KL30 (Klemme 30)

KL30 (Klemme 30), also called Terminal 30 or B+, is the permanent battery positive power rail as defined by the DIN 72552 automotive wiring standard. Unlike KL15, which only becomes live when the ignition is switched on, KL30 supplies voltage directly from the battery at all times — even with the key removed and the vehicle locked.

While the base KL30 rail is always live, BMW does not leave all modules powered permanently. The power management system uses relay-switched sub-terminals to control which modules receive power and for how long after the ignition is turned off, protecting the battery from excessive drain.

On E-series BMWs, this is handled by two relays in the power distribution boxes. KL30g is the time-controlled shutdown relay, managed by the CAS module. It powers modules like the CID, controller, sunroof, IHKA, DSC, EGS, ACC, HUD, and PDC, then switches them off roughly 30–60 minutes after the ignition is turned off. KL30g_f is the fault-dependent shutdown relay — a bistable relay managed by the JBE (junction box). It is normally on and only cuts power in fault conditions such as unauthorized bus wake-ups, the battery reaching the starting capability limit, or closed-circuit current violations.

On F-series vehicles (CAS4 / Boardnet 2020 onward), these were renamed: KL30g became KL30B, and KL30g_f became KL30F (the "F" stands for "Fault"). The FEM (Front Electronic Module) controls the relays for KL30B and KL30F on these platforms. The function is the same — KL30B is time-controlled, KL30F is fault-dependent — but the naming and controlling module changed.

Certain components remain on pure unswitched KL30 permanently, including the CAS module itself, the steering angle sensor (which must detect steering wheel movement even with the ignition off), and the alarm system (DWA). The Battery Safety Terminal (BST), mounted on the positive battery cable, can disconnect the B+ feed to the starter and alternator in the event of a crash, while keeping power flowing through the rear power distribution box so that hazard lights, power locks, and other post-crash systems remain operational.

In BMW fault code documentation, KL30 voltage is the system voltage that ISTA reports when reading modules. A low KL30 reading during diagnostics — typically anything below 12.0 V — can prevent modules from waking or communicating properly, which is why BMW service procedures require verifying battery voltage before programming or coding.