Auto Service Professional

OCT 2017

Magazine for the auto service professional

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18 | ASP October 2017 Technical to interpret what they mean. For instance, on Ford models, the maximum fuel pump duty cycle is 50%. is runs the pump at full speed to generate maximum pressure during wide-open-throttle or start-up. When the PCM commands 25% duty cycle, it's actually a command to turn the fuel pump off. e pump control module sends a diagnos- tic signal back to the PCM indicating actual duty cycle of the fuel pump. is provides a feedback loop so the PCM can determine if actual pump speed matches the commanded speed. ese signals are also displayed on a scan tool, and some scan tools allow you to issue pump speed commands with the tool. If pressure does not increase when the duty cycle is commanded high, either through a scan tool or at wide-open-throttle on a test drive, this could indicate a faulty pump. If fuel pump duty cycle remains high at idle just to maintain normal pressure, this can also indicate a bad fuel pump motor. However, at this point the only thing you know for sure is that the pump control module is working. You still need to test the voltage supply circuit under load before condemning the pump. If the engine won't start and/or if your scan tool won't display fuel pump commands, you can still diagnose the driver module with a voltmeter that can read duty cycle. You'll also need a service information system that has wiring diagrams and a good description of how the system operates. Just remember there will always be six wires: fuel pump power, control module power, control module ground, fuel pump ground, the command signal from the PCM and the feedback signal. By back-probing the connector and looking for the correct voltage and ground signals, you can determine if the control module is working. By the way, GM has always used the same wire colors for these circuits. Hold and flow Hold pressure is checked at the same time as fuel system pressure. After the engine is switched off and the fuel pump stops running, fuel system pressure will decrease slightly and then hold steady for at least five minutes. On many vehicles the hold-pressure specification For purposes of illustration, depicted here is a typical fuel pump control unit schematic. Photo courtesy of Motologic

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