Mercruiser 165/470 cooked a coil

[camper4lyfe]

I have a 1987 FourWinns with the dreaded 4cyl 165/470. Last summer the points burnt up and I replaced the points, condenser, rotor, cap, coil, wires, and plugs. It was due for a tuneup anyway. After all that, it ran like a top. We didn’t run it a whole lot after that, but it ran fine through the rest of last summer and the few times we ran it this year.

Yesterday it burned up a coil. I’m getting battery voltage to the + side of the coil, which to my understanding shows that the resistor wire is bad, so I went with an internal resistor coil. I replaced the coil and it’s running again (I have not taken it for a test drive yet, as the weather’s not looking great at the moment). On the surface it’s fixed, but I’m not convinced the whole problem is fixed. What would cause the coil to go bad?

One thing I did notice, while it was running at idle after replacing the coil, the tach jumped from 800rpm to 2000rpm. I don’t know if that’s an indication of anything or not, but it’s connected to the coil so maybe it’s something?

One additional bit of info that may or may not matter, I replaced the water cooled voltage regulator with a single wire alternator. The trigger wire for the alternative is attached to the + side of the coil. When we were testing voltages yesterday, we would get 12v at the + of the coil with the alternator wire disconnected. As soon as we connected the alternator, that voltage dropped to 8. Again? I’m not sure if that’s an indication of anything, but it’s some added info.

 

[Eric 3.7LX Alpha 1]

I have a 1987 FourWinns with the dreaded 4cyl 165/470. Last summer the points burnt up and I replaced the points, condenser, rotor, cap, coil, wires, and plugs. It was due for a tuneup anyway. After all that, it ran like a top. We didn’t run it a whole lot after that, but it ran fine through the rest of last summer and the few times we ran it this year.

Yesterday it burned up a coil. I’m getting battery voltage to the + side of the coil, which to my understanding shows that the resistor wire is bad, so I went with an internal resistor coil. I replaced the coil and it’s running again (I have not taken it for a test drive yet, as the weather’s not looking great at the moment). On the surface it’s fixed, but I’m not convinced the whole problem is fixed. What would cause the coil to go bad?

One thing I did notice, while it was running at idle after replacing the coil, the tach jumped from 800rpm to 2000rpm. I don’t know if that’s an indication of anything or not, but it’s connected to the coil so maybe it’s something?

One additional bit of info that may or may not matter, I replaced the water cooled voltage regulator with a single wire alternator. The trigger wire for the alternative is attached to the + side of the coil. When we were testing voltages yesterday, we would get 12v at the + of the coil with the alternator wire disconnected. As soon as we connected the alternator, that voltage dropped to 8. Again? I’m not sure if that’s an indication of anything, but it’s some added info.

The trigger wire for the alternator should be getting full 12 volts when running, which shouldn't be off the (+) of the coil, you could get it from the purple wire on the electric choke. The wire between the choke and coil should be the resistance wire which should lower voltage down to something like 8 or 9 volts to extend the life of the points. Having wrong voltage on the coil that should have a resistor on it either ballast or resistance wire I would think would also shorten it's life.

 

[camper4lyfe]

The trigger wire for the alternator should be getting full 12 volts when running, which shouldn't be off the (+) of the coil, you could get it from the purple wire on the electric choke. The wire between the choke and coil should be the resistance wire which should lower voltage down to something like 8 or 9 volts to extend the life of the points. Having wrong voltage on the coil that should have a resistor on it either ballast or resistance wire I would think would also shorten it's life.

I got thinking about it more, and I think the resistance wire went out last year which lead to the points burning up and everything else going south. By replacing everything but the wire, it left the problem in place by only treating the symptoms which lead to the coil burning up this year.