Chevrolet 10 Series 1968, Service Manual, Page: 459 / 713

ENGINE-ELECTRICAL 6Y-34
DISTRIBUTOR CARBURETOR
BUTTERFLY VALVE
SPINNER VALVE
AMBER
LAMP
IGNITION
SWITCH
LAMP BURN-OUT CHECKING CIRCUIT
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OIL PRESSURE
SWITCH
BATTERY
CARBURETOR ACTUATOR
Fig. 2 9 i—Engine Overspeed Warning System
Installation
Install the spark plugs in the engine with new gaskets
and torque to specifications. If torque wrench is not
available, tighten plugs finger tight and
1/2
turn more.
Plugs are of a 14 millimeter size and care must be exer­
cised when installing or the gap setting may be changed.
TRUCK ENGINE OVERSPEED WARNING SYSTEM
The purpose of this overspeed warning system is to
warn the driver of excessive engine speed in an over-run
condition (such as during down hill operation when the
wheels may begin to drive the engine) and thus enable
steps to be taken to correct the condition.
The system (fig. 29i) consists of an amber jeweled
warning lamp mounted in the cab, a vacuum sensing
switch in the spinner governor vacuum line, and a warn­
ing lamp burnout checking circuit. These components
operate in conjunction with the 2G Engine Governor
(Vacuum Spinner Type) described in Section 6 M of this
Shop Manual.
In operation, when the engine reaches its governed
speed (4000 rpm with standard transmission or with
Powermatic transmissions maximum under full load con­
ditions) the centrifugally operated spinner valve will
close, thus applying a vacuum on the carburetor actuator
diaphragm which in turn acts to close the throttle valves.
The overspeed warning system vacuum switch, being in
the spinner governor vacuum line, also senses the vac­
uum applied when governed speed is reached and acts to
close the circuit to the battery thus lighting the amber
warning lamp. The lamp burnout checking circuit con­
sists of a “'normally closed” engine oil pressure switch
which completes the circuit from the battery, through the
warning light, ignition switch and the oil pressure switch
itself, and back to ground. The warning light should light
up when the ignition switch is turned “ ON” but the engine
is “ OFF.” When the engine is started the oil pressure
opens the switch, breaking the checking circuit. The
warning light will then go out and remain out until such
time as the engine reaches its predetermined governed
speed. Thus the driver has a quick check of the condition
of the warning light bulb.
CHEVROLET TRUCK SERVICE MANUAL