Cummins ISX11.9 CM2250 – Service Manual 019-083   Tachometer Circuit

General Information

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The electronic control module (ECM) can supply an output signal to operate the vehicle tachometer.

The circuit is the tachometer signal wire and a return line in the original equipment manufacturer (OEM) harness.

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Resistance Check

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 CAUTION 

Do not use probes or leads other than Part Number 3822758. The connector will be damaged. The leads must fit tightly in the connector without expanding pins in the connector.

Disconnect the OEM harness from the ECM.

Disconnect the tachometer from the OEM harness.

Insert the test lead into the tachometer signal pin of the OEM harness connector and connect it to the multimeter probe.

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Locate the tachometer connector of the OEM harness.

Connect the other lead to the multimeter probe and connect it to the tachometer signal pin of the tachometer connector that is coming from the engine ECM. Refer to the OEM service manual for wiring schematics.

Adjust the multimeter to the resistance setting. 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 the tachometer signal pin in the OEM harness according to the vehicle manufacturer’s procedures.

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Check for Short Circuit to Ground

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Disconnect the tachometer from the OEM harness.

Insert the test lead into the tachometer signal pin of the OEM harness connector and connect it to the multimeter probe. 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 the prior checks, repair the wires which have incorrect readings. Refer to the OEM service manual for the repair procedures.

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Check for Short Circuit from Pin to Pin

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Disconnect the tachometer from the OEM harness. Insert the test lead into the tachometer signal pin of the OEM harness connector and connect it to the multimeter probe. Insert the other lead into any pin, except the tachometer switch return, of the OEM harness connector, and connect it to the other multimeter probe. Measure the resistance.

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

Measure the resistance from the tachometer signal pin to all other pins in the OEM connector. The multimeter must show an open circuit.

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Remove the test lead from the last tested pin, insert it into the tachometer switch return pin. Measure the resistance from the tachometer switch return pin to the tachometer signal pin in the OEM harness connector.

The multimeter must show an open circuit (100k ohms or more) at all pins. If any pin-to-pin check measures as not open, there is a short circuit between the tachometer signal pin and any other pin that measured a not open circuit. Repair or replace the OEM harness. Refer to Procedure 019-071 in Section 19.

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Last Modified:  16-Jun-2010