Mercruiser 120 Coil Overheating Like Many Other Members

[zellerj]

Great discussion. IF the alternator is supplying 13.8 V, to get 4 amps to the coil you need a 3.45 ohm resistance in the circuit. Would the difference between 3 ohms and 3.45 ohms be enough to allow the coil to get too hot?

 

[thumpar]

FWIW. I ran a pertronix with an internally ballisted flamethrower coil on a 120 for 5-6 years before I sold the boat. It was still on the water last time I saw it a couple years ago. I would run 25 miles in 100+ temps and never had an issue with overheating the coil. I taped up the resistance wire and ran a new full 12v line in the wire loom alongside it to the coil.

 

[lognum]

It drops the voltage at the coil positive by a principle known as voltage dividing. When you have 2 resistances in series, between a positive voltage and ground, the voltage at the junction of those 2 resistors will be a proportion of their resistances.

For example, if you have 2 equal resistances between 12 volts and ground, the centre point will be 6 volts. With our coil example, if the coil is 1.5 ohms and the resistance wire is 1 ohm, the point where they join, the coil +, will be 7.2v. (1.5/2.5*12v).

We also have Ohm's law to consider. With a total resistance of 2.5 ohms, a maximum current of 4.8 amps can flow. But as a coil is an inductor, the current will take time to reach maximum.

The module senses the current flowing, and a coil is usually fully charged at about 4amps. The module will try to set up the 'on' time until it sees 4 amps flowing before it fires the spark. If you add resistors, then the maximum current will never reach 4 amps.. (anything over 3 ohms resistance in a 12 volt circuit will have less than 4 amps flowing)...This is why adding ballast resistors will make it worse.

Hope this helps...

Chris......

Thanks again Chris. So does the purple wire that normally goes to the alternator (12V grounded) have anything to do with the issue, now that it terminates nowhere and is just taped off, now that a self regulating alternator is in place? Cheers.

 

[lognum]

FWIW. I ran a pertronix with an internally ballisted flamethrower coil on a 120 for 5-6 years before I sold the boat. It was still on the water last time I saw it a couple years ago. I would run 25 miles in 100+ temps and never had an issue with overheating the coil. I taped up the resistance wire and ran a new full 12v line in the wire loom alongside it to the coil.

Thanks Thumpar. And did the engine have the original wiring schematic set up, with the original alternator, or a self regulating alternator?

 

[thumpar]

Thanks Thumpar. And did the engine have the original wiring schematic set up, with the original alternator, or a self regulating alternator?

Everything was factory until I converted to the Pertronix ignition parts.

 

[achris]

The wire that is no longer connected because you have a single wire alternator (all alternators are internally regulated), will have no affect on the ignition system.

Chris......

 

[thumpar]

Are you sure it is the coil overheating and causing no spark? Have you checked for spark when it won't start? Could be vapor lock or vent issues.

 

[lognum]

Thanks again Chris. Okay, so if the purple wire, that is supposed to be a resistance wire, is giving out 10.8-11.6 V, it would need a ballast resistor before the coil, no? It needs to be down to 7V does it not? Somehow the resistance is not there, which is why I changed to an internally resisted 3ohm flamethrower coil, that still gets too hot. A sister boat has a similar set up, with the old original alternator and a Mallory distribution (full piece, EI). He has the purple resistance wire on the + side of the coil and the same flamethrower 3ohm coil that I have. He has 6.3V on the positive and 4.2V on the negative side of the coil and it runs great.

 

[lognum]

Are you sure it is the coil overheating and causing no spark? Have you checked for spark when it won't start? Could be vapor lock or vent issues.

Thanks Thumpar, but all checked. Definitely coil. It starts and runs perfect, until 48-55minutes, especially when ambient temp is high, then rpms drop and it dies. Wait 5 minutes and get 10 minutes of run time, till it stops again. Wait five minutes and so on and so on........

 

[lognum]

Thanks again Chris. Okay, so if the purple wire, that is supposed to be a resistance wire, is giving out 10.8-11.6 V, it would need a ballast resistor before the coil, no? It needs to be down to 7V does it not? Somehow the resistance is not there, which is why I changed to an internally resisted 3ohm flamethrower coil, that still gets too hot. A sister boat has a similar set up, with the old original alternator and a Mallory distribution (full piece, EI). He has the purple resistance wire on the + side of the coil and the same flamethrower 3ohm coil that I have. He has 6.3V on the positive and 4.2V on the negative side of the coil and it runs great.

BTW, his coil only gets warm!

