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  #1  
Old 05-27-2017, 01:07 PM
Drippy Drippy is offline
 
Join Date: Nov 2015
Location: Vermont
Posts: 89
Default Mag failed - i think - Mag coil wire

Guys,

I believe my bendix mag failed but wanted to ask for advice before shipping it down to tropic airpower for another rebuild.

I have 5 hours in the air and everything has been performing well, but today when I performed my Mag check prior to take off the mech mag failed. For info I have one non-impulse Bendix and with a light speed electronic.

I checked my P-lead and checks good in both key positions (off and selected). I noticed that the mag body was grounded so had someone spin the prop and noted it remained grounded throughout rotation after removing the ground wire and P-lead.

Next idea was points were not opening - so removed the mag back cover and rotated mag and confirmed points were opening. Checking conductivity across the two terminals it never dropped even with contacts/points open.

After messing around and with the help of a hangar mate A&P we figured out that the wire to the coil was constantly grounded. The question is "this isn't suppose to be grounded right? and this means I need to return the mag?"

To clarify this is the one wire that goes from the mag coil to the points connector.

Thanks

Last edited by Drippy : 05-27-2017 at 01:10 PM.
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  #2  
Old 05-27-2017, 01:33 PM
Carl Froehlich's Avatar
Carl Froehlich Carl Froehlich is offline
 
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Location: Dogwood Airpark (VA42)
Posts: 1,140
Default

Quote:
Originally Posted by Drippy View Post
Guys,

I believe my bendix mag failed but wanted to ask for advice before shipping it down to tropic airpower for another rebuild.

I have 5 hours in the air and everything has been performing well, but today when I performed my Mag check prior to take off the mech mag failed. For info I have one non-impulse Bendix and with a light speed electronic.

I checked my P-lead and checks good in both key positions (off and selected). I noticed that the mag body was grounded so had someone spin the prop and noted it remained grounded throughout rotation after removing the ground wire and P-lead.

Next idea was points were not opening - so removed the mag back cover and rotated mag and confirmed points were opening. Checking conductivity across the two terminals it never dropped even with contacts/points open.

After messing around and with the help of a hangar mate A&P we figured out that the wire to the coil was constantly grounded. The question is "this isn't suppose to be grounded right? and this means I need to return the mag?"

To clarify this is the one wire that goes from the mag coil to the points connector.

Thanks
A couple of thoughts:
1. It is very dangerous to not have the pLead grounded while you are pulling the prop through (i.e you have a hot mag) unless you have all the plugs out. More than one person has been killed doing this.
2. A coil that has one end connected to ground will read a low resistance on the other end, as in just a few mohms. Check that you are on a very low scale on your multi-meter.
3. The mag body is connected to the engine - which is grounded.
4. If you only have 5 hours on this rebuilt mag, call the place that did the work and ask them.
5. Do not rule out your ignition switch. Using it with a mag and a Lightspeed you could have wiring problems.
6. Did you do the timing check? Remember your mag is set at 25 degrees BTC and your Lightspeed is set at TDC (assuming your are running a stock 260hp IO-540).

Carl
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  #3  
Old 05-27-2017, 01:42 PM
Drippy Drippy is offline
 
Join Date: Nov 2015
Location: Vermont
Posts: 89
Default Carl

Thank you for the ideas

I actually removed the Mag from the engine for much of this testing and the A/P did the first one aircraft pull through - not saying that makes it right . . . just saying.

We checked the switch by removing the wires from the Mag and using the OHM meter so I think that may be correct.

We did the majority of the testing on the bench and by turning the Mag.

Do you know if the wire to the condensor/coil should be grounded to the the body of the Mag?
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Old 05-27-2017, 04:21 PM
lr172 lr172 is offline
 
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Location: Schaumburg, IL
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Originally Posted by Drippy View Post
Guys,

After messing around and with the help of a hangar mate A&P we figured out that the wire to the coil was constantly grounded. The question is "this isn't suppose to be grounded right? and this means I need to return the mag?"

To clarify this is the one wire that goes from the mag coil to the points connector.

