I'm reading in my car's repair manual that I need to check "power supply voltage (+BM)". It also uses the phrase: "power supply circuit (+BM)". What is BM voltage?

  • What is the year/make/model of the car? Needed for context. Aug 25, 2017 at 17:12
  • Battery Master... Main terminal on the battery or the fuse box... possibly...
    – Solar Mike
    Aug 25, 2017 at 17:20
  • @SolarMike - Sounds reasonable? Aug 25, 2017 at 17:29

1 Answer 1


https://en.wikipedia.org/wiki/CAN_bus#CAN_bus_monitor Control Area Network Bus (CanBus) is the data network protocol on modern cars that controls engine, transmission and other vehicle functions.

+BM is the positive supply reference input for a bus module on the CAN

That voltage must be within certain limits or there is a fault elsewhere in the system. The input doesn't have to be 12v, but it could be and probably is. One of the terminals on the module that set a fault is labelled +BM. You have been directed to put a high impedance volt meter on that terminal and verify that it is within limits.

https://www.toyotaguru.us/prius-2003-repair/check-voltage-between-terminal-bm-of-ecm-connector-and-body-ground.html This page shows which terminal the +BM is on a Prius Engine Control Module (ECM)

http://www.tundratalk.net/forums/tundra-problems-maintenance-repair-dealer-warranty-issues/205041-2003-p2118-after-dead-battery.html In this example the +BM voltage can be as low a 4v before a fault is set. This Tundra is fly-by-wire, it uses an elctronic motor instead of a cable to control the throttle position.

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