 

[thumpar]

Thanks Thumpar, but all checked. Definitely coil. It starts and runs perfect, until 48-55minutes, especially when ambient temp is high, then rpms drop and it dies. Wait 5 minutes and get 10 minutes of run time, till it stops again. Wait five minutes and so on and so on........

I know you say that it starts again after 5-10 minutes but have you checked for spark? Vapor lock can do the same.

 

[lognum]

Yes, thanks thumpar. Two mechanics also worked on it and thought they figured it out, but to no avail. It's an uneasy feeling, to think everything is alright, venture out in open water and then need to be rescued. Need someone smart with wiring and electronics. This only started when points were changed to EI. The issue remains the same and is just like others have posted. Coil gets too hot. Simple question; how do I stop that? The issue seems to be the purple resistance wire. Why would it not register 7V? I appreciate the input, but chasing down other possible reasons is what every other post ends up doing and I have yet to hear someone say that their coil is now running at the proper temp. It should not take placing it in a cooler area. There has to be a logical reason why I am still getting unrestricted voltage on a resistance wire. If I sound frustrated, it's because I am. I do not profess to be a mechanic or electrician, but I was able to get the engine running at least an hour at a time, which is a lot better than the guys I paid to work on it, by thinking objectively.
First coil, was a coil that needed external resistance, got too hot and stopped the engine. Purple, purple yellow and red wire from the ignition module on the +, grey tach and black wire from the EI module on the -. Then, from the advice in this forum, ran a 12v wire from the ignition, direct to the red wire on the EI module, kept the purple and purple yellow on the +, and the grey and black on the -. Coil got too hot to touch and cut out engine.
Second coil was internally resisted (Flamethrower 3ohm), did the same wiring, even trying the 12v lead from the ignition switch direct to the + of the coil, and same result. BTW, the motor was also changed from electronic gear shifting, to manual shifting with an interruptor switch, because of inconsistencies when shifting in and out of forward, neutral and reverse.
Right now, I have the wire from the ignition (12V) directly connected to the red wire from the EI module, the purple and purple yellow, on the + of the coil and the grey and black on the -. Engine starts great, runs smooth, gears shift fine. Ran it this past weekend, for an hour, pulled back in the slip, put it in neutral, went to tie the lines and it stalled. Wouldn't start. Tied off the lines (took 5 minutes), went back in, and it started perfectly, also shifting in and out of gear smooth. Coil was too hot to touch.

 

[lognum]

BTW, just got this message from Petronix;

Take the Ignitor red wire off the coil and remove the 12-volt power wire from the coil. Let the 12-volt power wire power the ignitor red wire off the coil. Let the resistor wire power the positive of the coil. This should keep heat out of the coil and still have a good running engine.
Your Pertronix Team,

 

[Fun Times]

What is all the voltage reading you're seeing at the coil compared to the wire combinations you have tried? Both engine on/running and off/key on.

Do you see around 13.8 volts while the engine is running at any of the power wire connections of the engine and helm gauge?

With the black wire disconnected from the alternator, is the alternator properly grounded now? May want to confirm it is.

Since you have the flame thrower coil that is internally resistant, it reads form pretronix that you shouldn't use the resistant purple wire but ran a direct 12v wire from the key switch.

But if you go back to trying the stock coil, you can use the purple resistance wire.....Which I'm thinking the purple resistance wire is no longer working correctly so it should be measured to know for sure it's status and/or repaired if you use a stock coil at all even if it's just for testing purposes.
https://forums.iboats.com/forum/engi...49#post1336749
https://www.justanswer.com/boat/31mk...lts-times.html
Overheating igntion coil - Marine Mechanic
RESISTOR WIRE KIT (TO REPLACE RESISTER WIRE) (#84-94227A2)


http://www.pertronix.com.au/troubleshooting.html

What type of coil can I use with the Ignitor? How do I check my coil's resistance? (12V negative ground only)


To determine if your systems coil is compatible with the Ignitor, some measurements should be taken prior to installation of the Ignitor. Caution… While performing this test, never leave the ignition switch on for more than 30 seconds at a time.

Set your voltmeter to a 15 or 20-volt scale. Attach an 18 or 20 AWG jumper wire from the negative coil terminal to an engine ground. Attach positive (red) lead of your voltmeter to the positive side of the coil, and the negative (black) lead to an engine ground. Turn the ignition switch to the run position. Now read the voltage at the positive coil terminal. Turn the ignition switch off. If the voltage measured is approximately 12 volts, no resistance wire is present. A typical resistance wire will provide 9 - 6 volts.

