Toyota Prius (3rd generation) system main relay operation



The information on this page is illustrative. It is not to be used for training purposes or as guidance or instruction. It is also incomplete. A full version of this content can be found in our PicoScope 7 Automotive software, which is downloadable from here.



The purpose of the test is to check the operation of the System Main Relay (SMR) within a third-generation Toyota Prius hybrid vehicle.

Illustrative System Main Relay (SMR) circuit diagram.

Illustrative System Main Relay (SMR) circuit diagram.

Further information

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:

  1. Initially, all the relays are open and there are no paths to the negative and positive HV sources from the respective SMR outputs.
  2. 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.
  3. 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).
  4. 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).
  5. 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.
  6. 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.
  7. After the startup process, the inverter is directly connected to the HV battery.

The SMR shutdown process is as follows:

  1. The HV negative post-charge relay is opened to prevent further current flow around the HV circuit.
  2. 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).
  3. The HV positive control relay is opened to totally isolate the inverter from the HV battery.
  4. 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.
  5. Eventually, all of the relays are open and there are neither positive or negative HV voltages at the respective SMR outputs.
  6. 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


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.

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1 comment | Add comment

Lwanga karim
February 18 2022

I have a Toyota Alphard hybrid and it reads power resources p3004 it doesn’t start and the system main relay doesn’t allow power to pass the positive relay doesn’t allow power to go through the inverter the 220 DC voltage stops on positive main relay

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Guided test: Toyota Prius (III) system main relay