Diagnostic Aids

NOTE: The following procedure is designed to account for intermittent codes. If no malfunction is uncovered using this procedure, it will indicate that the DFI system is OK at this time. Refer to NOTE ON INTERMITTENTS if further investigation is necessary.

When trouble Code 30 is set, the check engine light remains on for the entire ignition cycle and ISC operation is not tested again until the next ignition cycle. This means that an intermittent malfunction will appear as a hard failure during the ignition cycle in which it occurs. The ECM, even though it has recognized a malfunction in the ISC operation, will continue trying to control idle speed.

The ISC motor tries to control idle speed to a specified RPM whenever the throttle switch, located inside the ISC housing, is closed. The ECM determines whether the ISC motor is functioning properly by monitoring the resultant change in TPS readings as ISC motor operation is commanded.

Trouble Code 30 indicates that either of the following two conditions have occurred:

I. After commanding the ISC motor to retract for 15 seconds, the actual RPM remained more than 150 over that specified and the throttle angle remained greater than 2.5 degrees.

II. After commanding the ISC motor to extend for 15 seconds, the actual RPM remained more than 150 below that specified and the throttle angle remained less than 9 degrees. Possible causes of these conditions are:

A. A misadjusted TPS.
B. A misadjusted ISC motor.
C. A defective ISC motor.
D. A defective throttle switch.
E. A defect in wiring, terminals, etc.
F. An ECM which is unable to process the signal properly.

To begin diagnosis, determine if the throttle switch is working properly by performing the "switch tests". If Code .7.2 does not pass, refer to the Code .7.2 Diagnostic Decision Tree.
If Code .7.2 passes, then the next step is to verify that the ISC motor retracts and extends when commanded to do so by the ECM. To exercise the ISC motor, proceed as follows:
- A. Enter diagnostics and display "output cycling" Code .9. 6.
- B. Cycle the throttle switch to begin "output cycling".
- C. Visually check to see if the ISC motor extends and retracts.
If the ISC motor extends and retracts, then check for a binding TPS sensor. Also check the minimum air rate, TPS and ISC adjustments as described in Section 6 of the Service Manual. At this point, the checks and adjustments of the circuits which might set Code 30 have been completed, and Code 30 should not reappear. If no problems are found and Code 30 continues to be set, check for a faulty ECM connector or faulty ECM.
If the ISC motor fails to extend or retract, check the throttle linkage for free movement. If the throttle is not binding, disconnect the ISC connector and jumper pins A and B together. While in "output cycling," connect a test light between harness connector pin D and ground. If the test light flashes for 3 seconds and is off for 3 seconds, then circuit 425 is OK. Connect the test light between harness connector pin C and ground with pins A and B still jumpered together. If the test light flashes for 3 seconds and is off for 3 seconds, then circuit 426 is OK. If a test light across pin C and pin D flashes continuously, the problem must be in the ISC terminals or ISC motor. If the test light fails to light or flashes for 3 seconds and is off for 3 seconds then the problem is the ECM failing to provide a ground for the ISC motor. Check for a faulty ISC connector or faulty ISC motor.
If either circuit 425 or 426 fail to flash the test light, they should be checked for opens, shorts to ground or a short between the two circuits. If a short is discovered, the ECM may have been damaged and may not be able to control the ISC motor. If no opens or shorts are found, the fault must be in the ECM connector or or ECM itself.