The next step is to determine the resistance in the primary ignition. Label the wires attached to the coil terminals and note their appropriate location. Make sure that the ignition switch is off and disconnect all wires from the coil. Adjust your meter to the lowest ohm scale. If you are using an analog style meter make sure to zero the needle.

Measure from the negative terminal to the positive terminal. Write your measurement down.

Now the maximum system amperage can be determined. Divide your voltage measurement by your coil resistance measurement. This will give you the system current or amperage.

Four and six cylinder engines should not exceed 4 amps. Eight cylinder engines should not exceed 8 amps. If the total amperage in your system is higher than the amount recommended for your application, you should install a ballast resistor.
Example:
Voltage 12
Resistance 1.5
12 / 1.5 = 8
Total amperage is 8

Pertronix Flame-Thrower[SUP]®[/SUP] Coil Installation Instructions

NOTE: REMOVE OR BYPASS EXTERNAL BALLAST RESISTOR OR RESISTANCE WIRE WHEN INSTALLING THE RECOMMENDED FLAME-THROWER COIL, UNLESS THE PRIMARY RESISTANCE IS LOWER THAN SPECIFIED OR IF YOU ARE USING THE STOCK COIL.

1. To remove a ballast resistor (normally white ceramic blocks 3 to 4 inches long), disconnect all wires on both ends of the ballast resistor. Remove the resistor from the vehicle and splice the wires together at a single point.

2. The resistance wire is located between the ignition switch and the firewall on most applications.
• Locate the resistance wire, cut it out, and replace with a 12-gauge copper stranded wire OR:,
• Bypass resistance wire, connect a 12-gauge copper stranded wire from a 12-volt switched ignition source to the positive (+) terminal of the coil.

Instructions on how to do Power and Ground Tests, and Voltage Tests

 

[lognum]

Thanks Fun Times. Will try your suggestions and advise. Greatly appreciated.

 

[thumpar]

The main reason there is a resistance wire is to protect the points from burning up and not to keep the coil from overheating. The reason you see all the people say they have a coil overheat problem with no solution is because that was not what was happening. 99 times out of 100 it is not a coil issue when people think it is.

I have had had the same ignition setup on the same engine you are trying.

 

[lognum]

It drops the voltage at the coil positive by a principle known as voltage dividing. When you have 2 resistances in series, between a positive voltage and ground, the voltage at the junction of those 2 resistors will be a proportion of their resistances.

For example, if you have 2 equal resistances between 12 volts and ground, the centre point will be 6 volts. With our coil example, if the coil is 1.5 ohms and the resistance wire is 1 ohm, the point where they join, the coil +, will be 7.2v. (1.5/2.5*12v).

We also have Ohm's law to consider. With a total resistance of 2.5 ohms, a maximum current of 4.8 amps can flow. But as a coil is an inductor, the current will take time to reach maximum.

The module senses the current flowing, and a coil is usually fully charged at about 4amps. The module will try to set up the 'on' time until it sees 4 amps flowing before it fires the spark. If you add resistors, then the maximum current will never reach 4 amps.. (anything over 3 ohms resistance in a 12 volt circuit will have less than 4 amps flowing)...This is why adding ballast resistors will make it worse.

Hope this helps...

Chris......

Hey Chris. If I put the stock coil, which requires external resistance, back on and remove the current 3ohm Flamethrower coil, can I put a ballast resistor on the purple wire, before the coil?

 

[achris]

Hey Chris. If I put the stock coil, which requires external resistance, back on and remove the current 3ohm Flamethrower coil, can I put a ballast resistor on the purple wire, before the coil?

As long as you have full 12v to the module, yes.

 

[fishrdan]

It sounds like you have something that's failing when heat soaked. One way to determine the bad part is to quickly cool it down. Apply a chunk of ice to the side of the coil to quickly drop it's temp and see if it will fire back up and run for 15-20 minutes. If cooling off the coil doesn't get it to start, apply the ice to the bottom side of the distributor to cool the module, then see if it will run. (Don't drip melt water all over where it shouldn't go, electrical terminals or inside distributor.) Canned air tipped up-side down works too, but ice is cheaper.

Since you say you have to let it cool for 5-10, then it will run again for a short time... If you take 1 minute to cool 1 component down, and it runs for 15-20 minutes, and you can repeat this, you can determine which component is causing (or not) the problem.

A timing light connected to the coil - cap high tension lead will tell you if you have spark, or not. And either confirm it's an ignition problem, or something else, vapor lock as thumpar mentioned.

 

[lognum]

Thanks Dan!