ECU connector grounds

You can use the ECU mounting studs for grounding, as long as they do NOT touch the ECU. This is outlined in the 915iS Installation Manual, found here: https://www.rotax-owner.com/en/support-topmenu/engine-manuals#current-engines

GROUNDING CABLES

The wiring of grounding cables is crucial when managing engine electronics.

Two circuits must be distinguished:

• Airframe circuit
• Engine management system circuit (EMS circuit)

During engine start (dynamic condition, transient), these two circuits are connected. However, when the engine is running or turned off (and the Fusebox is not supplied with external power), the EMS circuit is electrically isolated from the rest of the aircraft. Therefore, the following components must be installed without any contact with the airframe ground:

• ECU
• Regulator A (mounted on Fusebox)
• Starter relay

The engine block must be connected to the airframe with a properly sized ground strap (minimum the same cable cross-section as the starter supply, but at least 10 mm²) to carry the starter current and prevent static electricity between the engine and airframe. For full lightning strike compliance, the rubber shock mounts must be bridged using ground straps (minimum 10 mm²).

Further clarity on this topic can be found in the Rotax 912iS Installation Manual:

Installation Position

The ECU can be installed in the engine compartment, cockpit, or an area protected from fire and water/humidity.

• Isolated – No connection to the airframe or engine
• Cables or engine ground must not touch the ECU
• Maximum bending radius for the wiring harness: 50 mm (1.97 in.)
• Strain relief for the wiring harness: ~100 mm (3.94 in.) behind the ECU connector