019-027   Diagnostic Test Mode Switch

General Information

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The diagnostic ON/OFF switch circuit signals the system that the operator is requesting to read any active fault code recorded in the ECM.

NOTE: Some OEM’s use a shorting plug rather than a switch.

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When the ECM receives the signal from the diagnostic ON/OFF switch, the yellow and red warning lights will come on and start flashing if any active fault code is recorded in the ECM. If both warning lights remain on and do not flash, there are no active fault codes present.

NOTE: The equipment must be stationary. If road speed is detected, the flashing sequence will not occur.

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Resistance Check

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If INSITE™ is available, monitor the switch for proper operation. If not, follow the troubleshooting procedures in this section.

Locate the desired ON/OFF toggle switch. Remove and tag the two connectors from the terminals on the switch.

Touch the multimeter probes to the terminals on the switch.

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Move the switch to the OFF position and measure the resistance. The multimeter must show an open circuit (100k ohms or more).

If the circuit is not open, the switch has failed.

Refer to the OEM troubleshooting and repair manual for the replacement procedures.

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Move the switch to the ON position and measure the resistance. The multimeter must show a closed circuit (10 ohms or less). If the circuit is not closed, the switch has failed.

Refer to the OEM troubleshooting and repair manual for the replacement procedures.

If the resistance value is correct, the switch must still be checked for a short circuit to ground.

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Check for Short Circuit to Ground

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Touch one of the multimeter probes to one of the switch terminals. Touch the other probe to chassis ground. Move the switch to the ON position and measure the resistance. The multimeter must show an open circuit (100k ohms or more). If the circuit is not open, the switch has failed. Refer to the OEM troubleshooting and repair manual for replacement procedures. If the switch passes all of the previous checks, the circuit must be checked for an open circuit, a short circuit to ground, a short circuit from pin to pin, and a short circuit to an external voltage source.

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Last Modified:  03-Apr-2003

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