Coax cables to coils

Another thread discussed the importance of having good, solid connections from the coax to the coils if you have a Lightspeed Engineering EI unit, which turned out to be an issue for me recently.

I had just returned from escorting the RV-1 to Lee's Summit and landed uneventfully. Put Smokey to bed without even cleaning off the bugs. I went out on Thursday for a quick flight around the patch (and clean off the bugs afterward, naturally) and was quite surprised when I did my run-up to find that the engine power dropped very dramatically when I turned off my left mag and ran on the LSE only.

I taxied back and pulled the cowling to change the oil and cogitate what could possibly have caused the lack of power. After all, LSE had just repaired and updated my Plasma II+ unit about 50 hours ago.

After changing the oil and filter, I started with the LSE unit and worked toward the engine. All seemed fine until I got to the connection of the coax to the coil -- the center wire was broken! Broke right at the crimp connector.

I stripped the center lead and crimped on a new connector. I decided to check the other connector as well and did no more than just touch the lead and the wire broke!

I had routed the coax around the center baffle support bracket to the right, then looped it back behind the coils, making a pretty sharp curve with the coax. There is obviously enough vibration when the engine shakes to have fatigued the center conductors where the connectors are crimped. I have reinforced the connector area with some sensor-safe RTV until condition inspection. Then I will re-route the coax cables and bring them DIRECTLY through the baffle and into the coils with no curves.

As a first-time builder, these are the kinds of things that are not second nature. With something as critical as electronic ignition systems, having a wire break due to vibration is not a good way to learn. Take a look at your installation -- if your coax cables make bends anywhere near the connectors that plug into the coils, you would be wise to either re-route the cables to eliminate the bends or reinforce and support the coax-connector interface.

Hope that makes sense. I didn't think to take any pics of my installation or the problem might have been evident to an experienced builder.

This is most notable seen on..
1. Use of coax that is not RG-400. Where the center conductor is not braided wire but solid core. Very bad
2. Installations that have not provided the proper strain relief.
3. Improperly crimped fast on (too tight or too loose)
4. Tinning the center conductor braid. (Bad in a high vibration environment)

As a technique when ever your doing your fwf poke around, and your grabbing everything, be sure you are grabbing those coax wires too.

The center conductor is braided, so it must be RG-400. When I re-do the installation through the rear baffle, I'll be thinking of a way to protect that fragile point where the rigid insulator over the center conductor meets the crimp-on connector -- that's the weak link and the focus of vibration.

There has to be a way to make that union RIGID to protect that center conductor. I'll think of something ...

Adhesive heat shrink.

Not Necessarily!

N8RV said:

The center conductor is braided, so it must be RG-400.

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Don't assume too much. Many quality coax cables use a stranded center conductor.

Generally RG-58 has a black outer jacket and RG-400 has a translucent, brownish jacket. Very distinctive. Some RG-58 has a stranded center conductor so you can't go by that.

For starters, RG400 usually has a translucent copper-colored outer insulation. RG58 (at least the stuff I've seen) is generally black. And then there's the solid vs. multi-strand center conductor.

That said, I test-drove a bunch of crimp connectors on sample pieces of RG400 before committing to the real thing. I didn't like any of the results, so I grafted pieces of tefzel wire to both the center conductor and the braid, then terminated the tefzel wire with crimp (Fast-On) connectors. The tefzel wire is much more flexible and allowed me to create service loops for strain relief in a small area. Much nicer.

Oh yeah, and what Dan said: adhesive-lined heat shrink everywhere.

Edit: I see Mark beat me to the RG400 description. Glad to see corroboration, I'm just waiting for someone to write that RG58 (or RG400) is available in chartreuse of some other designer color

N8RV said:

When I re-do the installation through the rear baffle, I'll be thinking of a way to protect that fragile point where the rigid insulator over the center conductor meets the crimp-on connector -- that's the weak link and the focus of vibration.

There has to be a way to make that union RIGID to protect that center conductor. I'll think of something ...

Click to expand...

