Ignition coil overheating - Mercruiser 140

[boozer1966]

1984 Mercruiser 140, Alpha MR drive, serial no. 6853440 - I have been chasing an electrical problem for about a year now and am at my wits end. It is not a fuel problem.

Symptoms are as follows: Boat starts and runs great for 30-45 minutes, then develops a miss. Miss can be made worse by applying any electrical load, for example, trim motor, blower bilge, etc. Engine will eventually stop running and after a cooling off period will restart and run erratically. I took he boat out yesterday, thinking that I had found the likely problem - some corrosion in the canon plug. Same experience - 30-45 minutes of running well, then problems. This time, I touched the coil, and it was clearly too hot.

Voltage readings to the coil are as follows:

Ignition off - 0 volts
Ignition on, but engine not running - 7.5 volts
Ignition starting - 12+ volts
Ignition on, and engine running - 11+ volts

Can a resistor wire go bad? That's what it seems like is happening? All voltage readings seems to be within spec except for 11+ volts to the coil when the engine is running.

A) Is this making my coil overheat when the engine is running?
B) What do I do about it?

Thanks in advance for any responses.

 

[alldodge]

Re: Ignition coil overheating - Mercruiser 140

The resistor wire can go bad but it usually opens up and engine won't start/run. Your correct 11V is to high. How long have you had the boat? Does it look like the resistor wire has been replaced?

 

[boozer1966]

Re: Ignition coil overheating - Mercruiser 140

The resistor wire can go bad but it usually opens up and engine won't start/run. Your correct 11V is to high. How long have you had the boat? Does it look like the resistor wire has been replaced?

I have only owned the boat for a few years, and I would guess the wire has not been replaced. I traced back/opened up the wiring loom back a few inches, and it all looks original.

 

[alldodge]

Re: Ignition coil overheating - Mercruiser 140

I have only owned the boat for a few years, and I would guess the wire has not been replaced. I traced back/opened up the wiring loom back a few inches, and it all looks original.

There should be 2 wires on the + side of the coil. Pull both wires off, turn ignition to ON and measure voltages on both wires independently. Want to find out if someone bypassed the resistor wire and it is just now showing up

 

[stonyloam]

Re: Ignition coil overheating - Mercruiser 140

What is your battery voltage when running? Just make sure you are not overcharging. Should be about 14 volts.

 

[boozer1966]

Re: Ignition coil overheating - Mercruiser 140

Battery charges at 14 volts. All other systems are normal when the problem.

 

[boozer1966]

Re: Ignition coil overheating - Mercruiser 140

There should be 2 wires on the + side of the coil. Pull both wires off, turn ignition to ON and measure voltages on both wires independently. Want to find out if someone bypassed the resistor wire and it is just now showing up

The purple wire going to the starter solenoid shows 0 volts when the ignition is on and 12 volts when turned to start. You can also pull it off the coil with the engine running with no effect. The other wire on the + side of the coil shows 7.5 volts when removed from the coil and the ignition in the on position.

 

[alldodge]

Re: Ignition coil overheating - Mercruiser 140

The purple wire going to the starter solenoid shows 0 volts when the ignition is on and 12 volts when turned to start. You can also pull it off the coil with the engine running with no effect. The other wire on the + side of the coil shows 7.5 volts when removed from the coil and the ignition in the on position.

Sounds like everything is working as it should, may just need to replace the coil

 

[boozer1966]

Re: Ignition coil overheating - Mercruiser 140

Sounds like everything is working as it should, may just need to replace the coil

But with the engine running I get 11+ volts at the coil.

 

[alldodge]

Re: Ignition coil overheating - Mercruiser 140

But with the engine running I get 11+ volts at the coil.

