Ignition coil overheating - Mercruiser 140

[boozer1966]

Re: Ignition coil overheating - Mercruiser 140

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



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

I hear ya. If I hooked up a 12 volt wire directly to the coil, the engine would run, but the coil would eventually overheat, hence the need for the resistor wire in the first place. I just can't figure out why I would have 7.5 volts from the resistor wire with the ignition on and engine not running, and 11+ volts at the same resistance wire with the engine running. That seems to be a problem that is upstream from the coil. But I will switch out the coil and see what happens. Any idea on where to buy resistance wire now that I have it exposed???

 

[alldodge]

Re: Ignition coil overheating - Mercruiser 140

I would get the coil first and if that fixes the issue then get this ballast resistor
Mallory Resistor For Coil 9-29900 - iboats
Ignition Coil - Sierra - iboats

Looks like the coil cost twice as much as I thought it was, up around 40 for the coil.

 

[Rob_NC]

Re: Ignition coil overheating - Mercruiser 140

...a question does the starter solenoid have a start circuit to the coil is it possible the start contact has fused and is sending continuous battery voltage to the coil while running ? If so it will overheat.

 

[boozer1966]

Re: Ignition coil overheating - Mercruiser 140

...a question does the starter solenoid have a start circuit to the coil is it possible the start contact has fused and is sending continuous battery voltage to the coil while running ? If so it will overheat.

I checked that. The wire coming from the starter is only powered when the key is turned to the start position. Thanks.

 

[boozer1966]

Re: Ignition coil overheating - Mercruiser 140

Any other ideas about why my coil is burning up?

 

[bruceb58]

Re: Ignition coil overheating - Mercruiser 140

If you have a resistor wire, I am assuming you have a points ignition. What is your dwell set to? If your dwell is too high, your coil can overheat.

 

[JustJason]

Re: Ignition coil overheating - Mercruiser 140

^^^^^^ Agree.

Also, which plugs are you running, at what gap? And how old is the cap, rotor, and plug wires. Is there any corrosion or pitting on the contacts?

 

[bruceb58]

Re: Ignition coil overheating - Mercruiser 140

^^^^^^ Agree.

Also, which plugs are you running, at what gap? And how old is the cap, rotor, and plug wires. Is there any corrosion or pitting on the contacts?

Those were my next questions as well.

 

[boozer1966]

Re: Ignition coil overheating - Mercruiser 140

Those were my next questions as well.

NGK BR6FS gapped at .035"
Brand new rotor, cap and wires
dwell set at 32 degrees

 

[boozer1966]

Re: Ignition coil overheating - Mercruiser 140

Just to satisfy my curiosity about it being the coil, I bought a new one and plan on replacing it if and when I can replicate the symptoms.

 

[bruceb58]

Re: Ignition coil overheating - Mercruiser 140

NGK BR6FS gapped at .035"
Brand new rotor, cap and wires
dwell set at 32 degrees

That's good unless damage was done before those items were changed. Very rarely do coils go bad but maybe that's the case here.

 

[JustJason]

Re: Ignition coil overheating - Mercruiser 140

Out of curiosity does your tach work? Is the gray wire still hooked up at the tach, or is that disconnected?

 

[LilRedNeckGirl]

Re: Ignition coil overheating - Mercruiser 140

have you checked the coil spark wire coming out the top@ spark plug variety? is it arcing? that could sure heat it up. past that, I would change out the coil. the voltage issue is beyond me, but a col swap is a few bucks and that, if nothing else, eliminates one potential issue/cause.

 

[boozer1966]

Re: Ignition coil overheating - Mercruiser 140

Tach works well. I thought maybe it was grounding out the coil, but it works fine. I also cleaned all the connections to it.

 

[JustJason]

Re: Ignition coil overheating - Mercruiser 140

You might want to start from scratch here. 11 volts isn't going to hurt the coil. If your misfiring when hot, check the timing at idle with a timing light. If the timing is fine, then it is not an ignition problem. If it is, then chances are it is just a bad condenser.

 

[boozer1966]

Re: Ignition coil overheating - Mercruiser 140

You might want to start from scratch here. 11 volts isn't going to hurt the coil. If your misfiring when hot, check the timing at idle with a timing light. If the timing is fine, then it is not an ignition problem. If it is, then chances are it is just a bad condenser.

Timing is good at idle and advance mechanism works well too. is there any way to check to see if something is grounding out the electrical system? Is that a possibility?

 

[JustJason]

Re: Ignition coil overheating - Mercruiser 140

Have you checked the timing when it is actually misfiring? Or when the engine is running fine, as it's 2 different things.

is there any way to check to see if something is grounding out the electrical system? Is that a possibility?

If you have a meter capable of doing it, you can check to see what the coil is pulling for current, and see if it increases when your having problems.

 

[alldodge]

Re: Ignition coil overheating - Mercruiser 140

Timing is good at idle and advance mechanism works well too. is there any way to check to see if something is grounding out the electrical system? Is that a possibility?

When the points are closed, this current goes directly to ground. When current flows from the ignition switch, through the windings in the coil, then to ground, it builds a strong magnetic field inside the coil. Current is limited by the ballast resistor and the primary windings. Every time the points open, the flow of current is interrupted through the coil, thereby collapsing the magnetic field and releasing a high voltage surge through the secondary coil windings.

If all is correct the voltage drop across the resistor will be around 7V and a bit higher when running. If the resistor increases or uses one with more ohms (R), then the voltage (E) drop will reduce along with current (I). The voltage being measured at the + terminal is the voltage drop across the ballast resistor and the resistance of the primary windings. If either the primary windings of the coil or the ballast resistor increases so will the voltage being read. If the resistance was decreasing or going to ground the voltage will reduce and the current would increase. Ohms law E=IxR

 

[boozer1966]

Re: Ignition coil overheating - Mercruiser 140

I get my cookies (and my ohms, I think), off the low shelf.

 

[JustJason]

Re: Ignition coil overheating - Mercruiser 140

Can I offer a suggestion? Throw the exiting coil away, replace the resistor wire with a regular wire and get an high energy coil. You won't even have to worry about it again and the engine will run better.

Is part of that suggestion keeping the points?