What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Bad "B" Ignition on Rotax 912ULS - Stranded KDLN - Any advice?

mwardle7

Well Known Member
Greetings:

On a flight from Salt Lake City to Ryan Field near West Glacier Montana, I stopped for fuel in Dillon, MT (KDLN). After fueling, I performed a runup. When I flipped the switch off for Ignition "A" the engine quit. The engine will only run if lane A is on. The switch for "B" does nothing, regardless whether it is on or off (although the green light on the switch is on when on and off when off).

I taxied back to the ramp. Fortunately I was able to get a ride to West Glacier, but the airplane is stranded. I either need to find a way to fix it in Dillon, or trailer it back to Salt Lake.

I tried the following with the limited tools I had:

  1. Swapped ignition modules. I ran the A1 and A2 connectors in to ignition module "B" and the B1 and B2 connectors into ignition module "A." It made no difference. I could run the engine with switch "A" on (but it died when switch "A" was turned off) and if I switched "B" on and off, it did nothing.
  2. I checked the continuity on the input side of the modules (connectors A1 and B1) for the white wires coming from the switches. Both behaved properly - with the ignition switches off there was continuity to ground, but when the switches were on there was not.
  3. I checked the ignition ground connections to the two bolts on the right intake manifold, including the jumper wire to the clamp on the aft bolt of the ignition module mount. All wires looked fine. I inspected all of the wires at the harness plug connectors and everything looked good.
  4. I did not test the trigger coils because I thought it unlikely that I'd lose two at the same time.
  5. I turned on the choke running at low RPM and 4000 RPM and the problem was the same with the choke on and off. I think it unlikely that it is a carb problem.
  6. After I gave up testing, I ran the engine on switch "A" to taxi the airplane to the tie down area. Lane "B" came partially alive. When I switched off switch "B" the engine ran really rough at low idle. I didn't feel comfortable running it that rough at high idle, so I turned off the engine.

I spoke to a friend of mine who had a similar problem on his RV-12 in Southern Utah. He had both modules die within hours of each other. I'm sure glad I didn't try and fly on one ignition! :eek: A gentleman camped at Ryan field said he had a friend who had to replace both ignition modules on a 912 as well. Apparently, they are a private label Ducati product, and when they get hot, they can die. Sounds eerily similar to our voltage regulator ...

I'll be driving back through Dillon later in the week. I need to decide if I'm going to trailer it, or try and fix it in place. Any advice? If you've had ignition problems on a Rotax 912ULS, please chime in!!!

Thank you so much!
 
I had one of the two ignition switches fail internally at @ 500 hours -- it was an intermittent failure. Replaced the failed switch with new and all is well.

Alternatively, there is much discussion on the Rotax Owner site on using an ice bag or heat to troubleshoot older-style ignition modules.
 
Last edited:
The ignition coils and CDI units are mounted on top of the engine which is tightly cowled on the RV-12. I open the oil access door when the airplane is parked to let the heat percolate out.



 
On a similar occasion, I found that the shielded cable going to the cdI units where the cable shelding was bared and clamped to ground was loose and had worn through the insulation of the internal wires and also grounded them, causing the failed B circuit. A quick wire replacement got me going again.
 
I had a 912 on a different aircraft that lost one ignition and it turned out to be a broken wire going to the CDI boxes. It was broken inside the insulation from vibration/movement and I only found it by pulling on the individual wires going to the connector. The wire would “stretch” because the actual wire was broken and the insulation was stretching. Repaired that wire & it was running great when I sold it. Long shot I know,just a FYI.
 
Getting warmer. I switched the red wires from the generator in the ignition harness from the "A" to "B" plugs and vice-versa. These wires power the ignition modules. The problem moved to lane "A." It's either a wiring issue to the generator, a bad ground in the generator, or a bad stator. Hoping it's not a bad stator $$$. Still more diagnosis to do. I'm going to check the resistance of both red wires going into the harness plug to ground, wiggle the wires, and see if the resistance changes. That test may reveal a broken red wire in the insulation. If that checks out, I'll check the stator grounds, which are here (Photo provided by Brett L. at Leading Edge Airfoils):

Wejk3_FGmduQ1L7TAjBZHpT0YNO9feCNa52qkwSo_9lYDYc1WF3lBUF1itj8HTjgjGAGgPYVRfETwk36tHuuoIAr6Mt98p1Xh9GwvbERwk2PCHu1cc0BOxt-FuwW2MCrg4PZ57unDw=w600-h315-p-k


Given how tight it is inside the cowling, I've ordered an inspection camera so I can see the stator grounds.

As a side note, I disconnected the pins for from the cockpit ignition switches, inserted two 18 ga wires stripped on both ends where each pin was removed. Then I ran the two wires into the cockpit. I started the engine and separately touched each wire on a ground in the cockpit. It was a makeshift ignition switch system that bypassed the cockpit switch system. It didn't make a difference in the behavior of the ignition, but I ruled out a bad switch or a bad ground on the ignition wires that the KAI has you ground on the bare cable clamp on the aft ignition module bolt. Note to self - get rid of the clamp and use a solder sleeve to ground the ignition wire shielding at some point in the future. The design on the clamp ground seems odd to me.
 
Ign problem

Don't ask why I know this. Look toward rear of ign box mounting on the under side. There is a small spacer tube to hold up the rear of the boxes. Is it in place ?
 
Don't ask why I know this. Look toward rear of ign box mounting on the under side. There is a small spacer tube to hold up the rear of the boxes. Is it in place ?

I'll check. Does the spacer affect ignition functionality? Can you point it out on the parts diagram?

