What's new
Van's Air Force

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

Inflight spark plug wire failure (Lightspeed/automotive style)

Paul Eastham

Well Known Member
Thought others might be interested in this recent maintenance issue, didn't see anything like it in the archives.

Was on a short fuel run a few days back when I noticed something painted yellow on my Dynon. Cylinder #4 EGT was hot, 1500 degrees versus the usual 1200. Everything else seemed normal. Was almost home so didn't debug further, despite Mike Busch's admonitions...

On the ground, a mag check showed rough running on the right (Lightspeed ignition) and cylinder #4 going cold. Easy diagnosis, no spark, right?

On disassembly, spark plug tested good and wire tested good. Upstream of that is the coil, but the other cylinder that's fed from the same coil was firing normally, so the coil seems good...gotta be connectors I figured. After a bit of wiggling the wire around at different angles I saw the resistance spike and eventually cut out entirely.

I put in the order to Amazon for new cable and connectors and crimp dies -- and while waiting for that to arrive cut open the bad cable. The crimp doesn't seem to have fully captured the black/white conductor (see photo - barely), and when I poked around in the crimp a bit with my multimeter probe, a piece of thin wire broke off and fell out. Turns out these cables use a wound thin wire instead of a central conductor, and I guess it failed.

Interestingly, in the lightspeed manual, it says replace these cables every 10 years. Time on this cable: 11 years! FWIW, the cable was secured at two spaces along the engine pushrod, though I think some vibration probably happens. You'd think the larger crimp would protect this area ... maybe the cable was just defective from the start?

kJAQr5lv5uqXaxUEQMmRhcG0fOL2eENeNODeG6jNRC_z8AtjvJr6OAcWRJQ21wwkZOdynCsKslVOONxkNBeTgw7EulkJ53aZSqjU2HJYsu1HjP1OCMFZbpFbsKH056ABCNvsChtZHGHV7Hb5qUgETc7jf9bsNEVqHERRzxYTpiea99l9IlBZCXN4_J5J9NelQ3strbelPpfEsnFrhgysLRazzShobARkyA79epIsK_i6_Jj_8OAAk8I-3DSPq1o6S65sW1mN3T61Us9YGh8e9EVkF9H52sCVjt4sd_vpS4tT47gMAQYZ69rFSW8q_z7WYOvPrnfe5PFabZutARWJyFZ9zP9SrKclRQbE7GrxXPUiu1DFqpJ2owJWY00Kikl8Xk_nYKSG-h_GFUU0ai5IIgDVKs4nCOzqKD2MkAwmFJZyb3ayGU3B3tReakrEG2DHiQj6r9dHLEjtF25jGKdM6-v4g8xrkZ67hMsZidEgNn5wnrepaYi4CKKeYESe7fQtF4BeAspQhfANbafTaOayAaiLAyL_CqPndm-Zssvv12XJZxCjyrDovEe8jMjBslc4fGwRx97wnWwKbtfWQBbVxHzvBR744dJqbuz5Q3IvwdLI5m7EJ-TnDzVXPyprtRqvMmZh8KNbGYHemZydX2fCfq9_iPz53pB3Vw=w545-h727-no
 
Last edited:
...looking a bit more in the archives, looks like a coil can fail for only one cylinder. So perhaps my reasoning is not quite right above, but hopefully reached the right conclusion anyway :)
 
Thanks for the heads up Paul. I ordered a new coil this week to replace the one connected to a wire that fell off recently, per the manual. I did not order a new set of wires, they are 14 years old but did not show any issues when I put a meter on them. Based on your experience I will replace the entire set.
 
I have made many sets of wires between hot rods, boats and airplanes and have found that some wire stock is more forgiving of assembly errors than others. Generally, the center conductor is folded back along the outside of the insulator jacket and then captured there by the closed back of the terminal (opposite the crimp). In some cases, the slightest nick of the center conductor will create enough of a stress riser to break outright when the conductor is folded back during the assembly/crimp process. In any case, it looks like yours was hastily assembled because only half of the crimp has captured the outer jacket. I'm suprised it lasted this long.
 
Attaching better photo. It looks wrong to me, but according to the crimping instructions (bottom right of page 2) apparently that's how it's supposed to be on this style terminal.

Pushing the conductor underneath is the procedure for single crimp terminals, so it says.

If you zoom in to the photo you can see the black wire wound around the center (toward the top) and the missing wire lower down. It seems to have failed right where the crimp starts to make contact with the conductor.

XKY7mLSSpZJ11817CYIYMl_LLykIyGZd7toJgOAb9RLjMsjJYZyimF0B9_nCsjRm5yPpNKroI00huM3ObnFX12HfmnU3Ply27rgit-OjL3sxZj3LWj-pZC9GUv6XNnNl85eP_ZP-kvIQ-xXPNaISubiN1-1FpW0GjLssSp3pCTV5sWEEyDRyQ6M3F1OdzZcv5A1nTJ6neYRD26N90n0lyqX9ZeL2VSYNUUNSSUhfiuNSk9VxhEk_yCIcZ1fQhHZlp63wYAmx8G6GkTWkCqvXayL_XqUM_Q6DYITzIAfGVzhnRHSsmlmmzdVJsTJXAK0KMc-k3gGeKwXJRmysBbH_FfVChG6VsDIp7WvCcW5GeghSPu4YE94D8miN4ubBD4Cka6xD3Jcexyb4Uq7uFrvKcdA52UFWPdOc91s2PSYDSZzKbzwWlz6HJ1fAZpxSyqDhuHMf-0oFCSM7uJolvKZBs_VNuFGCC8DHjd0mOs2pW2_eHnUmIOPUN11OwGl0eJOIga5ARpvk28DaegKS53kPAc_nUGK_ZZ7RnTvYBQALs67ZOjOzobtYQXW1QfEpnjLNLN9T4CFvnFPmHKLXe8JpcOw8Nt6jleu-3A_4E6eRqFXe5plfMOocVtXdErfz5_mIkCJPm_F7Z6RnQuR-HKkbyxM2CDyvWkRkeQ=w545-h727-no
 
Last edited:
As many of these as I've made, I have yet to run across the dual crimp versions. Good stuff there. Makes a lot of sense for the really large center conductor variants of the wire. Hard to say if the original crimp was sub optimal, but I guess 11 years' service is pretty respectable. Something to pay attention to at assembly though. Appreciate the thread!
 
