Diagnostic Overview-Fuel Pump Control Module
GENERAL DIAGNOSTIC INFORMATION: There are several faults associated with this system. The failure modes being monitored for are Loss of Communication, internal FPCM faults, system voltage faults, Thermal Protection, circuit faults, and system performance. On several vehicles, multiple or incorrect faults can set which can mislead diagnosing the vehicle. For this reason, it is a good practice to perform a total system diagnosis regardless of the DTC present for most faults.
PCM DIAGNOSTICS: The PCM directly monitors the Fuel Pump Relay Control circuit, FPCM Feedback circuit and the Fuel Pressure Sensor circuits for faults. The Fuel Pressure Sensor signal must be rationalized since the signal is monitored to perform fuel system pressure performance diagnostics. This is done by watching the signal at initial engine start up for both types of systems. On the fixed pressure system the normally stable fuel pressure is dynamic for the first few seconds. It will spike above the set point approximately 40-50 kPa (5.8-7.25 psi), then drop below the set point approximately 60-70 kPa (8.7-10.2 psi), and quickly dampen and settle at the set point 400 kPa (58.0 psi). At initial engine start up, when the engine speed reaches approximately 500 RPM, the Powertrain Control Module (PCM) measures the delta pressure change between the high pressure reading and low pressure reading and make determinations about sensor rationality. The diagnostic works the same on the variable pressure system as the fixed pressure system except that the system is in open loop at start up and the pressure is commanded to a preset known value to perform the diagnostic.
SYSTEM VOLTAGE DIAGNOSTICS: The FPCM monitors the Battery voltage supply coming from the Fuel Pump Relay to the module. A fault will set if the voltage is too high or too low. Typically a voltage high fault would indicate a charging system issue. The voltage low fault could also be a charging system issue, but may also be caused by resistance in the Battery supply circuit.
COMMUNICATION DIAGNOSTICS: Unlike other system faults, the Loss of Communication fault will typically set correctly. The PCM will set this fault when the FPCM PWM Feedback duty cycle is below 3% or above 97%. This can occur if the FPCM Battery voltage supply or ground circuits are open, or the FPCM PWM Feedback circuit is open or shorted.
FUEL PUMP PERFORMANCE DIAGNOSTICS: The PCM fails the P062A Fuel System Performance diagnostic when the actual fuel pressure is approximately 100 kPa (14.5 psi) below or 40 kPa (5.8 psi) above the desired fuel pressure. This fault usually sets along with other faults since the FPCM will default to 80% duty cycle for many failure modes. The entire system should be examined when this faults is set, including the Fuel Pressure Sensor and circuits. A sensor drifted out of range or high resistance in the signal or sensor ground circuit can cause a false reading on a normally operating system.
- The dual pump variable pressure systems can be tricky when diagnosing a performance fault. The fuel pressure may be good with one failed pump during most normal operating conditions. The condition may only be present under very heavy engine load conditions when both pumps are needed to produce the pressure and volume needed. If available on the scan tool, monitoring duty cycle for both pumps can help diagnosing the issue. If a failure is detected for either Fuel Pump or circuitry, the duty cycle will typically default to 80%. If Fuel Pump 1 (primary) is defaulted to 80%, Fuel Pump 2 may remain off during most operating conditions. However, if Fuel Pump 2 is defaulted to 80%, the primary pump will continue to operate at the lower duty cycle range. The reason for this is that the primary pump operates the jet pump to pull fuel from the secondary side of the Fuel Tank and also fills the Fuel Pump reservoir.