Engine Control Computer


If you remove and repair a ground connection, be sure you reconnect it to the same place. Do not change the length of the wire.

Static electricity from sliding around on the front seat is a concern when working around electronic components.Backprobing the computer under the hood is the only time you need to wear a ground strap.

  • A digital multimeter (dmm), also called a digital volt/ohmmeter (dvom), is the instrument used to measure electricity in electronic circuits.
  • A high impedance voltmeter must be used to perform tests on these systems. the integrated circuits in computer systems operate on very small amounts of current.
  • An analog meter (one with a needle) that operates on magnetism can load down a computer circuit and actually change what is happening in the circuit.
    • The meter seems like a short in the circuit, offering an easier path for electrical flow.

Electronics use very, very small amounts of electricity and are very sensitive to resistance in wiring. As little as 200 ohms resistance can cause a problem.

Testing the Circuit

  • A wiring diagram for the vehicle being tested must be used for these tests.
  • Backprobe the battery terminal at the powertrain control module (PCM) and connect a digital voltmeter from this terminal to ground.
Typical computer terminals and their identification. Reprinted with permission.
  • Always ground the black meter lead.
  • The voltage at this terminal should be 12 volts with the ignition switch off. If 12 volts are not available at this terminal, check the computer fuse and related circuit.
  • Turn on the ignition switch and connect the red voltmeter lead to the other battery terminals at the PCM with the black lead still grounded.
    • The voltage measured at these terminals should also be 12 volts with the ignition switch on.
    • When the specified voltage is not available, test the voltage supply wires to these terminals. These terminals may be connected through fuses, fuse links, or relays.
  • Computer ground wires usually extend from the computer to a ground connection on the engine or battery.
    • With the ignition switch on, connect a digital voltmeter from the battery ground to the computer ground.
    • The voltage drop across the ground wires should be 30 millivolts or less.
    • If the voltage reading is greater than that or more than that specified by the manufacturer, repair the ground wires or connection.
  • Not only should the computer ground be checked, but so should the ground (and positive) connection at the battery. Checking the condition of the battery and its cables should always be part of the initial visual inspection before beginning diagnosis of an engine control system.

A voltage drop test is a quick way of checking the condition of any wire.

  • Connect a voltmeter across the wire or device being tested.
  • Place the positive lead on the most positive side of the circuit.
  • Then turn on the circuit.
  • Ideally, there should be a zero volt reading across any wire unless it is a resistance wire that is designed to drop voltage; even then, check the drop against specifications to see if it is dropping too much.
  • A good ground is especially critical for all reference voltage sensors. The problem here is not obvious until it is thought about.
    • A bad ground will cause the reference voltage (normally 5 volts) to be higher than normal.
    • Normally, the added resistance of a bad ground in a circuit would cause less voltage at a load.
    • Because of the way reference voltage sensors are wired, the opposite is true.
    • If the reference voltage to a sensor is too high, the output signal from the sensor to the computer will also be too high.
    • As a result, the computer will be making decisions based on the wrong information.
    • If the output signal is within the normal range for that sensor, the computer will not notice the wrong information and will not set a diagnostic trouble code (DTC).

To explain why the reference voltage increases with a bad ground, look at a voltage divider circuit.

  • This circuit is designed to provide a 5-volt reference signal off the tap.
  • A vehicle's computer feeds a regulated 12 volts to a similar circuit to ensure that the reference voltage to the sensors is very close to 5 volts.
  • The voltage divider circuit consists of two resistors connected in series with a total resistance of 12 ohms.
  • The reference voltage tap is between the two resistors. The first resistor drops 7 volts, which leaves 5 volts for the second resistor and for the reference voltage tap.
A voltage divider circuit with voltage values.
  • This 5-volt reference signal will be always available at the tap, as long as 12 volts are available for the circuit.
  • If the circuit has a poor ground, one that has resistance, the voltage drop across the first resistor will be decreased. This will cause the reference voltage to increase.
  • In the figure below, to simulate a bad ground, a 4-ohm resistor was added into the circuit at the ground connection at the battery.
Voltage divider circuit with a bad ground.
  • This increases the total resistance of the circuit to 16 ohms and decreases the current flowing throughout the circuit.
  • With less current flow through the circuit, the voltage drop across the first resistor decreases to 5.25 volts.
The voltage divider circuit above with voltage readings.
  • This means the voltage available at the tap will be higher than 5 volts; it will be 6.75 volts.
  • Poor grounds can also allow electro-magnetic interference (EMI) or noise to be present on the reference voltage signal.
  • This noise causes small changes in the voltage going to the sensor. Therefore, the output signal from the sensor will also have these voltage changes.
  • The computer will try to respond to these small, rapid changes, which can cause a driveability problem.
  • The best way to check for noise is to use a lab scope.
    • Connect the labe scope between the 5-volt reference signal into the sensor and the ground.
    • The trace on the scope should be flat.
(Top) A good voltage signal. (Bottom) A voltage signal with noise.
    • If noise is present, move the scope's negative probe to a known good ground.
    • If the noise disappears, the sensor's ground circuit is bad or has resistance.
    • If the noise is still present, the voltage feed circuit is bad or there is EMI in the circuit from another source, such as the A/C generator.
    • Find and repair the cause of noise.

Testing a Ground Circuit

  • Measure voltage drop across the ground side of the circuit.
  • Connect the meter to the ground side of the sensor or actuator. This is the lead that returns to the computer.
  • Connect the negative side of the meter to the negative battery terminal.
  • Voltage drop in a sensor's ground circuit should be less than 0.1 V.
  • Power ground circuits should not exceed 0.3 V.
  • Isolate the problem by working your way down the circuit.
  • Ground side resistance that is too high will decrease voltage in the circuit. The computer will receive too low a return signal from the sensor.

Too much electrical current causes heat that can damage an electrical circuit. Voltage spikes are the biggest cause of electrical damage to an integrated circuit.

  • Voltage is limited to about 20 V. Most automotive computer systems operate on 5 volts.
  • If a circuit never exceeds its voltage or current limits, it is very reliable.
  • Semiconductors are designed to handle a limited amount of current.
    • A light bulb can draw 2.5 amps.
    • Transistors in computers draw 200 milliamps (0.2 A).
  • If too much current is applied in a reverse direction, it can force through and ruin a diode or transistor.
  • Computers can tolerate a high current surge for about 5 seconds.
  • If a sensor or wiring connection is unplugged while there is power to it, a spike can occur.
  • A spike while disconnecting a powered circuit lead can result in a spike of 50 volts or so.
    • This happens because electrons that were in motion before the circuit was broken back up at the connection.
    • When they try to push their way across the gap, the spike is created.
    • Manufacturers build safeguards for voltage spikes into their newer systems, but not enough to protect against disconnecting and reconnecting components.
  • Voltage spikes can also result from an arc welder. Always disconnect the battery before doing any welding.