Testing the Diagnostic Connector

TESTING THE DIAGNOSTIC CONNECTOR

Testing The Diagnostic Connector:

Use the following voltage tests at the diagnostic connector to help determine the reason that a vehicle will not perform diagnostic tests. Use a high-impedance digital voltmeter and refer to figure above.


ASD Relay Terminal (fuel-Injected) or Carburetor Solenoid Terminal (carbureted)

Connect the voltmeter positive (+) lead to the ASD relay (or carburetor solenoid) terminal in the connector. Connect the negative (-) lead to the battery ground terminal. Then turn the ignition key on.

- For a fuel-injected engine, you should read 0 volts on the ASD relay terminal for about 2 to 5 seconds with the key on. Voltage should then rise to open-circuit battery voltage (approximately 12 volts) as the computer deenergizes the relay.

- For a carbureted engine, you should read 0 volts on the carburetor solenoid terminal with the key on. With the engine running, you should read system voltage or approximately 6 to 9 volts as the computer modulates the solenoid.

If you do not get these voltage readings at the ASD relay or carburetor solenoid terminal in the connector, check the circuit connections at the relay or solenoid. The engine can still run with an open circuit to the terminal in the connector.

On 1985-87 cars, the ASD relay is built into the power module.

Diagnostic Connector Ground

With the ignition on, connect the voltmeter positive (+) lead to the ground terminal in the diagnostic connector. Connect the negative (-) lead directly to the battery negative (-) terminal. Do not connect the voltmeter negative lead to an engine or chassis ground. This test measures the voltage drop across the ground side of the diagnostic connector. An ideal system ground should have a voltage drop of 0.1 volt or less.

If the wiring is open to the ground terminal of the diagnostic connector, or at the connector Itself, It may not affect engine operation. It will affect scanner operation, however, because the scanner uses this terminal as a ground point. To check for an open circuit at the diagnostic connector ground terminal, connect the voltmeter positive (+) lead to battery positive and the negative lead to the connector ground terminal. You should read battery voltage. If the meter reads zero, the ground terminal is open.

Switched Ignition Voltage

With the ignition on, connect the voltmeter positive (+) lead to the battery voltage terminal at the diagnostic connector. Connect the negative (-) lead to the battery ground terminal. You should read battery voltage. This tests the battery voltage supply to the computer. You also can use the scanner READ J2 or FJ2 VOLTS test to check this voltage as explained previously. Also check the J1-J2 circuit splice near the controller.

Vehicle Communication Transmit And Receive

Refer to a wiring diagram and use an ohmmeter to check continuity in the transmit and receive circuits to the diagnostic connector. These are the lines that the scanner uses to receive computer data in the CODES & DATA mode and for sensor and ATM tests. If either circuit is open, the scanner cannot transmit a request to the vehicle computer nor receive data from it.