Merc 470 coil problem

[Boat guy 71]

Hi all, I am new to the Merc 470 and it seems to have a lot of non typical stuff. It runs like a top for maybe a half hour then runs rough and can even die. Have it narrowed down to a voltage drop on the coil +. Get the same drop on the solenoid+. 10.50 volts runs fine, 10.05 it doesn’t. Randomly fluctuates between there. Cool it off and runs fine again. Coil ohms was 2.5 and a 2 wire choke. Anyone had this issue?

 

[nola mike]

Hi all, I am new to the Merc 470 and it seems to have a lot of non typical stuff. It runs like a top for maybe a half hour then runs rough and can even die. Have it narrowed down to a voltage drop on the coil +. Get the same drop on the solenoid+. 10.50 volts runs fine, 10.05 it doesn’t. Randomly fluctuates between there. Cool it off and runs fine again. Coil ohms was 2.5 and a 2 wire choke. Anyone had this issue?

Your coil should be getting less than 10v.

 

[achris]

.... Have it narrowed down to a voltage drop on the coil +. Get the same drop on the solenoid+.

Have a look at a wiring diagram, you're measure both those points through a resistive wire... Of course you'll get lower voltage, that's what is supposed to happen.

In this diagram, '1' is the power in. Runs up to the connector on the choke (point '2'), then through a resistive wire (that dashed line). Points 3 are both the same, measure a voltage on either end of a (zero resistance) wire and it will be the same anywhere along the wire...

 

[SternDriveEd]

So it looks like your ignition system uses breaker points. Old school but dependable if you maintain it. With the key on and engine off, Coil positive voltage check should be 5 1/2 to 7 1/2 volts with the points closed. Coil negative voltage should be less than .2 volts with the points closed. That’s two tenths, which shows you that the breaker points are in good shape and are making good connection to ground. Lastly, when the engine is cranking over, the coil positive gets full battery power from a special wire connected to the relay. So if battery voltage goes down to 10 volts for instance, the coil positive should show the same voltage during cranking. By the way, for all these voltage tests keep the negative voltmeter lead connected to a good ground metal surface. Hope this helps.