7.4 Thunderbolt V coil voltage issue

[Celebrity 240]

1995 Mercruiser 7.4 was running fine and then suddenly shut off. After cooling off it would go for awhile and quit again. Did this several times on the way home. The coil would get very hot so I diagnosed it as being a bad coil. Put a new coil on and it ran for a few short outings and I assumed it was fine. Took a trip and got about 40 miles from home and it shut off again. Coil was blistering hot again. After several pit stops for it to cool made it back home and started looking for the real problem.

The only thing I can find is the voltage at the positive side of the coil is erratic, With key on I have 12.5 volts at the battery and at the coil. Start the motor and battery goes to 14.6 but voltage at the coil is all over the place from as high as 14.6 and down to 1.5 and everywhere in between.

Unhooked purple wire from coil and ran a jumper from battery to coil. With motor running I get a steady 14.6 volts to the unhooked purple wire but the coil running off of the jumper is still all over the map. I figure this unstable voltage is causing the coil to overheat and breakdown and then go again after it cools.

Checked all the grounds and found nothing. Any ideas what could be causing the unstable voltage. Sorry so long but I wanted to try to explain everything. Thanks in advance.

 

[Maclin]

Welcome Aboard!

*sorry misread the post advice would not be relevant**

 

[alldodge]

I am assuming the motor has a carb
What is the motor serial number?
What is the part number of the coil that replaced the original?

but the coil running off of the jumper is still all over the map.

A coil is constantly charging and discharging, which causes a spark (so to speak). Measuring voltage at the coil will not be steady for the average meter due to this factor

Place the voltmeter on the battery and measure the voltage as rpms increase. What is the variance found though the rpm range?

 

[Celebrity 240]

Have actually tried 3 coils all with the same results. The original Mercruiser coil. One from Sierra Marine that was supposed to be a direct replacement. And now a MSD Blaster. They will run for awhile and quit till it cools off. Voltage at battery is 12.5 till start up then goes to 14.6 through 4000 rpm. If run with a jumper I can measure voltage at the unhooked purple wire and it will show the same 14.6 through the rpm range.

 

[Celebrity 240]

And yes I forgot. Carb motor.

 

[alldodge]

Take a spark gap tester and look for the color of the spark

How old are the plug wires and plugs?
If plug wires loose resistance, this will cause the coil to heat up because there is less resistance in the wires. Plugs can have the same effect but not near like the wires. The ignition module can also add to the amount of current the coil will need to dissipate

 

[Celebrity 240]

Ok thanks. Probably be the weekend before I get a chance to check the spark. Plugs new the season. Wires (Mercruiser) cap and rotor new last year.

 

[alldodge]

Ok thanks. Probably be the weekend before I get a chance to check the spark. Plugs new the season. Wires (Mercruiser) cap and rotor new last year.

Then pull some of the wires and see what the resistance value you get and record length and ohms

 

[Celebrity 240]

Didn't get a chance to get down to the boat today but I will get those plug wires checked. Thinking about it all day I'm confused. Shouldn't the voltage at the positive side of the coil be constant same as the battery when running?

 

[achris]

Didn't get a chance to get down to the boat today but I will get those plug wires checked. Thinking about it all day I'm confused. Shouldn't the voltage at the positive side of the coil be constant, same as the battery when running?

It should. If you have fluctuations, check things like the key switch, the in-line fuse to the key switch, the 'kill' switch (if fitted), all the connectors in the engine to instrument harness plug/s. There's a voltage drop somewhere, you just gotta find it.

Chris......

 

[Celebrity 240]

Double checked all that and ran jumper from battery to coil with wire (purple) from switch unhooked. When running the unhooked wire has a steady 14.6 while the positive side of coil with the jumper still shows wacky voltage, Whatever is wrong has to be beyond the coil I would think. Thanks for reply.