Component Functional Description And Diagnostic Overview - Fuel Pump Relay
Functional Operation of the Fuel Pump Relay: Depending on the type of relay being used for the Fuel Pump Relay, the approximate resistance of the relay coil should be:
- 72-98 Ohms for a mini relay (larger square relay).
- 94-114 Ohms for a micro relay (smaller rectangular relay).
- 160-225 Ohms for a Printed Circuit Board (PCB) relay.
The coil resistance of a relay or solenoid can vary based on factors like temperature or age of the component. Typically a 10% variance is allowed on a new component at room temperature. Therefore, do not always assume that a component that is measuring slightly outside of the tolerance given is faulty. A faulty component usually will be noticeably out of tolerance.
Diagnostic Overview: With the ignition on and the PCM energized, battery voltage should be present at the Fuel Pump Relay Control circuit at the PDC.
- Ignition on, but no voltage would indicate a open or short to ground on the Fuel Pump Relay Control Circuit. Lack of voltage may also indicate the PCM High Side Driver (HSD) is not operable, but this is not common.
- Ignition on voltage present would indicate a functioning PCM HSD.
Typical High Side Driver Operation and Fault Detection: This type of driver circuit is generally used for relay control, solenoid control or a similar type of driver device. The PCM provides 12 volts to operate the device when switched on. The voltage could be constant or Pulse Width Modulated (PWM). The PCM also provides fault detection for the device, wiring and internal driver. Fault detection can be done by monitoring voltage on the circuit, current draw, or a combination of both. The graphic shows how the PCM performs diagnostics using an internal pull up diagnostic resistor and calibrated voltage reference (V-Ref) as a comparator for fault detection.
- Circuit Open and Circuit High Detection: The PCM monitors for an open circuit and short to voltage when the internal driver is switched off. A milliampere voltage is provided to the device through the internal pull up diagnostic resistor connected in series with the device. The diagnostic voltage is monitored between the pull up resistor and device, and compared to V-Ref. If the resistance in the device or circuitry becomes too large (approaches open) the voltage supply will increase on the comparator circuit and become greater than V-Ref, and a fault is detected. A short to Battery voltage will have the same effect. An alternative method of fault detection for an open that is used is to monitor current draw when the internal driver is switched on. If the module does not detect any current draw it determines that the component or circuitry is open.
- Circuit Low Detection:
The PCM monitors for a short to ground
when the driver is turned on. When switched on, the voltage to the device should be close to the 12 volt supply voltage. A short to ground will pull the voltage at the comparator circuit
below V-Ref and a circuit low fault is detected. An alternative method of fault detection for a short to ground
that is used is to monitor current draw when the internal driver is switched on. Excessive current draw detected would indicate a short to ground.