Auto Service Professional

APR 2018

Magazine for the auto service professional

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16 | ASP April 2018 Technical to 60 psi. e interesting thing is that the GDI systems are designed to actually start and run from low side pressure, albeit not very well. Even before returnless systems were introduced, the probability of having a fuel pressure test port has decreased over the years. If you can recall, the Honda PGMFI systems had a tab in the fuel pressure pulsator that would extend when system pressure was reached. On other systems it was truly a challenge to figure out how to manually test fuel pressure. On the Ford electronic returnless systems, there is no fuel pressure regulator. Fuel pressure is controlled electronically by a fuel pump control module. e control module hammers the pump with system voltage or ground side controls the pump at 9,500 times a second. e PCM sees the engine load and increases the duty cycle signal to the fuel control module circuit. e GM fuel pump control module will supply system voltage to the pump at 25,000 times a second. When the fuel pump control module wants to increase the low side pres- sure it simply increases the on time on a duty cycle scale to the pump. At any engine load condition the frequency signal never changes, but the duty cycle signal will increase as engine load and demand for low side fuel pressure increases. e PCM reads the engine load and uses the bus circuit to communicate this duty cycle signal to the fuel pump control module. ese duty cycle signals can be seen from the scan tool. As engine load increases so, too, will this signal. In addition, on GM systems there are two new scan tool parameters that are valuable in detecting weak or bad fuel pumps. ey are known as short term fuel pump trim and long term fuel pump trim. If all is well on the low pressure supply side the values will be very close to 1. Having numbers above 1 means the fuel pump control module is increasing the duty cycle command to keep the low pressure read- ings within spec. Numbers below 1 indicate that the fuel pump control module is reducing the duty cycle signal again to keep low side fuel pres- sure within specs (see Figure 1). e Ford electronic returnless systems also use a fuel pump control module. e Ford PCM reads the engine load and com- municates a fuel pump control (FPC) signal in a duty cycle scale. For example, let's say at idle under no-load conditions, your scan tool indicates a 30% duty cycle command from the PCM to the fuel pump control module. e Ford fuel pump control module will internally double this value and turn the pump on 60% on and 40% off. e voltage to the pump from the fuel pump control module is sent at 9,500 times a second. Ford fuel pumps are either ground side controlled or feed side controlled by the fuel pump control module. Obviously, as engine load increases, the duty cycle signal is increased but the frequency signal remains constant. On both Ford and GM systems the fuel pump control modules have the respon- sibility of detecting electrical faults and relaying these to the PCM. e Ford systems use a dedicated fuel pump monitoring (FPM) circuit to communicate these faults. e GM systems will use the two wire high speed bus circuit for communication between the PCM and the fuel pump control module. e good news is that on these electronic returnless systems, there is a three-wire low pressure sensor that reports low side fuel supply to the fuel pump control module, meaning that we can read this value from the scan tool. Figure 1: Fuel trim. Having numbers above 1 means that the fuel pump control module is in- creasing the duty cycle command to keep the low pressure readings within spec. Numbers below 1 indicate that the fuel pump control module is reducing the duty cycle signal to keep low side fuel pressure within spec.

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