Component Functional Description And Diagnostic Overview - EVP Relay
Functional Operation of the EVP Relay: Depending on the type of relay being used for the EVP, 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 relay commanded off , battery voltage should be present at the EVP Relay Control circuit at the PDC output.
- No voltage present would indicate either an open in battery supply somewhere from the source, through the relay coil to the output of the Power Distribution Center (PDC) or the LSD control circuit is grounded and the relay is energized. To determine if the relay is stuck on measure the voltage on the Output circuit with the relay commanded off. If voltage is present at the output circuit, but not present at the control circuit, it would indicate the control circuit is being grounded. If no voltage is present at the output circuit it would indicate an issue in the supply circuit path.
- Voltage present would indicate that supply voltage is present. Back probing and jumping the control circuit to ground should cause the relay to close if the coil resistance is within specification and the proper battery voltage is being supplied. If the relay doesn't close it would indicate low voltage supply or high coil resistance in the relay. If the relay closes it would indicate an open in the control path.
Typical Low 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 a ground to operate the device when switched on. The ground 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. For diagnostic purposes the PCM uses an internal pull down diagnostic resistor connected in series and a voltage reference (V-Ref) comparator for fault detection:
- Circuit Open and Circuit Low Detection: The PCM monitors for an open circuit and short to ground when the driver is switched off. When switched off, the available voltage passes through the device and the internal pull down resistor connected in series. The voltage at the comparator circuit should be close to Battery voltage since the majority of the voltage drop occurs through the diagnostic resistor. If the available voltage is less than the V-Ref, a fault is set. In this scenario the V-Ref would be slightly below Battery voltage. An alternative method of fault detection for an open or short to ground 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. Excessive current draw detected would indicate a short to ground.
- Circuit High Detection:
The PCM monitors for a short to voltage
when the driver is switched on. When the driver is switched on providing a path to ground through the transistor, the available voltage should be pulled low, near zero volts since the comparator circuit
is monitoring the ground side of the device. If the voltage is greater than V-Ref, a fault is detected. In this scenario V-Ref would be slightly above zero volts.
A load that has a resistance that is below manufacturer specification, or a second load device shorted to the low side driver circuit can cause excessive current draw on the internal driver. The driver will be switched off to protect against overheating and damaging the driver. In this instance the Circuit High fault may be detected because the available voltage on the comparator circuit is above V-Ref.