Idle position switch testing

^ Prior to testing or adjusting idle switch, make sure that the throttle position is set according to instructions in Throttle plate basic adjustment, given earlier.

A defective idle switch (open circuit) will cause high idle speed (approximately 1,200 rpm) and advanced ignition timing (12° before TDC).

— To test idle switch mechanical setting:

• Attach strobe type timing light according to manufacturer's instructions.

• Start engine and allow to idle.

• Open throttle plate approximately 1 mm while watching idle microswitch contact lever.

• Throttle switch should open before throttle plate begins to open.

• If switch is working idle speed will increase approximately 500 rpm and ignition timing will advance to 12° before TDC.

To test idle switch electrically:

• Engine should be stopped. Remove harness connector from switch.

• Connect ohmmeter leads to switch terminals (42 and 44).

• Open throttle approximately 1 mm.

• There should be continuity with throttle fully closed and no continuity in all other positions.

— To test idle switch harness and connector:

• Start engine and allow to idle.

• Disconnect idle switch harness connector.

• Engine speed must increase approximately 500 rpm.

• If engine speed does not increase check wiring to ECM.

• If no problems are found replace ECM.

— To test decel fuel shut-off function:

• Start engine and allow to idle.

• Disconnect idle switch harness connector and bridge harness terminals.

• Engine should start surging, indicating injectors are turning off. If not, check wiring between switch and ECM.

• If no problem is found with switch or wiring, ECM may be faulty.

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