019-083   Tachometer Circuit

The ECM can supply an output signal to operate the vehicle tachometer. The circuit is pin 17, the output signal, and pin 26, the signal return, in the OEM harness.

NOTE: The tachometer

must be capable of operating with a 4-VDC output signal from the ECM. Tachometers with only 12-VDC output singal capability will

not function on a CELECT™ ECM.

The tachometer circuit can be connected to the ECM two different ways, depending on the vehicle tachometer ground. If the vehicle tachometer is grounded internally, pin 26 is

not connected to the ECM.

If the vehicle tachometer is

not grounded internally, pin 26

must be connected to the ECM.

CAUTION  To reduce the possibility of connector damage, do not use probes or leads other than Part Number 3822758. The leads must fit tightly in the connector without expanding the pins in the connector.

Disconnect the OEM harness connector from the ECM. Refer to Procedure

019-071. Disconnect the tachometer from the OEM harness.

Insert a test lead into pin 17 in the OEM harness connector. Connect the alligator clip to the multimeter probe.

Locate the tachometer connector of the OEM harness. Connect the other probe of the multimeter to terminal A in the connector. Adjust the multimeter to measure resistance. The multimeter

must show a closed circuit
(10 ohms or less). If the circuit is

not closed, there is an open circuit or the wires in the tachometer connector are reversed. Repair or replace the wire connected to pin 17 in the OEM harness according to the vehicle manufacturer’s procedures.

Remove the lead from pin 17 and place it into pin 26. Remove the multimeter lead from terminal A and connect it to terminal B in the connector. Measure the resistance. The multimeter

must show a closed circuit (10 ohms or less). If the circuit is

not closed, there is an open circuit or the wires in the tachometer connector are reversed. Repair or replace the wire connected to pin 26 in the OEM harness according to the vehicle manufacturer’s instructions.

If the values are correct, the circuit

must still be checked for short circuits to ground and short circuits from pin to pin.

Connect the multimeter probe test lead to pin 26. Touch the other multimeter probe to the engine block ground. Measure the resistance. The multimeter

must show an open circuit (100k ohms or more).

Remove the lead from pin 26 and connect to pin 17. Touch the other multimeter probe to the engine block ground. Measure the resistance. The multimeter

must show an open circuit (100k ohms or more).

If the circuit is

not open in either of those checks, repair the shorted wires in the circuit according to the vehicle manufacturer’s procedures.

Connect the multimeter probe test lead to pin 17 of the OEM connector. Place the other multimeter probe test lead to pin 1. Measure the resistance. The multimeter

must show an open circuit (100k ohms or more).

Remove the test lead from pin 1 and place it into pin 2, then pin 3. Check all pins. Measure the resistance. The multimeter

must show an open circuit (100k ohms or more) at all pins.

Remove the test lead from pin 17 and place it into pin 26. Place the other multimeter probe test lead into pin 1 of the OEM connector. Measure the resistance. The multimeter

must show an open circuit (100k ohms or more) at all pins.

Remove the test lead from pin 1 and place it into all other pins. Measure the resistance. The multimeter

must show an open circuit (100k ohms or more).

If the circuit is

not open in any of these circuit checks in this section, the tachometer circuit is good. The problem is in the tachometer. Repair or replace the tachometer according to the vehicle manufacturer’s instructions.

NOTE: If the values are correct for all of the circuit checks in this section, the tachometer circuit is good. The problem is in the tachometer. Repair or replace the tachometer according to the vehicle manufacturer’s instructions.