Lane A/B Lights & Status

Thank you, Jeff, for your explanation.  

Regarding power / battery, I realized Rotax suggests a minimum 350 CCA battery.  My current battery in 270 CCA.  Regardless of whether it is battery or not, I've ordered a 405 CCA battery, as Rotax surely has a reason to recommend 350 CCA min.

I've tested continuity throughout the wiring affected by the 3 Relays.  All check out.  I have narrowed the problem (hopefully) to the connectors at the relay post position.  I've change out what I thought were the culprit connectors. However, the issue persists. I originally attached each wire separately to each of the 12 post on the 3 relays.  I am in process of installing a socket for each relay by connecting the 4 relay wires to each socket in hopes that I have a more consist and better connector configuration. 

Together with larger battery and installing sockets for relays, I will soon learn if this did the trick. I do like your suggestion of a 2-pole toggle switch. Since mine is Amateur Built, no issues with modification.

In my wiring configuration, the 3 relays are activated when pressing the starter button to provide temporary power (to Fuse Box), grounding (EMS to Airframe), and ECU HIC until the engine starts and takes over.  Regarding use of ECU backup, the backup switch is used only to run the fuel pumps to get fuel pressure up.  The backup is turned off before starting the engine. In the event of inflight engine shutdown due to fuel starvation, the backup is used to run the fuel pumps to build fuel pressure before attempting to restart engine inflight using the original start procedure.  Of course the backup can be used in situations such as mine, where the relays fail.  My preference is to stay on the ground until I get this figured out, and not to rely on backup start as my primary start procedure.

Thanks for taking the time to comment.

Drew

Drew,

if you look at the wiring diagrams in the 912iS installation manual there are no examples of relays being used in the start power circuit.  I’ve seen a wiring diagram for the RV12 912iS that does use a relay, but the relay is driven by a “start computer” rather than from the start position of the key switch.  This allows for specific timing of when the circuit is active.  I see two problems with the circuit you are using.

1. You should have stable fuel pressure before the engines turns over.  If you use the ECU backup switch to prime the fuel pump, then turn it back off, you don’t have any fuel pressure when you first engage the starter. It would come up quickly since it’s been primed, but that does not comply with the way Rotax describes the starting procedure. 

2. If you release the key switch from the start position too quickly after the engines fires the engine may stop running.  The start power circuit is intended to provide power to the ECU and other engine functions until the alternator is up to speed and stable, which may take a second or two after the apparent engine start.  

There seems to be lot of ideas by manufacturers to automate the start circuit and make the iS engine starting procedure the same as the carbureted engine.  However, in my opinion there are valid reasons for having the start power circuit manually controlled.

Thanks, Jeff.  In a previous reply above, I uploaded a copy of the recommended Sling2 wiring diagram for the 912iS.  As this is my first build and I am learning along the way, I may not have explained my situation properly.  If you are able to open the diagram file, perhaps you can compare my explanation to the diagram to see if I explained correctly.

Hi Drew,

My comments in the last two posts were based on the Sling 2 Diagram you posted.  I assumed that is how your aircraft was built.  For the reasons stated above, I don’t like the way they designed the start power relay circuit. But that’s just my opinion.  

You may have a problem with one of the start relays.  I know you said that it always  turns over and starts if you use the emergency battery switch, so that points me in the direction of a relay problem on the normal start circuit.  However, I don’t have an explanation for why the lane indications on your EFIS don’t go active when you use the emergency battery switch only.  After reading your post I tried that on my aircraft, specifically I activated the lanes with only the emergency battery switch and both lanes came on as well as the proper indications on the EFIS.