Thanks
The wire from the coil should go directly to the points and then bridged to the condensor. That primary coil circuit is ground by the points when closed and removes the ground when they open. The bridge to the condensor helps to absorb energy and prevent the points from arcing. The condensor also acts as a potential bridge to ground via your p-lead (connects to opposite side of condensor) to prevent the coil from firing. A shorted condensor will provide a constant ground on the primary circuit (condensor is electrically in contact with the mag case, i.e. ground) and prevent the points from firing the coil. Disconnect the p leads, then pull the lead from the condensor off the tab and put an ohmmeter on this lead and the other to ground to test for a shorted condensor. A read of an ohm or less indicates that it is shorted. If not, you need to investigate your p leads and check for shorts to ground on that circuit.

Please describe how you confirmed that the lead from the coil is "always grounded."

Did you pull you p leads immediately prior to the problem appearing? If so, you may have lost the fiber washer/insulator on the p lead post. This could cause a constant ground as well.

Larry
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Last edited by lr172 : 05-27-2017 at 04:37 PM.
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  #5  
Old 05-27-2017, 05:22 PM
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Tommycat Tommycat is offline
 
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Do you know if the wire to the condensor/coil should be grounded to the the body of the Mag?
Good thread HERE...

Resistance values of the windings are: coil measured .5 ohms primary/ 15k ohms secondary. From HERE.
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Last edited by Tommycat : 05-27-2017 at 07:07 PM.
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  #6  
Old 05-27-2017, 08:44 PM
Drippy Drippy is offline
 
Join Date: Nov 2015
Location: Vermont
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Default Test

Thank you great info. I confirmed the wire from the condensor to the points was grounding by disconnecting it from the points switch and touching one end of my ohm meter to the wire and the other to the case. I opened the mag with a+p buddy to check points operations and just kept progressing since the circuit was constantly grounded. Only after disconnecting condensor wire could I get it to open.
Anyway to fix a ground in condensor or coil wire without replacing it?

Last edited by Drippy : 05-27-2017 at 08:48 PM.
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  #7  
Old 05-28-2017, 09:18 AM
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Tommycat Tommycat is offline
 
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Anyway to fix a ground in condensor or coil wire without replacing it?
If after properly isolating the suspected components and verifying that the capacitor is shorted (no resistance to ground), and/or with having good leads and connectors the coil doesn't meet the resistance requirements. No repair is possible... remove and replace.
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  #8  
Old 05-28-2017, 11:13 AM
lr172 lr172 is offline
 
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Location: Schaumburg, IL
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Originally Posted by Drippy View Post
Thank you great info. I confirmed the wire from the condensor to the points was grounding by disconnecting it from the points switch and touching one end of my ohm meter to the wire and the other to the case. I opened the mag with a+p buddy to check points operations and just kept progressing since the circuit was constantly grounded. Only after disconnecting condensor wire could I get it to open.
Anyway to fix a ground in condensor or coil wire without replacing it?
Did you disconnect the p lead before the condenser test?. You needd to isolate the condensor to be sure the short is there and not the p lead wiring. If the condensor is shorted, you will need to replace it. No repair possible. They should be under $100
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  #9  
Old 05-28-2017, 01:51 PM
Drippy Drippy is offline
 
Join Date: Nov 2015
Location: Vermont
Posts: 89
Default P-Lead

Once again thanks for great info - as you can tell I am reluctant to send my Mag away for a few weeks just after getting started flying.

I am 100% sure I took the P-lead out of the equation by disconnecting it. When my A&P friend and I started disassembling the mag to check the points the P-lead came off.

During the trouble shooting I had both sides of the contact removed and was able to isolate the problem to the wire going to the condenser.

Any idea why it would go bad?
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  #10  
Old 05-29-2017, 01:00 AM
lr172 lr172 is offline
 
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Location: Schaumburg, IL
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Originally Posted by Drippy View Post
Once again thanks for great info - as you can tell I am reluctant to send my Mag away for a few weeks just after getting started flying.

I am 100% sure I took the P-lead out of the equation by disconnecting it. When my A&P friend and I started disassembling the mag to check the points the P-lead came off.

During the trouble shooting I had both sides of the contact removed and was able to isolate the problem to the wire going to the condenser.

Any idea why it would go bad?
Capacitors are long sheets of thin metal with an insulator on top that are then rolled and shoved in a tube. Any break down of the outer insulators will cause a short. A would expect a decent initial mortality rate.

I have a spare condensor that I would consider selling or loaning. It is NOS and would likely work in your application.

Larry
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