No Lane Lights, No ECU

Hi Rob,

The airframe power for starting the engine and running the fuel pumps comes from the start power switch or the backup power switch.  Both of these inputs are connected via X3 connector at the fusebox. The X3 connector only has 3 wire terminals.

Terminal 1 = 12+ volt input to fusebox from the back up power switch.

Terminal 2 = 12+ volt input to fusebox from the start power switch. 

Terminal 3 = 12+ volt charging output from fusebox to the airframe power bus.

You also need the B regulator ground plate grounded to the airframe ground buss.  The A regulator ground plate is isolated from the regulator B ground plate and these are only connected together when the start power or backup power switches are closed.  See attached diagram from section 92 of the 915 installation manual.  

The start power switch and backup power switch do similar things, both provide airframe power to the fusebox but are intended for different purposes.  You could use either to start the engine but normal starting should be done with the start power switch, which should be the momentary type so the switch is not left on after the engine starts.  Once the engine is started airframe power is disconnected from the fusebox by releasing the start power switch and the engine systems are then electrically isolated from the airframe and running on alternator A. 

I believe I have seen this information before, and recall during the installation following similar steps.  I will check again tomorrow to ensure it is as you have indicated.  

Thanks!

Rob,

I made a couple edits to my last post to make it more clear…. I hope. 

Thank you, Jeff. Much appreciated.  

I will ask AI to help me put together a troubleshooting process for these points.  I believe I should start with looking for power to the backup switch first and work backwards from there.  But we'll see...

Here are a few basic steps to get you started.

With an ohm meter, check resistance between the regulator B ground plate and the airframe.  Resistance should be low indicating a good connection between them.

With the backup power switch and start switches off (open) test between the regulator B ground plate and the regulator A ground plate.  Resistance should be infinite indicating no connection. This is important to make sure the airframe and engine ground systems are isolated.

Turn on the backup power switch and again test resistance between the regulator B ground plate and the regulator A ground plate.  This time resistance should be low indicating a good connection between the two plates.  If this works, open the backup power switch and close the start power switch and test again, the result should be the same. In other words, either the backup power switch or start switch should connect the two ground plates together.  If all of the above works, your grounding is correct. 

Next, unplug the X3 connector from the Rotax fusebox. Set your meter to measure 12v DC and touch the black lead to the regulator B  ground plate and the red to the #3 terminal in the X3 connector on the wiring harness (not the fusebox) There should be no voltage present.  Now turn on the master switch and backup power switch and test again and you should have 12 volts nominal.

Next, open the backup power switch and close the start power switch. Test for 12+ volts between the X3 #2 terminal and the regulator B ground plate.  You should see 12 volts nominal.

If all these steps test correctly you should have power at the Rotax fusebox when the backup power switch or start power switch is closed.  If not, work your way back to find where power is interrupted.