Mag. Check

Dale Campbell · #1 09-15-03, 02:03 PM

I just had a wire problem checking left magneto on front engine. When I check mags. before take off, I did not get a 100 RPM drop like normal, when I switched to left-right mag and back to both. The engine seemed normal other than that. I checked the rear and I did get normal 100 RPM drop. I than checked the front engine again. Then I remembered how the magneto works. You have to short a mag to get it to stop firing. I than pulled the front engine to idle and turned off the ignition. Guess what it keep on running.
I went back to the hanger and pulled the cowl and my suspicions
were right. The pig tail on the p-lead wire was broken, on the opposite mag I was checking, which is right. When you check left mag. you short out right mag. That could have cause a couple of problems. #1 if you pull a prop threw by hand, you could get killed. Thats why I always leave my mixture controls in full idle cut off position when parked or in hanger. #2 The other problem you could have is if you had a bad magnito, you would not even know it. So, make sure you see that drop in RPM when checking magnetos.

stackj · #2 09-15-03, 10:45 PM

Cessna has recommended for some time that you periodically test for exactly this condition by setting the engine to idle and switching both mags off.

Looks like they know what they are talking about.

You are one of the few that would realize that NOT getting a mag drop is a problem.

Good work!

Kim Geyer · #3 09-16-03, 10:44 PM

This test is also an ADthat should be done each 100 hrs.
I think it is AD 76-07-12 Bendix switch
Kim

Francisco · #4 10-03-03, 10:49 PM

today I tested the system. At Idle is shot down the master and the engines keept runing smothly...Cool

Francisco

Dale Campbell · #5 10-04-03, 09:32 AM

Francisco,
Do not rotate the prop by hand on the engine that kept running.
The ignition is always live. Fix that wire to the mag P lead.
Dale Campbell

kevin · #6 10-04-03, 04:20 PM

Francisco said he shut down the *master* and the engines kept running. That is normal. Nothing in the electrical system is required to make the ignition system run.

The test should be to turn both mags off on an engine while that engine is at idle. If the engine keeps stops, you are fine. If the engine keeps running, one or both of your mags are "hot", even when the switch is off. You should have this corrected immediately.

You can detect the same thing, (as described previously in this thread) by watching for "no drop" on each mag. When that happens, the other mag (not the one you selected) is hot all the time, and needs to have its ground checked.

Kevin

Francisco · #7 10-04-03, 09:59 PM

Dale, kevin is correct. But to tell you the truth when I first started the test, I turned the mags to off and sure enouhg the engine stoped. Then I realized that what I needed to turn off was the master switch, and it worked like it should. I did the test on bouth engines on (Bouth, Left & Right) it works like it should.

Francisco

kevin · #8 10-04-03, 11:46 PM

Francisco, I just don't understand why you keep mentioning the master switch. It has nothing to do with the test you are doing, and nothing to do with whether the engine stays running or not.

Kevin

Francisco · #9 10-05-03, 02:02 AM

Kevin you are right the reason I kept mentioning the master switch was because I was conduting a test to see if the engines would continue running with it off.

Did not mean to confuse you.

Francisco

Dale Campbell · #10 10-06-03, 08:46 AM

Sorry I read Francisco's reply to Mag Check wrong. I read master and was thinking Mag.