Not sure what type of crimp connector you are using but make sure you use a crimp connector with the extended metal sleeve that is designed to crimp on to the insulation surrounding the centre conductor AND the centre conductor. This connector requires a double crimp ideally with the correct tool so that both the centre conductor AND the insulation surrounding the centre conductor will be crimped. IMHO the crimp connector supplied with my Plasma III was inadequate as it only crimped the centre conductor.

Fin
9A

PIDG (Not)

Finley Atherton said:

<snip> IMHO the crimp connector supplied with my Plasma III was inadequate as it only crimped the centre conductor.

Fin
9A </snip>

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Second that. My Plasma II+ came with two yellow crimp connectors (for the shield wire) that appear to be genuine AMP PIDG. It also came with two red connectors (for the center conductor) that appear to be something else. I ordered some of the fully shrouded red AMP PIDGs to use instead.

RG400 ends

I use RG400 coax along with my ignition system. In my G3i installation manual pg. 24 shows a example on how to assemble terminal ends with crimp/heat shrink terminals to re-enforce the connection. Link:http://www.g3ignition.com/s1s2_manual.pdf

G3i Ignition said:

I use RG400 coax along with my ignition system. In my G3i installation manual pg. 24 shows a example on how to assemble terminal ends with crimp/heat shrink terminals to re-enforce the connection. Link:http://www.g3ignition.com/s1s2_manual.pdf

Click to expand...

Yeah, Thomas, that would be a big improvement over a simple crimp connector. It's important that the union of the center conductor insulator sleeve and the connector have NO exposed, unsupported wire. That's the weak link, and right where mine broke.

I'm betting that re-routing my coax so that it penetrates the baffles in a straight line to the coils, and ensuring that that weak link is solid and has no play will eliminate any worries of future wire breakage.

I need to find some Fast-On connectors with the sleeves or buy some adhesive heatshrink tubing like Dan suggested ...

N8RV said:

Yeah, Thomas, that would be a big improvement over a simple crimp connector. It's important that the union of the center conductor insulator sleeve and the connector have NO exposed, unsupported wire. That's the weak link, and right where mine broke.

I'm betting that re-routing my coax so that it penetrates the baffles in a straight line to the coils, and ensuring that that weak link is solid and has no play will eliminate any worries of future wire breakage.

I need to find some Fast-On connectors with the sleeves or buy some adhesive heatshrink tubing like Dan suggested ...

Click to expand...

I've used the method Thomas from G3i Ignition describes in his manual and it works very well. If the center lead breaks on a LS unit, you have a good chance the box has internal damage again.

Here are some (sort of crappy) photos of what I did. The center conductor and shield are grafted to tefzel wire. Those joints are covered with adhesive lined heat shrink, and immobilized against the back baffle with adel clamps. Tefzel wires, terminated with 90 degree Tyco/AMP terminals also with heat shrink at the wire end, have long service loops to eliminate any chance that the coax junctions ever see tension or vibration.

Aha!

Lars, those 90-degree connectors are just the ticket for my installation. However, given that I'm a rules follower, it never occurred to me to divert from the letter of the instructions. As I recall, the instructions said to not cover the shield wire strands with heat shrink. No clue why.

As for damage to the module from the wire breaking, it seems to be working just fine. I will keep an eye on it, though.

Gotta look for angled connectors now ...

N8RV said:

Lars, those 90-degree connectors are just the ticket for my installation. However, given that I'm a rules follower, it never occurred to me to divert from the letter of the instructions. As I recall, the instructions said to not cover the shield wire strands with heat shrink. No clue why.

As for damage to the module from the wire breaking, it seems to be working just fine. I will keep an eye on it, though.

Gotta look for angled connectors now ...

Click to expand...

Don, you are probably thinking about the note in the installation instructions regarding RG58, which will wither under the gaze of a heat gun:

Do not use heat shrink on the RG-58 wire (black, primary ignition wire used before summer 2002)

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RG400 insulation can take the heat.

As for the 90? connectors, they are Tyco/AMP 3-520133-2 "Ultra-Fast" terminals. The terminals are cheap, but unfortunately the official, correct crimper isn't- around $400 as I recall. Lucky me, we had one at work. There may be an alternate crimper that's less expensive but I haven't looked. They were a nice solution for me due to the proximity of that back baffle.