Interesting, rv6ejguy, your link says not to use a crimper for the conductor. My link says to use the crimper :) Also seems to conflict with the youtube videos that MSD have posted.

" Following is a new crimp procedure for the Dual Crimp Terminal. ... Do not use the MiniCrimper to crimp the conductor."
 
You can accomplish the closing of the 2 "wings" around the conductor in several ways. I find the end jaw on the MSD tool may not squeeze the wing quite tight enough on the conductor. You can start it with that jaw but I usually give the terminal an extra squeeze with some pliers. You want to be sure the spiral wound black conductor is firmly touching all 3 surfaces of the terminal.
 
I just did a crimp here using only the tip jaws of the MSD tool and it worked well with good retention and pressure. You usually have to push the conductor end down into the groove and hold it there with a small screwdriver as you start the crimp. If you scar up the spiral conductor it may burn up and it needs to be below the high point of the arc so it does not slip out as your crimp.

You can see the center point of the terminal is well below the center point of the insulation.

 
replacement interval

In the Inspection And Maintenance Procedures section of the LSE Plasma manual the recommended replacement interval is listed as 10 years or 500 hours. The latter came up for me last year. As many ignition primary wires as I've had break down over the years in high performance automotive applications I didn't want to chance it and replaced them at condition inspection time.

I used the MSD mini crimper I had left over from making wires for another project, wasn't aware of the ratcheting model. Goodie, another excuse to buy a tool... In any case those terminals are fiddly. I advise inspecting crimps under a magnifier in good light, at least if your eyeballs are as old as mine.
 
I didn't have/notice a failed wire in flight but when ferrying my -8 from TX to WA last summer with an 11'ish year old Lightspeed II installation on one set of plugs I did end up with an extended stay in Provo, UT, after noticing two cold cylinders during a run-up. Taxied back and proceeded to tear the thing apart on the ramp to start troubleshooting (... TAC Air is a great FBO if you've got an AOG, btw) and learned just how expensive a $5 length of RG400 can get...

Checked the plugs... all fine.
Checked all electrical/mechanical connections in the cockpit... all good.
Went through the Lightspeed troubleshooting guide in their User's Manual online... everything checked out (except for that whole "get a spark to jump between the coil leads" step -- to this day I've never seen that happen but know folks who swear they have).

For giggles, I decided to swap the coax lines between the two coils to see if the problem followed the "bad coil" to the other cylinders and... Ah HA!... it did. So, I called Klaus and told him what I was seeing. His guess was a bad coil (as evidenced by my coax swap) OR a bad crank sensor since mine was/is still the sort that fits in a mag hole. So, I traded him $thousands for an overnighted/new Lightspeed III system and an agreement that he'd take back whatever I didn't end up using as a return. Talk about great customer service!

Got the rushed shipment from Klaus and replaced the "bad" coil... no change.
Then I swapped the old LSII for a new LSIII computer... no change.
Then I swapped the crank sensor PCB... no change.
Then I swapped the computer/sensor harness... no change.
Then I was convinced it was the spade lugs on the coax cables... cut, strip, recrimp and no change.

I noticed my original ~11 year-old coax had a black jacket (RGwhatever) and the new kit from Klaus came with fancy new brown RG400. Hmmm. But, the continuity/resistance check performed per the troubleshooting manual said the coax is fine so I don't need to replace that... right?

After running out of things to swap I clipped several thousand zip ties, extracted the RGwhatever, and cut/stripped/crimped/installed a new RG400 replacement. Fired up the engine and... BINGO... the previously cold cylinders were now hot again. So, off I flew to WA with 2 new coils, a new LSIII computer (duct taped to the LSII mount!), and ONE new RG400 coax. Arrived without another issue.

After no more than 3 flights in WA I performed a run-up at the hangar prior to shutting down one evening and... hey!... those other two cylinders being fed by the coax I didn't replace in UT are not cold?? So, apart it all came and in went another $5 length of RG400. Problem solved. The time to failure on those 11 year-old RGwhatever harnesses was within 3 hours of each other!

While I had it apart (and to see just how much I *could* have sent back to Klaus for a refund) I stuck ALL of the old LSII stuff back in with the exception of those 2 new RG400 coax cables and... the darned engine ran like it was new. *sigh*

I now brag that I have some of the most expensive ignition system wiring EVER installed in an RV that I bought from Klaus (RG400 for ~$150/ft??) and that he threw in a spare LSIII ignition system for FREE! ;-)
 
Thanks for the video. That's what I ended up doing, although I found the black lacquer on the spiral conductor is pretty high-resistance and sometimes the crimp would not displace it enough to conduct properly. I ended up lightly sanding it off before crimping.

Not sure if that's right or wrong though -- after replacing the flaky wire on the cold cylinder I found a second that also seemed a bit flaky. Made a new one there that tests good, but the coldness has moved to that cylinder...now suspecting a coil problem.
 
...and, after reviewing some old photos, it looks like I swapped the leads on the coil during the replacement process. Since the cold cylinder has moved, despite rebuilding the wires and trying the old ones, I guess I had a failed coil all along.
 
Back
Top