Auto Service Professional

OCT 2017

Magazine for the auto service professional

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14 | ASP October 2017 Technical e only wear parts on this whole assembly are the motor's brushes. As electrical current flows through a circuit, it naturally gener- ates heat. Most of that heat is concentrated at the point of highest resistance, and in a brush-type motor, it's where the carbon brushes touch the rotating copper contacts (commutator bars). Even on a cold day the fuel pump motor typically runs at about 100 degrees Fahrenheit. is heat is carried away by fuel flowing through the pump, so when the low-fuel warning light turns on there's not much fuel left in the tank to absorb all that heat. Soon the ends of the brushes get so hot that microscopic particles of carbon burn off and stick to the copper contacts. If the brushes are overheated often enough, a coating of burnt carbon builds up on the contacts. is creates electrical resistance that reduces current flow through the motor and therefore reduces the motor's power. Eventually the motor becomes too weak to generate proper fuel pressure, and if the resistance gets high enough, the motor won't even run. Connectors As current flows through a circuit, the most common place for resistance to occur (other than the load) is in a connector. If the connec- tion is tight and remains clean, the resistance is almost nothing. However, if the contacts are loose, corroded, contaminated or simply not big enough to handle the current flow, resis- tance increases. is reduces or "drops" the voltage downstream of the connector, and heat concentrates at the point where the voltages are different; across the point of highest resistance. When a connector overheats, the plastic housing starts to warp and the problem tends to snowball; the contacts loosen or become corroded or contaminated, increasing resistance that generates even more heat and promotes corrosion that increases resistance even more. Voltage in the circuit is reduced "downstream" of the high resistance, so this reduces the voltage that reaches the pump motor. at's why heavy-duty connectors are used in a fuel pump circuit. Check valve ere are two fuel pressure specifications for every vehicle: system pressure and hold pressure. Hold pressure is maintained by the fuel pump's check valve when the engine is turned off. is makes it easier to start the engine. If the check valve fails or is jammed open by fuel contamination, the hold pressure will leak down. If that happens while the engine is cold, the starter will crank a bit longer while the fuel pump builds enough pressure to start the engine. If the engine is warm, the liquid fuel in the warm injector rail can evaporate, creating a "vapor lock" that prevents liquid fuel from reaching the injectors. Extended cranking might get the engine started, but Half the commutator bars are fouled with burnt carbon from the brushes. This causes high electri- cal resistance that reduces the motor's torque. Connectors are the most likely place to find high resistance and heat damage in a fuel pump circuit. Photo courtesy of Jim Morton

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