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. – Pᴀᴜʟsᴛᴇʀ2 Aug 25 '17 at 17:12
  • Battery Master... Main terminal on the battery or the fuse box... possibly... – Solar Mike Aug 25 '17 at 17:20
  • @SolarMike - Sounds reasonable? – Pᴀᴜʟsᴛᴇʀ2 Aug 25 '17 at 17:29

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.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.