KL15 (Klemme 15)
What is KL15
KL15 (Klemme 15), also called Terminal 15, is the ignition-on power rail as defined by the DIN 72552 automotive wiring standard. When the ignition is switched on, whether by turning the key or pressing the start/stop button, KL15 becomes live and supplies voltage to control modules, sensors, and accessories that should only operate with the ignition on. KL30, by contrast, is permanent battery power that remains live at all times regardless of ignition state.
How it works in BMW systems
The "KL" designation is not BMW-specific. It comes from DIN 72552, the German standard used across all European automakers for labeling electrical terminals. KL stands for Klemme (terminal). The most common terminals referenced in BMW documentation are KL15 (ignition on), KL30 (permanent battery), KL31 (ground), KL50 (starter), and KL R (accessory).
On E-series BMWs, KL15 is controlled by the CAS module. When the driver turns the key or presses the start/stop button without holding the brake, the CAS activates KL15 and power flows to ignition-dependent modules. On later platforms BMW moved this responsibility to body-electronics modules: the FEM (Front Electronic Module) on many F-series vehicles, and the BDC (Body Domain Controller) on newer F-series and G-series cars. These modules switch the KL15 relay along with related supplies such as KL15N, KL30B, and KL30F. BMW has also extended the base DIN standard with sub-terminals. KL15N is a relay-switched variant that activates shortly after KL15 and gives modules a five-second after-run period to save data when the ignition is switched off. KL15WUP is a wake-up signal sent by the IBS (Intelligent Battery Sensor) when the battery's state of charge drops and the DME needs to wake briefly.
BMW fault code documentation frequently references KL15 as a monitoring condition. Many faults are only evaluated when KL15 voltage is within a specified range, typically 9–16 V. If KL15 voltage is low due to a weak battery, modules may not wake fully, leading to communication errors during diagnostic sessions with ISTA or INPA. This is why BMW service procedures specify a minimum battery voltage at KL30 before running diagnostics.