Coil overheating

[robshepherd]

Hello All,

In a previous post last month I thought I had resolved my intermittent stall issues by replacing my ignition switch. That thread is here:

http://www.moyermarine.com/forums/showthread.php?t=7371

The problem persists, and now it seems that the coil is overheating and causing a shutdown after about 15-20 minutes. Motor runs wonderfully up to this point, then misses a few times, then gets really rough and dies. The coil will be almost too hot to touch after this event.

Right now I can't get it started, though I've got spark at the points -- but the spark from the coil wire when tested on the block appears very, very weak. Yesterday I replaced the coil AND condenser, but may have already damaged them to the point that one or both won't work.

Before I solicit help on the coil issue (and assuming I can somehow get her started again...) I have a couple of basic questions for you all. My goal is to get it started again and see if the coil overheats with the alternator disconnected from the coil, since I think I've ruled most everything else out.

1. How do I check the alternator's voltage output while running?
2. How do I check voltage while cranking?

I do have a multi tester. Just not so good (yet!) with this piece of equipment.

I'm ready for EI, but want to resolve this overheating coil issue before introducing more issues.

Thank you for any help! Kind regards, Rob

[hanleyclifford]

Sounds like you have fried the new coil most likely thru excessive voltage at coil +. The alternator is probably putting out 14 volts+ and if that is being delivered to coil+ the coil won't last long. If you can get the engine started you should immediately check voltage at coil +. If it exceeds 12 volts be prepared to put resistors in front of that terminal to bring that voltage down. Resistors can be found at Radio Shack cheap in 1, 2 and 6 ohm. If possible get a coil with internal resistance of 3 to 4 ohms. The alternator output should not go to coil +, nor the sensing wire

[robshepherd]

Two questions about voltage

Sounds like you have fried the new coil most likely thru excessive voltage at coil +. The alternator is probably putting out 14 volts+ and if that is being delivered to coil+ the coil won't last long. If you can get the engine started you should immediately check voltage at coil +. If it exceeds 12 volts be prepared to put resistors in front of that terminal to bring that voltage down.

Thank you! Two questions for you:

1. If the alternator is putting out more than 12 volts, shouldn't I have it looked at and corrected?

2. How do I check voltage at the coil+ while running? Do I simply put the positive side of the multi tester on the coil+ and ground the multi tester to the battery- ? Apologies for such a basic question!

Regards, Rob

[hanleyclifford]

Thank you! Two questions for you:

1. If the alternator is putting out more than 12 volts, shouldn't I have it looked at and corrected?

2. How do I check voltage at the coil+ while running? Do I simply put the positive side of the multi tester on the coil+ and ground the multi tester to the battery- ? Apologies for such a basic question!

Regards, Rob

Once the engine is running you can test alternator output voltage by putting ther meter + lead on the output terminal and grounding the negative anywhere on the block where you get a good connection. Alternator voltage of 14.1 to 14.3 is considered ideal by most but that much voltage will fry a coil. You are correct on #2. If you are running alternator output to coil+ that should be changed. Alternator output should go to the main buss bar or batt switch. Try testing voltages around the engine and it's components and you will see that voltage is not uniform throughout the dynamic system.

[romantic comedy]

Rob, alternators charge at about 14.3 volts. 12 volts would not charge a battery to full. A fully charged battery should read 12.6. The voltage to the coil (or resister) should be battery voltage.

Yes you measure voltage at the coil the same way all the time. One lead to the + side of the coil, and the other lead to ground.

[sastanley]

rob,
even though you may get the engine started again, it is possible the coil is no good. Likely, the heat caused an internal short that comes back to haunt you as the coil heats up and then it shorts out and the engine dies after running for 30-45 minutes. After the coil cools off the engine will often run again for a little while before the cycle repeats.

I highly recommend the "coil input information" thread --> here

It may help to answer some of your questions and then we can answer the remaining ones.

[robshepherd]

I highly recommend the "coil input information" thread --> here

Whoa.... Just spent the morning reading that thread/tome. I can see I need to take some readings today and figure out how much resistance I need to add, as well as get the alternator output off the coil +.

Kind of scratching my head though why I haven't had these kind of problems over the past few years with this boat. If anything we motor less than we used to.

I will be back with questions I'm sure. I also need to research how to wire alt. output to my 3 position battery selector. It's not obvious to me.

Thank you for the input! Regards, Rob

[sastanley]

Rob, There are a couple different ways to wire the charging circuit.

The simplest is to run a sufficient gauge wire (I think my boat had #8 for 35amp alt., but bigger is always better) from the output post on the alt. to the big terminal on the starter. The other end of that big cable should be connected to the common post on the battery switch. This will charge whichever battery you have selected on the switch.

One caution..when charging in "ALL" or "BOTH", a bad battery can suck the juice out of the other good battery, so I generally do not recommend that you run in that configuration..I would charge one battery at a time..if you set the throttle all the way to idle the alt. usually isn't putting out any charging amps and it is safe to shift between batteries but NEVER EVER thru "OFF", or you will blow the diodes on the alternator...

If you added in isolators or electronic charge controllers, etc. things would be connected differently, so we'll leave those out for now.

[robshepherd]

Possible resolution on coil overheat (with hopes!)

Boy, what a learning experience. I want to write up what I learned because it seems possible that my coil was overheating for a reason that was not mentioned in the 12-page thread on "coil input".

A couple of "bookkeeping" items first. What I thought was a coil output wire on the + terminal of my coil was actually the exciter wire. Ignorance is bliss... I learned this because I took my alternator in for a bench test. It's generating 14 to 14.1 volts, and the gent even checked the amperage draw on the exciter wire when I explained what I was working on. That wire is drawing 0.1 amps from the coil. He gave my alternator a clean bill of health.

