Stockish Looking relay ???

[monte0185]

After some more research I think it might be for the oil pressure some how. Not sure what exactly though.

[ray_mcavoy]

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.

[monte0185]

Originally Posted by ray_mcavoy

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.

I Could see this but its tied into the oil pressure system not the tach.

[ray_mcavoy]

Originally Posted by monte0185

I Could see this but its tied into the oil pressure system not the tach.

That is strange. The only thing (aside from the light or gauge) shown wired to an oil pressure switch on the factory wiring diagrams is the electric choke on some engines.

Here's the diagram from the manual showing the location of the TRC speed switch on the valve cover:


And the diagram showing how it's wired up: