Input Code EI72 - Throttle Switch Circuit: Notes

The throttle switch is part of the Idle Speed Control (ISC) motor. Terminal "B" of 4-wire connector on ISC is throttle switch (closed throttle input to ECM). The ISC throttle switch contacts are normally open, and close when throttle linkage contacts ISC plunger (closed throttle, ISC in control of idle speed).

The ECM sends a 5-volt signal to terminal "B" of ISC motor on circuit No. 427. When throttle linkage rests on ISC plunger, the throttle switch contacts close, shorting ISC terminal "B" to terminal "A" on circuit No. 450 (ground). The 5-volt signal from ECM is grounded at closed throttle, resulting in zero volts at ECM terminal C10. When throttle is opened, the throttle switch opens terminal "B" on circuit No. 427, resulting in 5 volts at ECM terminal C10.

1. With ISC plunger depressed, status should be "LO" (closed throttle). When plunger is released, status should be "HI" (open throttle).
2. Check throttle shaft for throttle plates binding, weak or distorted throttle spring, cruise and throttle valve cables for binding, throttle linkage, and check TPS for proper installation.
3. When ISC bottoms fully in retracted position, the throttle switch will open. ISC should be partially extended to control idle.
4. Never connect voltage source across terminals "A" and "B" of ISC. Damage to throttle switch contacts will result.
5. This step simulates the throttle switch shorting terminal "A" to terminal "B". If removing and replacing jumper causes switch to cycle, harness and ECM are okay. Fault is in ISC motor connector or ISC motor.
6. This steps checks for 5-volt reference signal to ISC.
7. A 5-volt reference from ECM to ISC is open or shorted to ground.
8. A 5-volt reference is okay, when checking for open circuit No. 450F from ISC terminal "B" to ground.