Battery Voltage is monitored by the ECM for fluctuations. It will adjust the output functions to compensate for a lower (11.7v) and higher (14v) voltage value. For example, the ECM will:

Modify Pulse Width Duration of Fuel Injection
Modify Dwell Time of Ignition

Fig 1: Identifying E39 Power Supplies Circuit

The E39 Power Supplies (KL15 and ECM Relay) are fused to the MS41.1 ECM.

The fuses are housed in the Engine Fuse Block located in the Electronics Box.

When KL15 is switched "on" the ECM is ready for engine management. The ECM will activate ground to energize the Engine Control Module Relay. The Engine Control Module Relay supplies operating voltage to the ECM and the previously mentioned operating components.

When KL15 is switched "off" the ECM operating voltage is removed. The ECM will maintain a ground to the Engine Control Module Relay for a few seconds.

Ground is required to complete the current path through the ECM. The ECM also:

Fig 2: Identifying Engine Control Module Relay Circuit

Internally links a constant ground (1) to the engine sensors.
Switches ground (2) to activate components.

When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

• You smell fuel, burning insulation, or see smoke.

• Brakes feel soft, pull hard to one side, or make grinding noises.

• The engine overheats, stalls repeatedly, or misfires under load.

• You are missing required tools, torque specs, or safe lifting equipment.

• You are not confident in the next step or safety outcome.

Safety Warning

Principle Of Operation

Use the Manual With the Right Hardware

When to See a Mechanic

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