ECM Input Code EI72 - Throttle Switch Circuit: Notes

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

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