Couple SDS CPI2 install questions

[David Carter]

Originally Posted by Bicyclops

Breakers are to protect wires, not devices. If you power both the ECU and the coilpack from one breaker, all the wires to them need to be large enough that the breaker pops before any of the wires burn. So, if it's a 15A breaker, all the wires need to be able to carry 15A.

Ed Holyoke

Good point, Ed. I had not caught that. The purple is 20 gauge & the CPI doc calls for a 2 to 5A breaker or fuse. The red is 18 gauge & calls for a 10A breaker or fuse. Looks like you would need to upgrade both wires to 14 gauge with a 15A breaker.

[rv6ejguy]

The purple wire only powers the CPU section. 2-5amp breaker, 20 ga wire which is already part of the harness.

[N546RV]

Originally Posted by rv6ejguy

The purple wire only powers the CPU section. 2-5amp breaker, 20 ga wire which is already part of the harness.

Right, but if the purple and red wires are tied together on the same circuit - as depicted in the aforementioned diagrams that demonstrate using a single switch to both remove power and/or activate the killswitch function - then that purple wire will be protected by a 15A breaker, rather than 2-5A.

This seems to be the concern being expressed here - it may be OK operationally to combine ECU and coil power feeds into a single circuit, but it raises issues with proper protection of the purple wire in the harness.

Of course, none of this applies if the system is set up as depicted in the installation manual, with two separate breakers for each ECU (one for the purple wire and one for red).

Put another way, David isn't calling out a deficiency with the harness as constructed, but a deficiency with the proposed single-switch setup.

[agirard7a]

Here is how I wired my system. Click on the image

[rv6ejguy]

Originally Posted by N546RV

Right, but if the purple and red wires are tied together on the same circuit - as depicted in the aforementioned diagrams that demonstrate using a single switch to both remove power and/or activate the killswitch function - then that purple wire will be protected by a 15A breaker, rather than 2-5A.

This seems to be the concern being expressed here - it may be OK operationally to combine ECU and coil power feeds into a single circuit, but it raises issues with proper protection of the purple wire in the harness.

Of course, none of this applies if the system is set up as depicted in the installation manual, with two separate breakers for each ECU (one for the purple wire and one for red).

Put another way, David isn't calling out a deficiency with the harness as constructed, but a deficiency with the proposed single-switch setup.

From Barry:

"Best to keep purple and red on separate circuits & breakers and use a double pole switch. The CPi2 should work if purple and red are tied together on one circuit but if the breaker or switch was not in good condition this could introduce some voltage fluctuation into the purple wire input and since this powers the cpu circuits, extra noise here is not a good thing to have. This is why separate breakers and a double pole switch are best."

Since coil power goes through the CPI2 red wire and there are large current surges as the coil charges, the decision was made in the design stage to separate CPU and coil power to reduce the likelihood of noise. Coils can be noisy devices. By combining these wires, you negate the design intention here. Might work ok, might not.

[N546RV]

Originally Posted by rv6ejguy

From Barry:

"Best to keep purple and red on separate circuits & breakers and use a double pole switch. The CPi2 should work if purple and red are tied together on one circuit but if the breaker or switch was not in good condition this could introduce some voltage fluctuation into the purple wire input and since this powers the cpu circuits, extra noise here is not a good thing to have. This is why separate breakers and a double pole switch are best."

Since coil power goes through the CPI2 red wire and there are large current surges as the coil charges, the decision was made in the design stage to separate CPU and coil power to reduce the likelihood of noise. Coils can be noisy devices. By combining these wires, you negate the design intention here. Might work ok, might not.

And now we've kind of come full circle. Barry's words here reflect what I get out of the installation manual, that separating the power leads is best practice. This, however, seems to contradict this proposed wiring diagram which is said to have been vetted by Barry.

To be clear, I'm not trying to belabor a point here, just wanting to clarify what looks to me (and apparently, to a few others) as mixed messages. I'm not sure where the disconnect might be happening here - maybe the "vetting" was not the "two thumbs up" endorsement I imagine, but something more akin to "well the plane won't explode if you do it that way..."

So I'm really just seeking to resolve this disconnect - and not even really for my own sake. For me, best-practice and separating the feeds seems like the right decision. But I'd also like to be sure that anyone stumbling across this discussion down the road comes away with the right idea.

I appreciate your responses. I'm pretty sure I've said it before, but I'll say it again: your involvement in the community here is a huge factor in my decision to go with the CPI2 for my build.

[rv6ejguy]

I think you make a good point here. Most of our recommendations for systems and wiring lean towards best practice. Often other ways of doing things also work fine but sometimes they don't and we end up on the phone trying to sort out how to fix issues that crop up.

We've seen some vexing problems over the years, many caused by worst practice on installations that were only revealed when we reviewed photos or video of the installations. Tie wrapping sensor cables to spark plug wires was one (done by an A&P and avionics tech!). Injectors in backwards was another memorable one.

Best practice gives you the best chance of avoiding any weird issues.

[Chkaharyer99]

As a CPI2 owner/operator I’ve been following this thread with interest. As said previously I wired my dual channel system per plans. I do however appreciate the points made about having the purple and red wires tied together.

I currently have two 16 gauge red wires coming off the + side of my primary battery. One wire for each channel. The wires are Each individually protected immediately after the connection to the battery with Fuses. Then it’s wired per SDS plans.

If I’m getting this right, the suggestion would be to run two additional wires from the power source, in my case the positive battery terminal, to supply power for the purple wires?

I’ve had no issues with current scenario but prefer to use the “best practice” approach if I can discern what that actually is.

A revised wiring diagram might help.

[rv6ejguy]

I'd say if your installation is working well, no need to change. The proof is in the pudding as they say.

We've seen a large variation in electrical noise between airplanes and installations over the years. Where one is wired not with best practices, we may see no issues while another wired with best practices may have multiple noise issues. There are no guarantees...

[Chkaharyer99]

Originally Posted by rv6ejguy

I'd say if your installation is working well, no need to change. The proof is in the pudding as they say.

We've seen a large variation in electrical noise between airplanes and installations over the years. Where one is wired not with best practices, we may see no issues while another wired with best practices may have multiple noise issues. There are no guarantees...

Thanks for the quick response Ross.

My CPI2 works great as wired per diagram.

On another note, never had a fuse blow on CPI2.