GM 6.5L V8 – Electrical Systems – Fast Idle/Cold Advance Circuit

The fast idle/cold advance circuit has the components shown in Figure 8-28. When the ignition switch is in the CRANK or RUN position, power through the GAGES fuse is present at the contacts of the temperature switch.

At temperatures below approximately 85° F (29° C), the temperature switch contacts close, providing power to both the fast idle and HPCA solenoids. As part of the starting procedure, the vehicle driver must first press the accelerator pedal to allow the fast idle solenoid plunger to extend. When it is ON, the fast idle solenoid (mounted on an external injection pump bracket) holds the throttle shaft away from its low idle position slightly to increase engine idle speed.

At the same time, the HPCA solenoid (mounted inside the injection pump governor cover) unseats the valve of the housing pressure regulator to provide timing advance (see page 7-44 for more information). When engine coolant temperature exceeds 95° F (35° C), the temperature switch contacts open, disabling the system.

The fast idle solenoid has a resistance of 20 ohms, and the HPCA solenoid resistance is 24 ohms. Diagnosis of the HPCA solenoid is similar to the fuel shutoff solenoid, with initial checks involving a “click” test.


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