cold engine. As the sensor warms internal resistance deceases), the voltage signal will decease and the VCM-A will interpret the lower voltage as a warm engine.

Start-Up Engine Coolant Temperature - Range ('4O°C - 151 °C), -40°F to 304°F - The engine coolant temperature when the engine is first started is displayed. This value can be used to determine if VCM-A or IGN has a loose terminal or if there is a voltage loss.

Engine Coolant Temp Sensor - Range (0.00 - 4.98) volts - The Engine Coolant Temperature (ECT) sensor is mounted in the intake manifold and sends engine temperature information to the VCM-A. The VCM-A 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-A 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-A will interpret the lower voltage as a warm engine.

Trans Fluid Temp - Range (-40°C -151 °C) - The VCM-A converts the resistance of the transmission fluid temperature sensor to degrees. Trans fluid temp is used to determine "Hot Mode" Torque Converter Clutch (TCC) operation.

Intake Air Temp - Range (-40°C - 151°C) - The VCM-A converts the resistance of the Intake Air Temperature (IAT) sensor to degrees. Intake air temperature is used by the VCM-A to adjust fuel delivery and spark timing according to incoming - air density.

Intake Air Temp Sensor - Range (0.008 - 4.98) volts - The VCM-A converts the resistance of the Intake Air Temperature (IAT) sensor to degrees. Intake air temperature is used by the VCM-A to adjust fuel delivery and spark timing according to incoming air density.

Manifold Absolute Pressure (MAP) Sensor - Range (0.00 - 4.98) volts - 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-A 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) - Range (0.00 - 15.18) volts - 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.).

Manifold Absolute Pressure (MAP) - Range (10 kPa - 104 kPa) - The Manifold Absolute Pressure (MAP) sensor measures the change in the intake manifold pressure from engine load and speed changes. As intake manifold pressure increases, the air density in the intake manifold also increases and additional fuel is required.

Barometric Pressure (BARO) Sensor - Range (0 kPa - 62 kPa) - The BARO reading displayed is determined from the MAP sensor at ignition "ON," engine "OFF," and WOT conditions. The BARO reading displayed represents barometric pressure and is used to compensate for altitude differences.

Throt Position Sensor (TPS) - Range (0.00-5.00) volts - Used by the VCM-A 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 Position Angle - Range (0- 100) % - Computed by the VCM-A from Throttle Position (TP) signal voltage (throt position) should read 0% at idle, 100% at Wide Open Throttle (WOT).

HO2S 1 Bank 1 - Range (0 - 1107) mv - Represents the heated 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."

HO2S 2 Bank 1 - Range (0 - 1107) mv - Represents the HO2S 2 (catalyst monitor) output voltage. Should fluctuate less than the HO2S 1 signal when operating in "Closed Loop."

02S Cross Counts - Range (0 - 255) - The number of times the heated oxygen sensor voltage crosses over the rich/lean threshold during a one second interval.

Loop Status Range (Open - Closed) - "Closed Loop" displayed indicates that the VCM is controlling fuel delivery according to oxygen sensor voltage. In "Open Loop," the VCM ignores the oxygen sensor voltage and bases the amount of fuel to be delivered on TP sensor, engine coolant, and MAP sensor inputs only.

Long Term Fuel Trim - Range (0-255) - 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-A is compensating by adding fuel (increased injector pulse width).

Short Term Fuel Trim - Range (0-255) - Short term fuel trim (formerly fuel integrator) represents a short-term correction to fuel delivery by the VCM-A in response to the amount of time the heated oxygen sensor voltage spends above or below the 450 mV threshold. If the heated 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-A to add fuel. If the heated oxygen sensor voltage stays mainly above the threshold, the VCM-A will reduce fuel delivery to compensate for the indicated rich condition.

Fuel Trim Cell - Range (0 - 255) - 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 - Range (Yes - No) - 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."

IAC Motor - Range (0 - 255) - Displays the commanded position of the idle air control pintle in counts. The greater the number of counts, the greater the idle air passages opened. Idle air control should respond fairly quickly to changes in engine load to maintain desired idle RPM.

Desired IAC - Range (0 - 255) - This indicates the number of counts (steps) that the VCM-A is commanding.

KS Activity - Range (- - Yes) - A change in the displayed value indicates a knock signal is being detected by the VCM-A.

KS Retard - Range (97° - 256°) - This indicates the amount of spark advance being removed to diminish spark knock.

Ignition 1 - Range (0.00 - 25.5) V - This parameter will indicate the ignition voltage measured at the VCM-A.

