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. The communication faults are detected directly by the PCM and will typically set correctly. See Communication Diagnostics explanation below.

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. The pressure 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 400kPa (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 highest pressure reading and lowest 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 or ground 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 which causes the actual pressure to be higher than the desired set point. The entire system should be examined when this fault 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.

The FPCM fails the P064A  Fuel Pump Control Module Performance when excessive current draw is detected on the fuel pump circuitry. The FPCM will default and reduce the duty cycle to the fuel pump to the lowest possible duty cycle while the issue is present.

For most vehicles the fuel pressure desired and actual values, as well as the Fuel Pump output duty cycle data can be viewed on the scan tool. These readings can be very useful during the diagnosis of the system.

TECH TIP:  A vehicle will typically have one of two issues;

1. The vehicle has no fuel pressure/pump is not operating.
   • The first thing to check for is a Loss of Communication fault. The reason is that the same Battery supply and ground circuit that are used to power up the FPCM are also used for the Fuel Pump motor to operate. A loss of Battery supply, or a poor chassis ground for the module will cause both issues.
   • The next thing to look for is an output from the FPCM to the pump. This can be done with the scan tool or by checking the duty cycle output from the FPCM. If there is any output from the FPCM to the Fuel Pump, the pump should be operating and creating fuel pressure. If the pump is not running then the Fuel Pump and circuits between the FPCM and Fuel Pump should be focused on during diagnostics. If the Loss of Communication fault is not present the Battery supply and chassis ground are most likely good.
2. The vehicle runs normally, has a DTC or DTCs present, and the output duty cycle to the Fuel Pump is defaulted. In most cases, the output is defaulted to 80% and the actual pressure is greater than the desired pressure.
   • The first thing to check is for a Loss of Communication fault. If the vehicle is running with a Loss of Communication fault, and an 80% output duty cycle, it would typically point to an issue with the FPCM PWM Feedback circuit. A loss of Battery supply or a poor chassis ground would usually cause no Fuel Pump operation and a no start condition.
   • If there is no Loss of Communication fault then this condition is most likely present due to an issue with the command circuit, an over current draw or over temp condition with the Fuel Pump.