Speed Switch?

[erichevy]

Anybody know what the speed switch on a 79 GMC does? I can't seem to find anything when I try to Google it... thanks in advance!

[kwmech]

Emissions---- I believe it would not allow vacuum advance until the trans hit final drive in an automatic

[erichevy]

Originally Posted by kwmech

Emissions---- I believe it would not allow vacuum advance until the trans hit final drive in an automatic

Thanks for responding.... wonder what it would do in a manual trans truck like mine...

[ray_mcavoy]

That's part of the TRC (Throttle Return Control) system that's described on page 6E-26 of the 1979 service manual. Thanks to hatzie you can download a copy of that manual --> http://67-72chevytrucks.com/vboard/s...d.php?t=545416

Description from the manual:

The TRC system used on heavy duty emissions vehicles consists of three major components:

(1) Throttle Lever Actuator - Mounted as part of the carburetor assembly, this device opens the primary throttle blades a preset amount in excess of curb idle when engine vacuum is applied to it. This actuating vacuum is controlled by a separate solenoid control valve.

(2) Solenoid Vacuum Control Valve - Mounted separately from the carburetor, this off-on valve is held open above a present nominal engine speed by a signal from an electronic speed sensor. The valve when open allows a vacuum signal to be applied to the throttle lever actuator as long as the preset engine speed is exceeded.

(3) Electronic Speed Sensor - Mounted separately from the solenoid vacuum control valve, this switching device monitors engine speed at the distributor and supplies a continuous electrical signal to the solenoid vacuum control valve as long as the preset engine speed is exceeded.