Toyota Prius (III) system main relay
The purpose of this test is to check the operation of the System Main Relay (SMR) within a third-generation Toyota Prius hybrid vehicle.
High Voltage (HV) electrical systems have high voltage cabling, stored electrical energy, retained hazardous voltages, and the potential to affect medical devices.
Risk of damage to equipment and property, risk of electric shock, personal injury, or death, and risk of explosion and fire.
- Read carefully and understand the hazards and risks described above.
- Only work on HV systems if you are a qualified technician, have received appropriate type-specific training from a qualified electrician, and hold valid accreditation.
- Do not work with HV systems if you have been fitted with a pacemaker or other electronic medical devices.
- Always refer to the vehicle manufacturer’s information sources to identify the actions you must take to prevent danger.
- Use equipment and accessories in accordance with their safety guides.
- Always wear appropriate Personal Protective Equipment.
How to perform the test
- Connect a test lead to Channel A and the SMR HV positive control circuit (labelled SMRB), with its earth fly lead connected to chassis ground.
- Connect a test lead to Channel B and the SMR HV negative pre-charge control circuit (labelled SMRP), with its earth fly lead connected to chassis ground.
- Connect a test lead to Channel C and the SMR HV negative post-charge control circuit (labelled SMRG), with its earth fly lead connected to chassis ground.
- Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
- Start the scope.
- Put the Toyota Prius vehicle into its READY (on) mode.
- With your waveforms on screen stop the scope.
- Use the Waveform Buffer, Zoom and Measurements tools to examine your waveforms.
READY off to READY on
Further guidance
The HV Control Module (HVCM) uses the SMR to control the HV connections between the HV battery and the motor inverter. The HVCM monitors the HV current flow to ensure the HV inverter supply is correctly established, maintained, and shut down. This allows:
- Regulated charging (during start-up) and discharging (during shut down) of the inverter capacitors.
- Direct HV circuit connections between the inverter and HV battery after startup.
- Diagnostic feedback to the HVCM of any potential HV circuit faults.
The SMR startup process is as follows:
- Initially, all the relays are open and there are no paths to the negative and positive HV sources from the respective SMR outputs.
- The HV positive control relay is closed to create a direct path from the HV positive input to the SMR positive output. As the HV negative pre-charge and post-charge relays (SMRP and SMRG) are initially open, there are no paths from the HV negative input to the SMR negative output and there should be no current within the HV circuit.
- The HVCM closes the HV negative pre-charge relay if there is no current through the HV positive circuit (which, otherwise, would indicate a circuit fault).
- Current flows and charges the inverter capacitors via a resistor. The resistor limits the current and charging rate (and also prevents arcing across the relay contacts).
- The HV circuit current decreases toward zero (the SMR output voltage will increase toward the nominal HV battery voltage) as the capacitors become fully charged.
- When the HVCM detects that the HV circuit current is close to zero, it opens the HV negative pre-charge relay and closes the HV negative post-charge relay. This creates a direct path from the HV negative input through to the SMR negative output.
- After the startup process, the inverter is directly connected to the HV battery.
The SMR shutdown process is as follows:
- The HV negative post-charge relay is opened to prevent further current flow around the HV circuit.
- The HVCM carries out an active discharge process by discharging the inverter capacitors through the motor windings (the motor acts as a load to regulate the discharge).
- The HV positive control relay is opened to totally isolate the inverter from the HV battery.
- The HV negative pre-charge relay is momentarily closed and opened. If current flows whilst the relay is open, an HV positive voltage must be present, indicating a fault in the HV circuit.
- Eventually, all of the relays are open and there are neither positive or negative HV voltages at the respective SMR outputs.
- After the shutdown process, the inverter is totally isolated from the HV battery.
Potential faults
- Possible HV circuit shorts to earth
- Seized (welded) relay contacts
Potential symptoms
- Vehicle not entering its READY mode
- Diagnostic Trouble Codes (DTCs)
- Dashboard warning lamp illumination
Disclaimer
This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.
