SonexBuilders.net

I've got an issue that I just can't seem to solve. Looking for any insight...

Setup:
Hummel 2400cc VW conversion with 1 slick mag and 1 electronic secondary ignition
MGL EFIS Xtreme linked to an MGL RDAC XF
Sonex CHT Probes
MGL EGT thermocouples


Problem:
When the engine is running, the engine instruments are very unreliable, meaning they jump all over the place. It is most noticeable in the CHT/EGT however the oil pressure and oil temp jump all over as well. This problem only occurs when the magneto is firing. In other words, with the engine running and mag grounded(off), running only on the secondary ignition the problem completely goes away. Flip the mag switch back on and the problem comes back immediately.

Trouble shooting so far:
Disconnected p-lead from mag isolating the wiring. Disconnected all items from RDAC and connected one at a time. The problem seems to be noise from the mag coming in from the thermocouples. I have tried 1 thermocouple at a time and the problem doesn't happen with just 1. I can plug up to 3 of them (any 3) at a time and the problem doesn't exist. As soon as I plug in the 4th thermocouple (any of the remaining 3) the problem comes back. MGL sent me a replacement RDAC to see if that would fix the problem...no luck. Rerouted CAN Bus and power supply wiring between EFIS and RDAC.

The engine has automotive type plug wires on both ignitions as well as automotive type resistor spark plugs on both ignitions. My next step is to try a new set of plugs. I am also having a set of shielded aviation type plug wires sent to try. I am at a complete loss as to where to go next. I have tried everything I can possibly think of and nothing seems to fix the problem.

Any suggestions?

I would first call MGL

I have talked to MGL multiple times. They are pretty much out of ideas as well

Hi Sean, Just some things I would check. Don't know if the Extreme has a backup battery, but if it does, does the Extreme act up while powered up from backup alone? Have you tried powering the Extreme from an independent from the aircraft source? Do you have a ground connection from the RDAC XF to the engine block? Do you have a 20,000Mfd capacitor across your 12V supply?

Good luck!

Rick
Sonex 524

Without seeing a wiring diagram it is hard to give advice but it sounds like a grounding issue to me. Are all grounds run to a common point (don't use the airframe as a ground). Do you have a heavy gage dedicated ground wire running from the engine block to your common ground point? Does the RDAC ground to that same point? Does each mag have a dedicated ground wire running from it (not just the p-lead)? The red annodize on the accessory case will actually act as an insulator and can cause things that you think are grounded to act funny because of a weak or intermittant ground.

Keith
#554

The system works perfect without the engine running and with it running only on the secondary electronic ignition. The RDAC is powered from the avionics bus and so is the EFIS. I do have a common ground bus where the battery, and the engine are grounded to with 4awg wire. This is the same grounding location as the RDAC and the magneto. I have tried changing the grounding location of each of the items with no success. I even powered the RDAC from a completely separate power source but keeping a common ground. Nothing I have done seems to make any difference. It almost acts like the noise is jumping from the magneto case itself to the thermocouple wires. When I get the shielded plug wires I should be able to confirm whether that is true or not. I also ran a separate ground wire from the heads to the ground bus just to make sure that the heads weren't floating due to the cylinders being painted, etc.

Hi Sean,

Have you had a look at your magneto?
I assume its new but I"d be checking the points for burning and arcing. The internal capacitor is serviceable and connected. Making sure nothing is loose inside.
Magnetos produce a lot of energy, if there is something not quite right, it could be producing extra electrical "noise" or arcing over inside the cap.

Edit:You don't mention P leads are shielded. P lead grounding... from Aeroelectric http://www.aeroelectric.com/articles/MagnetoSwitchOptions.pdf

magnetos and the magneto switches. Spark plug wires and magneto p-lead wires are the ONLY wires in the airplane which are ALWAYS shielded. Further, p-lead shields should be grounded to the airframe/engine only at the engine end. The shield is used to provide a ground for the "GRD" terminals at the switch but no other connection to the shields should be made at the instrument panel end (I assume Bob is referring to a standard aircraft key switch)

Cheers
Tony

This is how I'd wire in the magneto with a toggle switch, but not knowing whats hanging off your magneto.. its a guess :)
A good quality switch is probably a good idea as well.

That's exactly how mine is wired. I bought, read, and followed the advice written in the Aerolectric Connection. The first thing I did when trouble shooting was unhook the p-lead and mag ground from he mag completely leaving the mag "hot" the whole time. Problem didn't go away. I have not checked the magneto. I am starting to wonder. It is new. I spoke to Scott Casler (he built the engine) and he said that if the capacitor was bad, the engine would have a miss and wouldn't run correctly. I know nothing about overhauling a mag but I guess I need to learn.

On the hummelbird group there was some magneto discussion. Not sure which mag you have - but if it an old military surplus slick mag from a GPU the condensers are known to go bad with age. Might go along with what Tony is saying. Scott Casler would know.