I searched, but didn't see an answer to this. I understand Lane B must be on to monitor the OP on the Garmin EFIS during the Rotax oil purge procedure (sensor connects to B). This would seem to also have ignition B on, firing half of the plugs when the prop is spun. Because either the bottom or top plugs are removed from the cylinder, should those plug leads be grounded, left open, or left hanging with the plugs inserted and threads grounded so they can fire? Youtube videos seem to assume a manual OP gauge, so this isn't addressed. I'm not sure if the Rotax ignition modules care if they are open circuit or grounded, and exactly which plugs are fired by Lane B.
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Monitoring Oil Pressure with EFIS during Purge Procedure
- Thread starter BobN54
- Start date
rcarsey
Well Known Member
The coil fires both of it's attached spark plugs simultaneously.. I think there is less risk of the coil burning out than a coil designed to connect to only one plug—where the high voltage has nowhere to go and may start arcing inside the coil. That said, these coils are priced like they have 24k gold inside them.. so it’s probably worth the price of using some safety wire to ground the pulled out plug.
Also, you could leave the ignition off and hook up a volt meter to read the oil pressure sensor.. you’d have to know some specs on the sensor.. max psi.. then do some math to find out what voltage you’d see at some target psi you’re looking for.
Also, you could leave the ignition off and hook up a volt meter to read the oil pressure sensor.. you’d have to know some specs on the sensor.. max psi.. then do some math to find out what voltage you’d see at some target psi you’re looking for.
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Important to note that wasted spark (dual terminal) coils do not spark to ground. They form a circuit, energy leaves one terminal, jumps across both plugs and grounds or returns via the other terminal. Engine ground is used only to complete the circuit between the two plugs. If only one plug from the coil is reoved, then The other can be grounded.
They won’t arc internally if there is nothing connected at the terminals, though can overheat if done for too long. Minutes not seconds.
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rcarsey
Well Known Member
Ahh ok so if one plug is not hooked up.. the other isn’t going to fire.. yeaa don’t do this then!!
True for std dual terminal coil, which I assume is the case but know nothing about rotax.
However if you put the plug back in the wire and ground the removed plug back to the case, then both plugs will fire as you have restored the path.
Remember also the 12is Rotax ECU won't fire the plugs until a minimum RPM is obtained which isn't reached by turning prop by hand. One of the reasons starter is to be engaged until engine RPM reaches a minimum speed (1500 rpm) not to mention prevents sprag damage. (see OM_912 i_ED2_REV2 Sept 2025) It does not act like conventional mag which fires with every turn of prop through ignition stroke. I suggest following the instructions in SI-916i-003R1_915i-003R2_912i-004R3_912-018R4_914-020R4.pdf which I've attached and you'll have no problems. There are a couple of videos on the web that are not entirely accurate. The best is the Rotax BRP video but it references the SI for definitive step by step instructions. Do not use starter to turn the engine over during purge procedure!
Yes, I am using the published Service Instructions and Maintenance Manual. However, the Rotax procedure does not account for the scenario of an EFIS connected OP gauge. The procedure doesn't even call for the master or a lane switch to be on at all. They assume a mechanical OP gauge. That's the point of my original question.
WJaviation
Well Known Member
If the ECU checks "bad", red warning lamps remain on, which in the case of only a single lane being activated (lane B for oil PX monitoring, Lane A off), I don't believe that the ECU would fire any spark down either plugs upon hand rotation. I cant possibly imagine that Rotax would "overlook" digital engine monitor vs mechanical gauges when running this procedure. At the very least it would have a Note, Warning, or Caution regarding spark plug condition with Lane B engaged.
I think the hand propping spark grounding issue is a ULS issue/consideration.
I think the hand propping spark grounding issue is a ULS issue/consideration.
Bob - As docjjrmd said, the 912iS won’t fire the plugs until the engine reaches something like 400 rpm. I did the purge before first engine start a couple of months ago, with no issues (Garmin EFIS). Just turn on the master and the Lane B switches. You have to watch the OP gauge pretty closely, as the pressure drops to zero within about 10 seconds, but as long as it registers on the gauge you’re good to go.