Fuel Pump - Range (0.0 - 25.5) V - This parameter is a reading of the voltage going to the fuel pump. The VCM-A uses fuel pump volts as a reference to compensate for variation in fuel pressure due to voltage.

Spark Advance - Range (-96° - 255°) - This is a display of the spark advance calculation which the VCM-A is programing to the ignition system. It computes the desired spark advance using data such as engine temperature, RPM, load, vehicle speed and operating mode.

Spark Control - Range (Advance - Retard) - Indicates amount in degrees of advance (+) or retard (-).

Ign 1 Low - Range (No - Yes) - Indicates low voltage on Ign 1 circuit.

Ign 1 High - Range (No - Yes) - Indicates high voltage on Ign 1 circuit.

Loop Status - Range (Open - Closed) - This position will indicate whether the engine fuel 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 heated oxygen sensor becomes active.

OC Overtemp - Range (Yes - No) - Air fuel ratio enriches from 14.7 to 1 to 12 to 1 to help cool catalytic converter.

Rich/Lean Indicator - Range (Rich - Lean) - Indicates whether exhaust heated oxygen sensor voltage is above (rich) or below (lean) the 450 mV heated oxygen sensor threshold voltage. Should change constantly indicating that the VCM-A is controlling the air/fuel mixture properly.

Intake Tuning Valve - Range (On - Off) - The tuning valve is electrically controlled by the VCM-A based on RPM and TP sensor inputs, for accurate switching capability at optimum switching points to enable either a "split" or "single" plenum condition.

Evaporative Emissions System (EVAP) Vacuum Switch - Range (On - Off) - Senses vacuum in EVAP system.

4WD Low - Range (yes - No) - Indicates to VCM-A when driver selects 4WD Lo Range.

Air Fuel Ratio - Range (0.0 - 25.5) - 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 Pintle Position - Range (0.00- 4.98) V - Represents the linear EGR pintle position sensor output voltage. Should be under 1 volt at idle.

NC Request - Range (Yes - No) - This parameter will indicate if A/C is being requested (control switch "ON").

Trans Range SW - Range (Low 1) - Tech 1 displays Park/Neutral, Reverse, Drive 4, Drive 3, Drive 2, Low and Invalid - These values represent the decoded sequence of the Transmission Range (TR) pressure switch assembly circuits and are used to determine manual valve position.

A/B/C Range - "ON/OFF," they are used to detect which gear has been manually selected, engine must be operating.

TCC Forced Off - Range (Yes - No) - When brake pedal is depressed or vehicle speed is lower than limit on TCC engagement criteria, the TCC will disengage.

TCC Enable - Range (Yes - No) - In this position, the tool will indicate when the TCC has been commanded by the VCM-A to turn "ON." This does not necessarily mean that the clutch was engaged but only the VCM-A grounded the circuit internally. The best way to determine if the clutch has engaged is to monitor engine RPM when the TCC comes "ON."

Brake Switch - Range (Open - Closed) - 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-A signal line. When brakes are applied, the VCM-A receives a zero (0) volt signal on the signal line.

1-2 SOL/2-3 SOL - An active or inactive command state of the trans solenoid or upshift or downshift.

DTC Counter - Range (0 - 255) - Counts the number of Diagnostic Trouble Code (DTCs) stored in the VCM-A.

Engine Run Time - Range (0 - 23:18:06.08) - The amount of time the engine has been running during this ignition cycle.

EVAP Duty Cycle - Range (0 - 100) % - A proportional signal used to control EVAP canister purge function. 0% implies the valve is commanded fully closed while 100% implies that the valve is fully open.

EGR Duty Cycle - Range (0-100) % - The VCM-A cycles the Exhaust Gas Recirculation (EGR) valve "ON" and "OFF." The "ON" time (duty cycle) of the EGR solenoid valve, expressed as a percent, determines how much the exhaust gas is recirculated.

Actual EGR POS - Range (0 - 100) % - Represents the linear EGR pintle position signal in percent of travel. 0% displayed indicates a closed EGR valve (EGR flow not allowed).

Desired EGR POS - Range (0- 199.9) % - The EGR pintle position which is being commanded by the VCM-A should be very close to "ACTUAL EGR POS."

TCC Duty Cycle - Range (0 - 100) % - This is the relative position of the Pulse Width Modulation (PWM) solenoid that controls TCC apply pressure. 100% is low pressure, 0% is full line pressure.

TCC Slip Speed - Range (-50 - 50) mph - The RPM difference between engine crankshaft speed and torque converter turbine speed. When TCC mode is "ON" there should only be a small amount of slip.

MPH Km/h - (0 - 255) mph-kph - This is a VCM-A 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 (kph).