Fuel Pump Case Study

89 Jeep Cherokee 4.0 Noisy Pump

89 Jeep Cherokee 4.0 87k miles, fuel pump noisy. This pattern has little information, engine running, signal captured at pump side of ballast resistor.

Going directly to fuel pump with 12 volts (at starter relay) gave this pattern, looks like 19.5ms, pulling 3 - 3.8 amps, 3076rpm. Although going direct to the pump bypassed most of the pump circuit, it does make the pump pattern more readable, and allows getting what we want from it. The pump being noisy is enough to indicate replacement, I wanted to go in depth on this job to find out just what makes these things howl.

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