Code EI72 - Throttle Switch Circuit: Notes

The throttle switch is part of the ISC. Pin B of 4-way weatherpack 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 resets 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 (0) volts at ECM pin D15. When throttle is opened, the throttle switch opens pin B on circuit No. 427, resulting in 5 volts at ECM pin D15.

1. ISC plunger depressed and released should allow switch test to pass. If "EI72" does not pass, check for ISC circuit problems.
2. Check throttle shaft for throttle plates binding, weak or distorted throttle spring, cruise and TV cables 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 a voltage source across pins "A" and "B" of the ISC. Damage to throttle switch contacts will result.
5. Disconnect ISC motor, and jumper between pin "A" to "B" on harness. Remove and replace jumper, noting status. If harness and ECM are okay, fault is in ISC motor connector or ISC motor.
6. Check 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.