CDI interference?

[Ron Gray]

i have a conundrum which I’m hoping someone on here may shed some light on. My RC gear is FrSky and the setup in question is using an X8R Rx. This has been fine until today when testing various props on my petrol engine which has an RCexl CDI ignition system. I started the engine but within a couple of minutes I lost all controls. So my first thoughts were Rx battery so I replaced that but had exactly the same result. Next thought was the Rx switch so I bypassed that with the battery directly connected to the Rx, same result. So maybe the Rx was faulty so I replaced that with another one, same result. Re-bound the Rx, same result. Created a new model, same result. So now I can only think that for some reason the CDI unit has a fault. I should add that if I unplug the battery from the Rx and plug it back in again the Rx starts working but will die after the engine has been running for about a couple of minutes. Just before losing the connection, I have noticed that the control surfaces start to move with no input from me!

[Denis Watkins]

Check with Cymaz, Ron

**LINK**

[Engine Doctor]

Swap out the CDI and see if it stops fault. Are any leads running near CDI or ignition circuit ? Worth checking as servo leads can act as aerials for the interference.

[Shaun Walsh]

Does the CDI unit have any form of interference suppression built in, could it be failing after a few minutes use and then reverting to normal operation when the engine is turned off? From experience with car CDI units they put out a lot of voltage and produce a fat spark but can also produce a lot of radio interference.

[Ron Gray]

Unfortunately my other CDI unit are for different spark plugs so I've had to order a new one (£53!!!). No leads run past the CDI unit but the RCexl kill control is next to it. But having said that, it was perfectly OK up until today!

[Former Member]

[This posting has been removed]

[Bob Cotsford]

Check the braid on the HT connection, this needs to have a good connection to the plug cap and to ground in the CDI unit.

I used to use the Rcxcel clones from Hobbyking, iirc I did have to swap the cap on one, that just needed a decent size iron to solder the braid to the cap.

It might be worth flashing the later X8R firmware as there was an issue around spark interference recovery times if my memory serves me right.

[Ron Gray]

HT braid to cap connection is supplemented with wire wrapped around cap, just to make sure! I flashed the Rxs with EU firmware about 6 weeks ago. Haven’t tried changing the plug so will give that a go later on this week.

Thanks for all suggestions will post back when I’ve tried other things.

[Engine Doctor]

Has your engine ever been turned over with ignition on an no plug fitted into the cap ? This can damage the coil in the cdi unit . Nofix for this only option is to replace cdi. An opto isolator , if you can get hold of one fitted between the rx and servos would totally isolate your receiver from any interference getting to it via the servo leads as they can act as a aerial.

[Ron Gray]

@ED - engine never turned over in the situation you have described. I use an Rcexl switch / isolator but no other opt isolators, where would I get one?

The new CDI unit has been delivered so I will be testing it hopefully today.

Edited By Ron Gray on 14/12/2018 11:07:42

[Ron Gray]

For those that are interested, an update. I installed the new CDI unit and after fully charging the ignition and Rx batts ran the engine without any problems. Great, all sorted, or so I thought! As I wanted to run a test using a different prop I started it up again and went through the test, recording revs at WOT and at idle, then I started to get the same problems as before. Engine started to not respond, control surfaces started to move, then the engine cut out and I couldn’t re-start it. Somewhat frustrated I double checked everything, Rx battery and ignition battery showing 98% on the tester. I now turned my attention to the battery switch and tested voltage before and after the switch, no difference. Next I looked at the telemetry on the TX in particular the Rx voltage. This was showing 5.9v, so looked ok. Then I decided to check the voltage whilst applying a load so wiggled the throttlevand rudder stock around and this is where I saw the problem, with a few seconds the voltage dropped to below 4.5v at which point the servo responses became somewhat erratic. Switched it all off and rechecked the Rx batt voltage, back up to 5.8v. Bearing in mind I had replaced the Rx batt with another one (see my original post) I then swapped the batts back and retested and I got exactly the same results! I have now replaced the NiMh batts with LiFe 2s ones and after further extensive testing am happy to report that the problem has gone away - phew. So the problem I had was that 2 NiMh packs were defective.

2018 has been a rather expensive one for me as I have lost 4 ‘planes, 2 of them due to battery failures, 1 due to a faulty battery switch and the one that hurt the most, my Hurricane which was completely my fault due to frying servos using a flight pack of too higher voltage. I’m glad that I found the cause of this latest problem before the ‘plane took flight again!