Models With Mechanical Speedometer

1. Raise and support vehicle so that drive wheels are free to turn.
2. Connect suitable voltmeter between pin 2 of red ECM connector and ground, then turn ignition on, place gearshift in neutral, and manually rotate drive wheel.
3. If voltage varies between .6 and 11 volts, road test vehicle in fourth gear at 55 MPH and enter diagnostics. If code 24 is still stored as a ``hard failure,'' replace ECM. If code 24 is no longer indicated as a ``hard failure,'' there is an intermittent failure in the system.
4. If there is no voltage variation, disconnect buffer amplifier, turn ignition on and connect voltmeter between pin A of harness connector (ECM side) and ground. If no voltage is recorded, inspect circuit 961 for an open or short to ground. If wire is satisfactory, fault is in ECM or ECM connector.
5. If voltmeter reads 10 to 12 volts, connect voltmeter between pin C of harness connector and ground. If no voltage is recorded, inspect 10 amp ignition fuse. If fuse is satisfactory, repair open in circuit 639. If fuse is blown, repair short in circuit 639.
6. If voltmeter reads 12 volts, connect voltmeter between pins B and C of connector harness and ground. If no voltage is recorded, inspect circuit 151 for an open and repair as necessary.
7. If voltmeter reads 12 volts, reconnect buffer amplifier, connect voltmeter between pin G of sensor harness connector and ground and manually rotate speedometer drive gear mechanism. If voltage varies between 0 and 5 volts, replace buffer amplifier.
8. If there is no voltage variation, connect voltmeter between pin F of speed sensor harness and ground. If no voltage is recorded, replace buffer amplifier.
9. If voltmeter reads between 4 and 6 volts, connect voltmeter between pin E of speed sensor harness and ground. If voltage measured is within one volt of pin F, replace buffer amplifier.
10. If voltage is more than one volt less than pin F, check speed sensor. If speed sensor is satisfactory, repair speedometer.