Ignition wiring

[JRo]

I had quite the problem diagnosing a faulty "B" ignition on my RV-12 on first engine run. Swapped modules, checked wiring, pulled off glare shied & checked switch(es), etc. Finally found problem where the "kill" wire is grounded with a clamp holding many other items near the ignition modules. Learned from an expert that a clamp is no way to ground a shielded (co-ax) cable. In my case, the clamp had squeezed the shielding so hard that it grounded against the central conductor of its own cable, rendering that ignition inoperative.

The right way to do it (which we did and solved the problem) is to clip and twist the shielding separately, attach a lug, and ground properly to a screw.

Save yourself this immense nuisance. The only "field" wiring on the ignition for the RV-12 builder is to attach 2 white wires to existing plugs. These are the "kill" wires for each module, "A" & "B". Take the time to route and ground these wires properly, or you, too, may end up with a $28K engine that runs like **** from the get-go.

 

[NASA515]

Good info and totally agree. Grounds are the secret - in airplanes and many other things, and good grounds (earths for you guys who talk funny) are hard to come by.....

Don't forget your "elixir" - DeOxit.

Bob Bogash
RPI grad - near Albany.......
N737G

 

[E. D. Eliot]

Good catch

Don't they run these engines before sending them to the US? I assumed that they did - you know how that word breaks down. Since I don't have my engine yet, could you post a picture of the clamp/ign. module are involved please? Thanks for the heads up!

 

[rgmwa]

I always thought good grounds were what you got from good coffee beans.

 

[NASA515]

Right on, Mate!

Bob

 

[Tony_T]

Page 46-12

Don't they run these engines before sending them to the US? I assumed that they did - you know how that word breaks down. Since I don't have my engine yet, could you post a picture of the clamp/ign. module are involved please? Thanks for the heads up!

Here's yer picture, refer to figure 1:

The OP had the clamp pinch the white wire shield hard enough to short to the inner conductor, probably not a common or likely occurrence, but it is well to know that it could happen and something to check before blaming an ignition module.

T

 

[Tony_T]

I always thought good grounds were what you got from good coffee beans.

I love this forum

 

[KALEWIS]

Had the same thing happen to us. The clamp pinched the wire to the inner conductor. Be prudent when doing this step. It is a tight fit and any misalignment could pinch a wire. Live and learn.

 

[JRo]

Tony T's picture is good! Please save yourself the trouble and DON'T run this cable under a clamp to ground it. If the ignition doesn't fail initially, it will eventually. Safety of Flight item. Get someone who really understands wiring to help. Split the shielding from the co-ax, attach a lug, ground separately, and shrink-wrap all

 

[rvbuilder2002]

Please save yourself the trouble and DON'T run this cable under a clamp to ground it. If the ignition doesn't fail initially, it will eventually.

I don't think this is a valid statement.

I wont disagree that maybe it is possibly true.
It is probably possible to route the wires through the clamp in such a way that the clamp would excessively crimp the wire(s). The intended way (and the only way they will really fit) is for them to nest in the recessed area between the two heavy cables.

As a result of this thread, I inspected the wires on N412RV (red demonstrator). It is now approaching 900 hrs. total time.
The exposed shield area of the wires are still perfectly round with no indication of being excessively squashed/compressed.

 

[JRo]

it is absolutely a valid statement. Running a coax cable under a metal clamp to ground it is not appropriate. It's not how it's done, plain & simple. It may (or may NOT) work for a period of time. If we were discussing a clock or a light, I wouldn't object. This is our IGNITION.

JRo

 

[rvbuilder2002]

In a sense, the installation of the two wires in question is doing what Rotax is already doing.

The clamp in question already has two different cable bundles passing through it, with bare shielding on the exterior (factory installed by Rotax). The bare shielding is clamped by the all metal clamp, tying it to ground.
I agree it is not exactly the same because it is a wire bundle with shield, instead of a single wire, but still very similar.
The same is being done with the builder added P lead wires.

I will agree that the wires should be located in the clamp so that it is not being crimped (I verified it is not difficult to locate them properly), and I feel comfortable that in the long run, it is not destine for certain failure (see previous post about no evidence of problems or damage after almost 900 hrs).

 

[rgmwa]

I have positioned mine on top of the two externally shielded wires as clearly shown in Fig 1 of post #6. In the photo in Fig 3 they are shown on the far side of the clamp, but there is nothing in the instructions to say that the position is significant, only that the clamp should make good contact. Mine are OK as far as I can see, but I assume I should now relocate them as per the photo to comply with the intended position between the two large wires and not on top of them?

 

[Mich48041]

I will agree that clamping a coax is not the proper way to ground a shield. A good electrical connection requires pressure which is usually effected by a spring or threaded fastener. In this application the shield does not carry any significant current. The purpose of the shield is to prevent ignition noise in the avionics. The shield is not continuous from the engine to the instrument panel. A poor connection between the coax shield and the clamp will not affect the operation of the engine. If the connection fails, the worse thing that will happen is that there could be noise on the radio.
As mentioned in previous posts, clamping the coax too tightly can damage the coax and cause the ignition to be shorted out. Coax that is sharply bent or compressed might be fine initially. But it can eventually short out because the center insulation flows under pressure over a period of time.
I will inspect the ignition coax and clamp next time that I have the cowl off.
Joe Gores

 

[rvbuilder2002]

As mentioned in previous posts, clamping the coax too tightly can damage the coax and cause the ignition to be shorted out. Coax that is sharply bent or compressed might be fine initially. But it can eventually short out because the center insulation flows under pressure over a period of time.
I will inspect the ignition coax and clamp next time that I have the cowl off.
Joe Gores

Joe, I entirely agree with your comments regarding coaxial cable, but wanted to clarify that the wires we are discussing aren't coax cables. It is a single conductor shielded wire. The inner conductor is insulated with Tefzel insulation (the same as most of the other wire used in the airplane), not a typical coax dielectric (which is much more fragile and crushable).

 

[Mich48041]

Joe, I entirely agree with your comments regarding coaxial cable, but wanted to clarify that the wires we are discussing aren't coax cables. It is a single conductor shielded wire. The inner conductor is insulated with Tefzel insulation (the same as most of the other wire used in the airplane), not a typical coax dielectric (which is much more fragile and crushable).

You are right Scott. Most RV-12s are not going to have a problem with the ignition wires as long as they are parallel with the other cables inside of the clamp and are not squashed. I installed mine according to the plans and will leave them be unless it turns out to be a problem.
See you at OSH.
Joe Gores