_6meTp29O1KQDDpVdV7wcUUohsQL4W6sdTdxkaT4HEpr09YJakY5o8Mm9L8WERLw-MomzsEHP7DpYO1m1YK4R9tPj8fcY5CvI5RgJrm9YS7BWWO_R4sAEMe5QR8NGzpxukDGUxiobQ=w2400


I'm back home in Salt Lake and will be driving back to KDLN to trailer the airplane home tomorrow. :(
 
Last edited:
I can't speak to John's "small spacer tube" issue but a number of us have found the aluminum bracket (#14) to be cracked through -- likely due to vibration, misalignment, etc. I suspect that such a broken bracket would not have an impact on your problem but it is good to look for nonetheless.
 
I can't speak to John's "small spacer tube" issue but a number of us have found the aluminum bracket (#14) to be cracked through -- likely due to vibration, misalignment, etc. I suspect that such a broken bracket would not have an impact on your problem but it is good to look for nonetheless.

My RV-12 broke bracket #33 in the schematic. Either of them can break, if the engine vibrations are bad. Royal PIA to change out. $12 part at Aircraft Spruce.

Balance carbs, balance prop, and make sure pitch of both blades of the prop are in spec, also.
 
Missing spacer

Here is more of my story. When the rear of the ign box assembly drops down, one of the coils rest on the engine mount. It can wear thru the plastic coil covering and start to short out that coil. Gives intermittent ign firing for that coil. Just a maybe. Another issue to look for. ( reference #43 in parts pic above post ).
 
Last edited:
I found my #14 bracket cracked after reading about it on the forum. I don’t know how long it was broken, but the bracket had worn away at the bolt head after it broke loose. I replaced mine with a home made one that has slots instead of holes to eliminate stress from minor misalignment. After several years it is still intact.
 
Aircraft has successfully been trailered home. Did a "wire wiggle" while testing resistance on both red wires from the stator; no changes in resistance observed.

In order to get to the stator grounds, I'm going to have to take the engine off the mounts. It will probably be a week or two before I get to it, but I'll post the results when I have them.

So grateful for everyone's suggestions!
 
Looking at the Heavy Maintenance Manual, on page 65 there is wiring diagram. Reference
number 8 is labeled "plug receptacle 2-pole". That connector could be a potential
trouble spot. Have you measured the resistance from that connector to ground?
On page 64, it says the resistance to ground at that connector should be between
3.2 ohms and 4.5 ohms. How many ohms did you measure?
 
Found another more recent heavy maintenance manual. See section 74-00-00 page 25. (page 279 out of 470)
It does not show the single pole connector. If there is no convenient place to measure voltage or resistance,
I have attached a voltmeter probe to an xacto knife and shoved the sharp blade through the wire insulation,
causing minimal damage to the wire or insulation.
 
I pulled the stator and it is being tested. I'll post once I have the issue resolved.

By the way, I did test all of the resistance values in the Heavy Maintenance Manual and they were all within spec.
 
Mwardle7,

What was the end result of your ignition issue?

I'm happy to report that the airplane is flying again. It was a bad stator. Unfortunately, it was a saga to get to the final diagnosis.

After speaking with a Rotax technician about the possibility of a failed stator, I was advised to ship it to the Rotax shop to be bench tested. Before I sent it off to be tested, I checked all resistance values per the Heavy Maintenance Manual. Everything checked out within spec. I was frustrated about the prospect of sending a good stator off to be tested, but having no other ideas I loosened the engine from the mounts while suspended from a hoist, rented a flywheel puller and "protection mushroom", and removed the stator. I checked all resistance values again and everything looked good. I also looked for any signs of wire breakage, bad crimps, etc. I didn't notice anything amiss.

I sent the stator off to the Rotax shop (which will remain anyonomous). It was tested and returned as being fine. So, I reinstalled the stator, reattached the engine to the mounts, and did a runup. I had the same. exact. problem. The engine would only run on Ignition A.

Befuddled, I called another Rotax shop; this time I tried one in Florida .... They said to replace the stator. The tech explained that you can get resistance values within spec, but when the wires are put under load (more than the minuscule load from a multimeter) they can fail.

I really didn't want to replace a stator that I could not diagnose as being bad. So, I tried my own experiment. Don't try this at home, or if you do, do so at your own risk. It isn't part of the Rotax manual procedures or anything recommended by a Rotax technician. Here's the do-at-your-own-risk-and-safety procedure: I put the probe of the red lead on my voltmeter into the ignition module connector where each red charging cable plugs into the module so has to check voltage from each red charging cable going into the ignition module, while it was under load, which is important because otherwise you can get a deceptive reading because the wire potentially isn't carrying all of the current if it isn't under load. I grounded the black lead to the intake manifold. I put my multimeter set to AC (not DC) voltage, because the flywheel magneto stator generates AC voltage. I then ran the engine, shutting down to switch leads from the A side to the B side. The A side showed just under 30 volts. The B side bounced between 11-15 volts. Bingo! Problem diagnosed. Please note that I am told that after after going through the charge coils, the ignition system can create 300+ AC volts, which is deadly. Be careful. Again, try this procedure, if you must, at your own risk.

Now that I knew the stator was bad, I ordered a new one. This time I attempted removing the stator without loosening the engine from the mounts. It worked. It's really tight, and you have to rent the "short" flywheel puller. But it's doable if you have a variety of socket extensions, universal joints, good lighting, a bucket full of patience, and a smart, lovely wife with small hands.

After installing the new stator, everything worked perfectly.

I still don't know if I had a wiring issue or a stator issue. The new stator came with new leads, so I'll never know. I'm grateful to have the problem behind me and for all that I learned along the way. I hope that this can be helpful to someone in the future.
 
Back
Top