DTC 26

Fig. 18 Throttle position sensor wiring circuit:

The ECM supplies a 5.0 volt signal to pin B of the idle speed control motor. When the engine is operating at idle speed, the throttle linkage closes the throttle switch and lowers the voltage in the circuit to zero volts. When the idle switch is closed, the ECM and idle speed control motor will control engine speed under all operating conditions. Code 26 indicates that the voltage signal from the idle switch is too low when the engine is operating above idle speed or that the voltage signal from the throttle position sensor is too high when the engine is operating at idle speed. Possible causes of this condition are:
a. Defective idle switch.
b. Defective wiring or electrical connectors.
c. ECM unable to process signals properly.
d. Improperly adjusted or sticking throttle position sensor.
e. An open circuit in circuit 476. 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 and parameter .0.1 simultaneously. If throttle switch fails to open upon application of throttle, investigate circuit for malfunction. If throttle angle ever jumps above 13° with throttle closed, investigate that circuit for malfunction.
To diagnose a Code 26, proceed as follows:
1. If a hard Code 15 is stored with a hard Code 26, perform the following preliminary checks:
a. Disconnect coolant temperature, TPS and BARO sensor electrical connectors. Measure voltage between 12 volts and circuit 476-X on each sensor harness connector.
b. If zero voltage is measured at all sensors, check circuit 476-X for an open. If circuit is satisfactory, check for faulty ECM or ECM connector and repair or replace as necessary.
c. If zero voltage is not measured at all sensors, proceed to step 2.
2. Turn ignition on and enter diagnostics. Observe ``Throttle Switch'' light above ``Lo'' button on EEC control head. If ``Throttle Switch'' light goes out, proceed to step 3. If light remains lit, refer to ``Code .7.2, Throttle Switch Circuit'' diagnosis.
3. Ensure TPS does not bind and that the TPS and ISC are properly adjusted. If TPS and ISC are satisfactory, proceed to step 4.
4. Turn ignition on, enter diagnostics and display parameter .0.1. Under Code 26 condition, value of parameter .0.1 will be greater than 13° at closed throttle.
5. Disconnect TPS electrical connector and note parameter value. If value is between (-)10 and (-)7, proceed to step 6. If value is (-)6 or more, check circuit 417 for a short to voltage. If circuit is satisfactory, check for faulty ECM or ECM connector and repair or replace as necessary.
6. Measure voltage between harness connector pins B and C. If voltmeter reads 5 volts, check for loose connections in sensor terminals or a faulty TPS. If no voltage is measured, repair open in circuit 476-X.