Agree and if the coil was working correctly you should not get this reading, it should be lower. Could measure primary and secondary and see what you find. How to Check an Ignition Coil | eHow Also check to see if your coil is marked as external resistance is required

 

[boozer1966]

Re: Ignition coil overheating - Mercruiser 140

Agree and if the coil was working correctly you should not get this reading, it should be lower. Could measure primary and secondary and see what you find. How to Check an Ignition Coil | eHow Also check to see if your coil is marked as external resistance is required

Why would a bad coil cause a reading of 11+ volts on the + side of the coil and I only get that reading when the engine is running. I only get 7.5 volts on the + side of the coil with the ignition on and the engine not running.

 

[airshot]

Re: Ignition coil overheating - Mercruiser 140

Had a similar problem on my 83 3.0 and ended up finding an improperly wired battery isolator switch to make a long story short. After a complete rewire of the dual battery system and a new coil everything is great once again. Mine would take longer than your to go haywire mabey a couple hours and then not start for quite awhile, but when it restarted it ran great for another hour. Just a suggestion here. Good luck

 

[alldodge]

Re: Ignition coil overheating - Mercruiser 140

Why would a bad coil cause a reading of 11+ volts on the + side of the coil and I only get that reading when the engine is running. I only get 7.5 volts on the + side of the coil with the ignition on and the engine not running.

OK, start from scratch.
When the engine misfires pull a plug wire and see if you get good spark. If you don't, check points, plugs, wires and see if there is anything which is not allowing good spark.
When cool the coil may show 7.5V, once it gets hot the coil breaks down and does not supply correctly, hence the 11V

Edit: Looks like the issue has been resolved

 

[boozer1966]

Re: Ignition coil overheating - Mercruiser 140

I get different voltage readings on the + side of the coil depending on whether or not the engine is running. When running, the 11+ volts seems to be overheating the coil. How do I correct that? Can I install an automotive ballast resistor?

 

[alldodge]

Re: Ignition coil overheating - Mercruiser 140

I get different voltage readings on the + side of the coil depending on whether or not the engine is running. When running, the 11+ volts seems to be overheating the coil. How do I correct that? Can I install an automotive ballast resistor?

Think of current flow as water in a hose. A resistor is a restriction and should reduce the flow. You get full flow before the restriction, less then full after "if there is use". If the coil was using what it should, the voltage should reduce down, if it does not reduce the coil is not using voltage being supplied.

If the wire is pulled off the coil and measured with the ignition on should read battery voltage, or close to it. Your reading 11V with the motor running and it should be around 7 to 8V. The coil is either not using the voltage (current) being supplied or there is a bad connection.

A coil doesn't cost much, and while I cannot state this is your issue, it should be no more then the resistor, and the resistor I will say is not your issue

 

[thumpar]

Re: Ignition coil overheating - Mercruiser 140

Try this. Pull the wires off the coil on the + side. Put a jumper from a 12v source to the + side and start the engine. Now measure the voltages on the original wires to the coil. Make sure the wires are secured and won't touch anything while doing this. I wonder if somehow the full voltage from the starter is feeding when the engine is running.

The coil it self cannot pull more voltage than what is sent to it.

 

[alldodge]

Re: Ignition coil overheating - Mercruiser 140

The coil it self cannot pull more voltage than what is sent to it.

I'm trying to keep it as simple as possible without getting into current and load variables. Current is the amount equal to the load (more simplicity here), no load across a resistance equals no current and voltage equal to supply. So if your going to advise that I am in error. please explain how and I will concede

 

[thumpar]

Re: Ignition coil overheating - Mercruiser 140

Amps increase with lower resistance or the pull. Voltage comes from the supply. Ohms law.

 

[boozer1966]

Re: Ignition coil overheating - Mercruiser 140

I just want to figure out how to keep my coil from overheating.

 

[alldodge]

Re: Ignition coil overheating - Mercruiser 140

Amps increase with lower resistance or the pull. Voltage comes from the supply. Ohms law.

Exactly E=I*r or I=e/r and r=e/I :facepalm: Think about it a bit more

I just want to figure out how to keep my coil from overheating.

boozer, your coil should not be getting hot, it is. Your ballast resistor wire should be about 3 ohms. If you have 3 ohms across the wire and read 11 volts, the coil is the issue