Theory Of Operation
The Engine Stop/Start (ESS) system utilizes two Starter Relays to operate the Engine Starter Motor. The Body Control Module (BCM) controls one relay with a Low Side Driver (LSD). When the BCM controlled relay is energized, it provides Ground to the other Starter Relay controlled by the Powertrain Control Module (PCM). The PCM controls engine cranking by energizing the second Starter Relay. The Starter Motor can be disengaged by de-energizing either relay. The relay names and DTC descriptions may not match and can be confusing. In the PCM diagnostics, the relay controlled by the PCM is always considered starter relay 1 and the BCM controlled relay is considered starter relay 2.
The control circuits for both relays are monitored for circuit faults. The PCM also monitors and performs diagnostics on the output to the Starter Motor on the Ignition Crank Sense circuit. In addition to the control circuit diagnostics for each relay, the PCM also performs diagnostics to detect if one of the relays is stuck in the closed position.
- Ignition Crank Sense Diagnostics: The PCM monitors the Ignition Crank Sense circuit voltage when engine cranking is commanded. The Ignition Crank Sense circuit is connected to the Starter Solenoid Feed circuit inside the PDC. During an engine cranking event the PCM expects to see Battery voltage at the Ignition Crank Sense circuit. If no voltage is detected a fault is set.
- Stuck Relay Diagnostic: After the engine is initially started, the Powertrain Control Module (PCM) will command one of the Starter Relays closed and the other Starter Relay open and monitor the Ignition Crank Sense circuit for voltage. It will then open the closed relay and close the open relay and monitor the Ignition Crank Sense circuit for voltage. If there is voltage present on the feedback circuit during either of these tests, then the relay that was commanded off when voltage was detected is determined to be stuck closed and a performance fault is set against that relay.
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.
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.