My 1992 GMC K2500 chatters the fuel pump relay when I turn the key on.
And... the CEL stays constantly on.
Any thoughts?

1. What is battery voltage? (better be 12.6--12.7)
2. What is voltage at the power distribution panel on upper right side of firewall behind plastic cover? (better be within .10 volts of battery voltage. SAME as battery voltage is best.)
3. What is voltage at fuel pump relay pin F with key ON and relay active? (probably blk/wht wire) (better be 0.0--it's a ground)
4. What is voltage at fuel pump relay pin D with key ON and relay active? (probably grn/wht wire)

First guesses: Low battery voltage or high resistance in fuel pump relay trigger circuit--either + side or ground side.

I can't remember if the relay trigger is "12 volt" or not.