I bought mine from Snap-on 20-25 years ago. They possibly might still carry them. Mack or Matco also carries them now. I think KD still makes them.
1976 and later cars-the fuel pump is triggered (through relay) by the sensor plate microswitch behind sensor plate air horn. When you crank, sensor plate lifts up, switch turns on energizing relay.
Black and red relays are basically the same. Red relays are dioded and internally fuzed at 30 amps. (Diodes are used to suppress voltage spikes from inductive loads like motors) Black relays have no diodes and (I believe) are internally fuzed at 25 amps. For testing purposes they are interchangeable.
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