912 Excessive Mag Drop

by colin benham in 912 / 914 Technical Questions 11 years ago

I have had two problems with the ignition. One I sorted, one remains. Problem one was the engine did not shut down when the mag switches were turned off. (Turned the engine off by flooding the carburettor with the choke). I found that the plastic connector (mag to earth and plugs) had sufficient play to allow the connection of the earth not to complete under certain circumstances. Solved by the use of two cable ties to take up the slack.

The second remaining problem is the mag drop is sometimes excessive on test. When it fails, I jiggle all the wires and restart and retest the engine, the mag drop at 3000 rpm is normal but I cannot replicate it so I don't know which of the wires is the propblem.

I am assuming there is a break in one of the wires but they are so fine I would like to avoid splicing in new connectors to all the connections if possible.

Suggestions welcomed.

Re: 912 Excessive Mag Drop

by MARK JACKSON » 11 years ago

MARK JACKSON

12 332 11

Hi Colin

If you don't want to repair the poor connections then you will need to replace them or not use the aircraft.

This is the only answer I could come up with.

Have you thought of anything else you could do.

Mark

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Re: 912 Excessive Mag Drop

by Roger Lee » 11 years ago

Roger Lee

128 5.28k 4.98k

Hi Colin,

Hunting electrical ghost can certainly be trying. What is the normal mag drop you talked about? About all you can do is continue to hunt for the problem wire. I don't know how your engine setup is, but I many times chock the wheels, set the brakes and put someone in the pilot seat to control the throttle or kill the engine if need be and start to work each wire while it's running. If it is too dangerous for you then I wouldn't do it, but some engine setups aren't too bad to work with. Just sounds like a bad connection or a wire has been trapped and rubbing against the engine or has a bad connection in it's socket.
This is a tough call here on the forum to hunt down a specific cause.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: 912 Excessive Mag Drop

by colin benham » 11 years ago

colin benham

1 1 0

Thanks for the replies. At 3000 rpm with everything working well there is a 1 – 200 rpm drop on testing each of the mags. Which I have been advised is what is expected and in limits.

When things are not working well, there is an 800 – 1000 rpm drop on one mag test and the engine begins to run very rough and then reverts back to smooth running after I stop the earth test. After I stop the engine, play with the wires at the plastic connectors where they join wires from the mags and restart, it goes back to 1 – 200 rpm drop on test.

If always had this rev drop every time I tested the mags I would assume it was a lead or plug but the engine seems to be running fine before I get the massive rpm drop so it is only on test does it show itself.

Thoughts please?

Colin

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Re: 912 Excessive Mag Drop

by bill martin » 11 years ago

bill martin

10 757 0

Colin,

It takes a little work to do it, but you can unsnap and remove each wire from the plastic plug. There is a small tang on each male and female connector inside the plastic plug. Depress this tang and the wire and connector will slip right out of the plastic housing. You can then check each wire and wire plug for corrosion and tightness. Afterwards, just smear a little dielectric grease on each wire and snap it back in.

If it is the plug, that should cure it.

Bill.

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Re: 912 Excessive Mag Drop

by Hans Meier » 11 years ago

Hans Meier

2 15 0

Hello,

I had a similar problem on my older 912. It turned out to be a broken charging cable leading to the electronic module. (BRP-Rotax Maintenance Manual Fig. 74/ 8 ) It was broken next to the plug. It was at the 2 single-pin plugs, not at the 4 Pin plug connectors. When I pulled at the plug the cable got longer. The isolation extended and the connection inside was lost. I cut off male and female plug and soldered it together. After that all was OK. Except when I want to change the electronic module. Then I would have to crimp some new plugs. Maybe it was due to vibration.

Good luck!

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912 Excessive Mag Drop

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