Duel Tank Switch

[chengny]

I agree, they didn't change to the 5 wire control around 1981 maybe. Those are motorized valves and stay put when the power is removed.
So, only that later style valve actuator requires a momentary switch - it would not last long with a standard toggle/latching switch.

And the use of a momentary switch in the control of the single wire (solenoid type) actuated valve would keep defaulting to the factory tank when released.

I have a bad habit of not checking the model year of the truck being discussed and missed the 1978. I try to limit my input to the 81-87 series and I also occasionally enter general automotive/engineering discussions.

 

[chengny]

I think it was a single wire controlling a solenoid actuated valve (NO to the factory tanks by the spring - and held open to the auxiliary tank by powering the solenoid coil) from the earliest designs up until around 1981?

From then until the advent of TBI systems it was a 5 wire control circuit that operated the Pollak valve.

After 1987, when TBI was the only game in town, the control circuit was again changed back to a latching type. I don't know why - maybe because of the increased fuel pressure and the pumps being moved to suction side of the valves there was a concern that the pump pressure could inadvertanly shuttle a motorized valve?

Schematics for the two designs:

 

[SlickGTP]

Hmmm, you learn something new every day... I knew the ones for the older ones were the single wire solenoid type, didn't know they had switched temporarily in the 80's to a motorized version.

Thanks chengny!