"Scan" Tool Limitations, Uses & Positions
``SCAN'' TOOL LIMITATIONS & USES
The ``SCAN'' tool allows a quick check of switches and sensors that input to the ECM. The data only updates every 1.25 seconds, which makes the tool less effective than a voltmeter for detecting an intermittent that lasts less than 1.25 seconds. However, the ``SCAN'' tool allows for manipulation of wiring harnesses or components under the hood while observing the ``SCAN'' readout. This assists in locating intermittents with the engine off.
The ``SCAN'' tool is also a useful and quick way of comparing operating parameters of a poorly operating engine with a known good one.
The ``SCAN'' tool has the ability to save time in diagnosis and prevent the replacement of good parts. The key to using the ``SCAN'' tool successfully for diagnosis lies in the operator's ability to understand the system being diagnosed and the limitations of the ``SCAN'' tool.
``SCAN'' TOOL POSITIONS
Not all ``SCAN'' tool positions are applicable to all engines. Positions not applicable to engine being serviced will be indicated by tool.
Mode
Function of this mode differs by tool manufacturer. In most cases it is used to place the ECM in different operating modes.
Injector Pulse Width
In this position the reading is given in milliseconds which is the on time that the ECM is controlling the injector(s).
Closed Loop/Open Loop
This position indicates 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 is activated.
Exhaust (Rich/Lean Indicator)
This position indicates whether oxygen sensor is sensing a rich or lean exhaust and is usually displayed as RH or LN.
Trouble Codes
This position will display any trouble codes stored in the ECM memory.
TPS (Throttle Position Sensor)
Values read in this position will be the voltage as seen by the ECM. The voltage should be the TPS specification with the throttle closed and go up to approximately 5 volts with throttle wide open.
Oxygen
In this position the reading will be read out in millivolts (mv) with a range from 100-999 mv. If the reading is consistently below 35, the fuel system is running lean as seen by the ECM and if the reading is consistently above 55, the system is running rich.
PROM ID
In this position, information is used for assembly verification only. PROM ID is useful only when the vehicle is equipped with the original ECM and PROM or MEM-CAL.
RPM
This position displays engine RPM. This position is often useful when extra reference pulses are suspected. A sudden high RPM indication while at a steady throttle would indicate electrical interference (EMI) in the reference circuit. This interference is usually caused by the ECM being too close to the ignition secondary wires on open distributor ground circuits.
MPH
This position displays vehicle speed and is useful in checking TCC lock up speed or speedometer accuracy.
Coolant Temperature
This position displays engine temperature in degrees Centigrade. After engine is started, temperature should rise steadily to approximately 90 degrees centigrade, then stabilize when thermostat opens.
Manifold Air Temperature (MAT) Sensor
This position displays temperature of intake manifold air. Tool should read close to ambient air temperature when engine is cold and rise as underhood and engine temperature increases.
Park/Neutral Switch
The indication in this mode varies by manufacturer as described in the tool operating manual. The reading switches when the gear selector is moved from neutral to drive.
Torque Converter Clutch (TCC)
In this position, the tool indicates when the TCC has been commanded to turn on by the ECM. This does not necessarily mean that the clutch was engaged but only that the ECM grounded the circuit internally. The best way to determine if the clutch has engaged is to monitor the engine RPM when the TCC comes ``On.''
Integrator & Block Learn
Normal reading in these modes is approximately 128. A higher reading indicates that the ECM is adding fuel to the base fuel calculation and a lower number indicates that the ECM is taking fuel from base calculation. The integrator is short term corrective action while the block learn portion (long term correction) will only change if the integrator has seen a condition which lasts for a calibrated period of time.
IAC (Idle Air Control)
In this position, the numbers indicate what position the ECM thinks the IAC valve is in. The ECM moves the IAC in counts and these counts are what is displayed on the ``SCAN'' tool.
Cross Counts
In this mode, the activity of the oxygen sensor is displayed by how many times the voltage of the sensor has passed by the midpoint in the cast 1.25 second period. The number will add up to 255, then start over.
Power Steering Pressure Switch
This position displays the state of the switch and may vary with tool being used and type of switch installed on vehicle. The reading changes when the steering is moved against the stops.
Manifold Absolute Pressure (MAP)
The MAP sensor produces a low signal voltage when manifold pressure is low (high vacuum) and a high voltage when pressure is high (low vacuum).
When ignition is on with the engine off, 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. Reading should be within 400 mV of each other.
MAP sensors have a colored plastic insert visible in the connector cavity. Sensors with the same insert color are identical in calibration. The harness electrical connector should also be the same as the insert color.
A/C Request-A/C Clutch
The A/C Request position displays the status of the A/C signal line to the ECM. The A/C Clutch position displays ``on'' when the ECM energizes the compressor clutch.
Airflow
On 1987-88 models, this display indicates air flow in grams per second (gm/sec). Readings should be the same as for the Mass Air Flow (MAF) sensor position (4-7 gm/sec) if no malfunction exists in the MAF circuit. If trouble Code 33 or 34 is set, the reading will remain fixed at the value that the failure has detected.
BARO-Barometric Pressure
On 1987-88 models, barometric pressure is displayed as a voltage value of 2.5-5.5 volts, depending upon altitude and barometric pressure. BARO reading should be equal to MAP reading before engine is started.
Batt-AD Bat
On 1987-88 models, display reads battery voltage at ECM ignition feed.
CCP-Canister Purge
On 1987-88 models, display reads status of ECM command for canister purge solenoid. Reading of ``on'' indicates ECM signal to solenoid. On some systems, duty cycle is displayed from 0-100 percent.
Desired RPM
On 1987-88 models, indicates RPM to which ECM is trying to control idle speed.
EGR Signal
On 1987-88 models, the SCAN tool monitors only the ECM drive command to the EGR solenoid. On models with Pulse Width Modulated EGR, reading is duty cycle of ECM signal from 0-100 percent. On models with EGR solenoid, ``on'' or ``off'' status of solenoid is indicated. Normal reading with engine running at idle should be 0 or off.
EGR Position
On 1987-88 models, indicates position of EGR valve pintle as voltage reading. Normal reading is 0.3 - 2 volts.
Fan
On 1987-88 models, monitors ECM command to engine cooling fan. Displays ``yes'' when ECM energizes fan relay. Indication should be ``no'' when coolant temperature is below 106° C.
Knock Retard
On 1986 engines equipped with ESC, this mode will indicate if the ECM is recognizing the retard signal from the ESC module and taking appropriate action. If knock is being detected, the numerical display on the tool will count up to 255, then start over.
On 1987-88 engines the display indicates the number of degrees the ECM is retarding the EST spark timing.
Knock Signal, 1987-88
Indicates knock sensor input to ECM. Display is ``yes'' when knock is detected and ``no'' when no signal received.
MAF-Mass Air Flow Sensor
On 1987-88 models, indicates the amount of air passing sensor in grams per second (gm/sec). Normal readings are 4-8 gm/sec. If Code 33 or 34 is set, reading will display ECM default value.
A/C Request-A/C Clutch
On 1987-88 models, the A/C Request position displays the status of the A/C signal line to the ECM. The A/C Clutch position displays ``on'' when the ECM energizes the compressor clutch.
Shift Light
On 1987-88 models, the shift light displays ``Yes'' when the ECM is commanding the shift lamp to light.