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Old 06-20-2018, 07:03 PM   #1
erichevy
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Speed Switch?

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!
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1979 GMC C35 - Farm Work Truck
1986 GMC C25 Crew Cab
1987 SWB R10
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Old 06-21-2018, 12:57 AM   #2
kwmech
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Re: Speed Switch?

Emissions---- I believe it would not allow vacuum advance until the trans hit final drive in an automatic
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Old 06-21-2018, 12:19 PM   #3
erichevy
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Re: Speed Switch?

Quote:
Originally Posted by kwmech View Post
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...
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Old 06-21-2018, 08:48 PM   #4
ray_mcavoy
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Re: Speed Switch?

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.
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