DTC 27

Fig. 21 Idle speed control throttle switch wiring circuit:

If this code is not displayed during the third pass of diagnostic codes, the problem is an intermittent failure. To help isolate the problem, operate vehicle while observing throttle switch status light. If throttle switch ever fails to close upon release of throttle, investigate circuit for malfunction. Also check for misadjusted minimum air rate, binding throttle linkage or binding ISC plunger.
The ECM supplies a 5 volt signal to pin B of the idle speed control motor. When the engine operates above idle speed, the throttle switch is open and the ECM increases voltage in circuit 427 to 5 volts. When the idle speed switch is open, the idle speed control motor will not control idle speed because engine speed is controlled by the driver. Code 27 indicates that the throttle switch remained open during the last ignition cycle and has not yet closed during the present cycle. Possible causes of this condition are:
a. Defective idle switch.
b. Defective wiring or electrical connectors.
c. ECM unable to process signals properly.

To diagnose a Code 27, proceed as follows:
1. Turn ignition on and enter diagnostics. Observe ``Throttle Switch'' light above ``Lo'' button on ECC control head. If light comes on, proceed to step 2. If light remains off, refer to ``Code .7.2, Throttle Switch Circuit'' diagnosis.
2. Exit diagnostics with ``Throttle Switch'' light on. If Code 27 remains as a hard code, replace ECM. If Code 27 is now an intermittent, refer to note above.