1973-1991 Dual fuel tank systems theory of operation

[squarebodyaddict]

Originally Posted by hatzie

The fuel pump relay may not be passing full voltage through the relay switch contacts anymore. They can get dirty just like ignition breaker points.

Back probe relay Terminal A and Terminal E to a good ground.
Terminal A is the input voltage from the ECM.
Terminal E is the output voltage to the tank switch and selected fuel pump.

Thanks for the Info Hatzie.
I tried 4 used relays and 1 brand new relay, all had same result... 12.4V back probing Terminal A and 8.9-9.1V on Terminal E, grounded direct to battery.

I'm not great at finding info in these forums, I feel like I've searched every possible combination of word search... would like to know any suggestions for a next step.

P.S. Some Clarification:
91 V3500 454: While testing the A terminal (key on) 12.3+/- volts. Testing the E terminal I get 12.3V+/- at initial key on and while cranking, but falls back to 9Volts +/- when not cranking/not triggering fuel pump....
Interestingly enough I did the same test on my 91 V2500 Suburban 350 it gives similar readings except when testing E terminal key on when not cranking/not triggering fuel pump it then gives a 9 MILI VOLT reading.
Any Idea why the V3500 gives 9Volt reading while the same test on the V2500 gives different reading of 9 Mili Volt???

Also I checked firewall and Engine Ground both seem perfect.