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Old 13th Nov 2009, 21:58   #1 (permalink)
Thread Starter
Join Date: May 2007
Location: Norfolk
Posts: 26
Question Bendix 3000 dual Magneto - radio interference

Hi all,
I have a SAUER engine which is fitted with the Bendix 3000 dual Magneto. One of the Mag circuits is generating huge radio interference (which goes away when that side is selected off).

Anyone have any suggestions as to what could be causing this? Its only done about 10 hours since a complete professional rebuild, which is doubly annoying...

microlightgary is offline   Reply With Quote
Old 13th Nov 2009, 22:18   #2 (permalink)
Join Date: Jan 2007
Location: California
Age: 58
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Here is a link to a good article, John also has alot of good information on other subjects on his site.

slatch is offline   Reply With Quote
Old 13th Nov 2009, 22:33   #3 (permalink)
Join Date: Jun 2003
Location: EuroGA
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I would start with replacing the capacitor ("condenser") on that half-mag. Better change both while it is open.

If that doesn't do it, change the ignition harness.

I have the same problem but it's only just audible when on the ground. It wasn't the condenser... but it usually is, I am told.
IO540 is offline   Reply With Quote
Old 14th Nov 2009, 04:25   #4 (permalink)
Join Date: Aug 2006
Location: Ontario, Canada
Age: 56
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I support what IO540 says. Add to that, to diagnose a suspect ignition harness, fly as high as you can get (8000 to 10000 feet if possible). If you start to get rough running that gets worse as you fly higher, it's time for a new harness. The less dense air at higher altitude enable the high voltage to jump through a crack in the ignition harness insulation more easily than going all the way to the spark plug, hence the rough running.

Take really good care of that mag. The C182 RG I used to fly had a dual mag, and the one gear which drives both halves of the mag, was found to have two consecutive teeth missing at overhaul. I'm amazed it worked at all!

Pilot DAR
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Old 14th Nov 2009, 08:02   #5 (permalink)
Join Date: Jun 2003
Location: EuroGA
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IMHO the single shaft dual mags (I have one of those too!) are the weakest link of that whole engine. Never skimp on the overhaul intervals, and always use the most reputable overhaul company.

There are two issues with an ignition harness: breakdown of the insulation between the inner conductor and the shield (which can become evident if one does an LOP mag test at altitude) and a problem with the grounding of the shield (which is where the interference would come from).

One needs to be careful doing mag tests at altitude, due to risk of backfire and exhaust damage. When switching to one mag only, be prepared to shut the throttle to idle quite rapidly if a duff plug (rough running) is evident, then switch the mag switch to BOTH, and only then advance the throttle back to the cruise setting.

I had another weird interference issue: the Shadin flowmeter was reading about 7% too high. It took me years to chase it down, and sure enough it was caused by unshielded wiring to the flow sensor turbine, picking up muck from the ignition harness, within the part of the wiring located forward of the firewall. It was an amazingly consistent 7% too... The unshielded wiring was done by one of UK's best known avionics shops, too...
IO540 is offline   Reply With Quote

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