After frying a brand new coil with 15 minutes of run time, and also after reading the coil input thread, I purchased another coil with the intent of getting the motor started so I could take the appropriate measurements so I could determine the size of additional resistor to add. I expected the motor to fire immediately with the new coil, but she didn't. Back to checking for spark, etc. All good in the primary ignition, but I could not get a spark from the coil wire to the block, when the day before I could. After much head scratching I finally noticed that the end of the coil wire was charred black. I thought "ah ha!", and immediately tried the test by substituting a spark plug wire for the apparently bad coil wire. HOLY SPARK from the coil, and now I know that the spark I was previously observing was very, very anemic. Again, ignorance is bliss. I fashioned a new coil wire from extra length of a plug wire, put it all together and she fired immediately and ran very smoothly. I should also note that I decreased the points gap from .019 to .017.

Is it possible that I had a damaged, or slowly failing wire between the center of my coil and center of distributor? Could this be the cause of my coil overheating and failure? I didn't suspect this because the wire set is a good one and only 1 year old. Before replacing this wire the motor seemed to run "fair", but now it's running perfectly. No occasional misses, AND it has a LOT more power. I can only estimate, but I would say 40% or more power than previously. It's dramatic.

Ok. The MEASUREMENTS

3 ohm internally resisted coil
Coil voltage w/ key in on pos: 12V at coil +
Coil voltage at cruise RPM: 14V at coil +
This yields a 4.66 amp system at cruise--higher than recommended, it seems

THEN

3 ohm int. resisted coil, with .85 ohm resistor ahead of coil + (multi meter indicates 1 ohm of resistance here)
Coil voltage w/ key in on pos: 9V at coil +
Coil voltage at cruise RPM: 12V at coil +
This yields a 3 amp system at cruise, nicely below the 4 amp maximum

So, with all this being said, and the motor running perfectly for a couple of 45-minute sessions, the coil is still VERY hot to the touch. I don't have a heat gun like some members, so I can't provide better data.

Would it be worth trying more resistance ahead of coil + now, or wait for a while to see how the coil fares? Any thoughts?

Thank you for taking the time to read this, and I look forward to comments!

Regards, Rob

[ndutton]

Nice progress.

I'd like to see some heat measurements before we get all excited. Please check out Harbor Freight Tools and pick up an infrared thermometer.

http://www.harborfreight.com/automotive-motorcycle/diagnostics/non-contact-infrared-thermometer-with-laser-targeting-60725.html

I have a theory about that coil wire and its relationship to the subsequent overheating coil but it's entirely speculative, not worthy of this forum.

[hanleyclifford]

If you are cranking at 9 volts and the engine starts you are about as low as can be achieved and still start the engine. Your cruising voltage of 12 volts does leave some room for additional resistance. However, if you do this you may get cranking voltage below 9 volts and not be able to start. The solution may be an override or bypass circuit via the "R" terminal on the starter which is only energized when the key is all the way over to "start". A coil that is too hot to touch does not augur well for the long term if you plan on cruising.

[robshepherd]

Nice progress.

I'd like to see some heat measurements before we get all excited. Please check out Harbor Freight Tools and pick up an infrared thermometer.

Oh boy... but I kind of knew this would be coming. I'll pick up a thermometer. After I'm done with the coil I suppose I could use it to calibrate the oven temp dial to the actual oven temperature. Haha.

I notice that my coil bracket is metal, and attached to the block. That HAS to transmit heat from the block to the coil through the bracket. If it's a problem now, why wasn't it (presumably) over the past 40 years?

I think that I'd like to try moving the coil to an alternate position away from the block and give it the "touch test" tomorrow. It was on the list for today, but the weather was so darn nice we just HAD to get out for a bit! Picture below with Yuki at the helm and the tall ship Lady Washington in the background. They got into irons shortly after this picture!

[robshepherd]

Cranking voltage vs. "on" voltage

If you are cranking at 9 volts and the engine starts you are about as low as can be achieved and still start the engine.

Thank you. The only measurement I don't have is voltage at the coil + while cranking. My 9v measurement is just with the key in "on" pos, but engine not running nor cranking.

I can get some help tomorrow by using a "cranker" (friend) and "measurer" (me).

If I understand correctly, the measured voltage should be higher when cranking than it is with key in "on" pos. -Rob

________________________________________________

Ok. Misunderstanding on my part. I read the following by Hanley and missed the whole bypass concept. Yes, it makes intuitive sense that voltage should be lower while the starter is doing its job.

"Kelly - Sometimes, especially in cold weather, engines can have trouble starting because cranking voltage can be as low as 9 or 10 volts. If ignition voltage is being further reduced by resistors ahead of the coil it is possible that insufficient voltage will be available to fire the coil. The remedy? Bypass the coil resistor during the cranking cycle by running a wire from solenoid "R" which is activated only when cranking. Once the key (or button) is released "R" goes dead and the coil must draw thru the resistors."

[robshepherd]

Analog vs. Digital meter

Guys,

It occurs to me that my voltage readings may not be as good as they could be. I'm using an older analog multimeter--the type where you select a range (ex X1, X25, X100), and as such, it's not easy to get a perfectly accurate reading due to the shared scales for the different ranges.

Is this important? It seems like it probably is since the percentage difference between these small numbers (before adding resistance and after) can be rather large.

Would I be better served by a digital unit? I really love analog stuff. Just a disclaimer there...

Thanks, Rob

[hanleyclifford]

I liked analog cell phones.....but seriously, you need a good digital meter to do this exercise properly.