The 1947 - Present Chevrolet & GMC Truck Message Board Network - View Single Post

ray_mcavoy · 12-31-2013, 07:46 PM

According to the factory repair manual, that is the "speed switch" for the TRC (Throttle Return Control) system. The TRC is supposed to open the throttle a little bit while the truck is coasting in order to reduce hydrocarbon emissions.

That "speed switch" gets it's RPM reference from a connection to the HEI TACH terminal. It's contacts are supposed to close above 1900 RPM and send power to a vacuum solenoid. That in turn switches vacuum on/off to the throttle lever actuator mounted to the carburetor.

12-31-2013, 07:46 PM	#4
ray_mcavoy Registered User Join Date: Nov 2009 Location: Sherman, ME Posts: 2,358	Re: Stockish Looking relay ??? According to the factory repair manual, that is the "speed switch" for the TRC (Throttle Return Control) system. The TRC is supposed to open the throttle a little bit while the truck is coasting in order to reduce hydrocarbon emissions. That "speed switch" gets it's RPM reference from a connection to the HEI TACH terminal. It's contacts are supposed to close above 1900 RPM and send power to a vacuum solenoid. That in turn switches vacuum on/off to the throttle lever actuator mounted to the carburetor.