Even if you know it is good, check it.

[DENNY]

Sooooo, I had an issue a few weeks back with a 400 Rpm drop on my Right mag. Pulled/cleaned/tested the plugs all good harness looked fine, timing was off about 15 degrees so figured I had an issue with the points. So pulled the Mag and sure enough the plastic block that rides on the cam had been melted on the point side of the block, little bit of pitting on the points but not bad. Figured might have a bad condenser. Called my mag guy and he agreed, so new points and condenser went in and problem solved. 3 hours of flight time latter and now total loss of Right mag. Also really did not want to turn over, it did that just about spin but stop thing, 5-6 times then she went over the top and started. Now I have been having some issues with intermittent starter performance. I replaced the 10 year old battery this spring and 3 years ago I did the bottom end because of a bad cam, put new cable from battery to starter and made sure the ground strap to the motor mount was clean and tight. I check it closely at annual every year. So now pull the mag and same problem melted plastic block. Called the mag shop and said I had the same problem They said "You know we saw that once a few years back with a new set of mags we sent out, the plane had a bad engine ground". Now I know my ground was good because I put it on myself!! But I am standing right next to the plane so while still on the phone I reach up and give it a feel . Sure enough engine mount side was loose. Looks like when I used the mount to lift the front of the plane with engine hoist to put my ski brackets on I bent the tab over and bolt was not as tight as it should be. The two give away keys is the poor starter performance and the Right mag was the only one with melted point cam follower. Just for fun who can say why it was just Right and not Left mag with the problem. Web you have to wait and give the others a chance.
DENNY

 

[skywagon8a]

Left mag grounded off during starting.

 

[wireweinie]

Oh man!!!

 

[frequent_flyer]

Left mag grounded off during starting.

So, if one one mag has an impulse coupling, which one is it? Do Continental and Lycoming have different configurations when only one mag has an impulse coupling?

 

[frequent_flyer]

Just for fun who can say why it was just Right and not Left mag with the problem. Web you have to wait and give the others a chance.
DENNY

Well I'll stick my neck out. Lycomings usually have the impulse coupling on the left mag. Right mag P lead is usually grounded by the start switch or by manual selection of the mag toggle switch.

With the P lead grounded the P lead becomes a path for current that should have flowed through the engine ground lug. That's a lot of current and something's going to get hot.

 

[skywagon8a]

So, if one one mag has an impulse coupling, which one is it? Do Continental and Lycoming have different configurations when only one mag has an impulse coupling?

In DENNY's example it is the right one. In reality you could use either position. The side with the longer mounting studs gets the impulse and the spacer. You could use two impulse mags if you so desired.

 

[frequent_flyer]

In DENNY's example it is the right one.

I would like to see his confirmation of that. The P lead of the impulse mag is not grounded during starting and nothing abnormal happens in the mageneto with the open P lead during starting. The magneto that is overheated is the one with the grounded P lead.

The abnormal current path is through the magneto primary coil when the points are open and through the contact points when they are closed.

 

[cubflier]

I agree with FF. The engine starter is sourcing enough of it's current through right mag p lead to melt things. I hope your starter is still happy.

Jerry

 

[wireweinie]

And . . . .

Web

 

[BC12D-4-85]

Every day is ground school here

Gary

 

[DENNY]

As usual Skywagon8a and Frequent flyer nailed it. When starter is working the Right mag is grounded allowing current flow through the points. This engine came with dual impulse slicks, I changed out to standard Bendix setup left impulse shortly after I started flying it. If I ever put a battery on the engine side firewall of a cub I think I will run a dedicated ground cable to the engine block. DENNY

 

[wireweinie]

I get to be Paul Harvey!

The rest of the story, is that Denny has a key switch that is still connected to airframe ground. The best way to wire P-leads is with single conductor, shielded wire. Use the center conductor to connect the P-lead terminal to the correct tab on the switch. Then use the shield as a conductor to connect the mag body to the ground terminal on the switch. With no connection to airframe there is no path for cranking power to back feed through the mags/P-leads.

Web

 

[DENNY]

I get to be Paul Harvey!

The rest of the story, is that Denny has a key switch that is still connected to airframe ground. The best way to wire P-leads is with single conductor, shielded wire. Use the center conductor to connect the P-lead terminal to the correct tab on the switch. Then use the shield as a conductor to connect the mag body to the ground terminal on the switch. With no connection to airframe there is no path for cranking power to back feed through the mags/P-leads.

Web

It seems I learn every time Web types about electrons!! I will pull that switch this winter and see what was done with the connections. THANKS Web!!
DENNY

 

[wireweinie]

When you pull the switch, let me know and I'll print up new P-leads for you.

Web