mercruiser 140 hp no spark from coil

gatorG6

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Jul 1, 2013
Messages
6
Please help really want to take the kids fishing this weekend, having issue with getting spark from my coil and starting.
Issue started while boating a couple weeks ago idled rough then would not start / no spark. let it sit overnight and it fired right up and ran all day. Took out the folllowing week and it shut off for no spark again towed in this time. have been working on this for a couple of weeks with no resolve.
Sometimes the spark seams good from the coil and it starts right up other times (most times on the water) it doesnt spark at all or spark is very week and wont start the engine.
I have replaced the following: (kinda pokeing and hoping it is a 78)
Points / condenser
distributor cap / rotor
coil
key switch
starter selenoid
ground wire to battery
I have also read a ton of threads and have tried running a wire from the battery + to the coil + and rolled over to bypass the resistance wire, this did not work either



what voltage should I have on the + side of the coil when the key ****ch is on? and what would cause it to be low?

Thank You
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Re: mercruiser 140 hp no spark from coil

Voltage at the positive side of the coil will depend on whether the points are open or closed. It's all about voltage dividers. The coil is one resistance, the resistance wire is another. The point between the 2 is the positive terminal of the coil. If the points are open, no current flows, the voltage will be battery voltage anywhere on the circuit. With the points closed current will flow so the voltage on the positive terminal will be a proportion of battery voltage, determined by the ratio of the 2 resistances. Usually around 7 or 8 volts...

Pull the cap off and spin the engine over. You should see a small spark between the point contacts. If it's a large spark, you may have a condenser problem, no spark could be a supply or coil. If you have the small spark, then the problem lies in the high tension side of things, coil, leads, cap, rotor. Also make sure the starter solenoid is supplying full (12) volts during cranking.

As you have found out, throwing parts at it doesn't always (in fact seldom) fixes things.... Just makes the parts suppliers richer.

Chris......
 
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stonyloam

Vice Admiral
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5,827
Re: mercruiser 140 hp no spark from coil

Check your distributor ground. If the ground is making poor contact you will not get proper current flow through the coil = weak spark.
 

sti1471

Petty Officer 1st Class
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Messages
312
Re: mercruiser 140 hp no spark from coil

How have you tested the spark from the distributor/coil? Besides testing the spark by whether the engine starts or not, have you tested the spark visually?
 

gatorG6

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Re: mercruiser 140 hp no spark from coil

Tested the spark visualy by removing the plug wire from the distributor, putting a spark plug in the connector, rolling engine over while grounding the plug.
 

achris

More fish than mountain goat
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May 19, 2004
Messages
27,468
Re: mercruiser 140 hp no spark from coil

You need to test directly from the coil with a spark plug to the engine block. That will eliminate the rotor and distributor cap from the equation. The most effective troubleshooting procedures always work by halving the system. In this case halving the high tension side is checking for spark directly from the coil tower. And don't forget that the high tension lead from the coil to the centre post on the dissy cap can go faulty too.

Chris.......
 

gatorG6

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Re: mercruiser 140 hp no spark from coil

Thnaks all,
Checked the grounds, jumpered to good ground with no resolve.
testing directly from the coil with a spark plug to the engine block is how I am determining spark, tried 2 different wires to eliminate wire fault.


The spark issue is on the high tension side as far as I can tell No spark from the coil. Checking the + side of the coil with the key on I have about 5.5volts, when I roll the engine I have 8.5 - 10 volts but no spark from coil wire. After seeing the voltage to the coil as a test I ran a jumper from the + on the battery to the + on the coil to supply full 12 volts durring roll over, this did not produce spark from coil either??



Noticed while checking spark, when I first hit the key I would get spark, durring roll over no spark, then when the key is released I would see spark??? Other times nothing at all for spark.
Thought of a bad wire or contact but checked continuity on all wires and couldn't find a break / intermittent.
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Re: mercruiser 140 hp no spark from coil

Remove the tacho and shift interrupt switch wires from the negative side of the coil (leave only the wire going to the points), then re-check. If you still have no spark, then remove the wire that goes to the points and put a short length of 'scrap' wire on the negative of the coil.... Now strike that against a good ground. If that produces a spark, the problem is the points or condenser, or the breaker plate not grounded properly....

Chris......
 

gatorG6

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Jul 1, 2013
Messages
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Re: mercruiser 140 hp no spark from coil

Remove the tacho and shift interrupt switch wires from the negative side of the coil (leave only the wire going to the points), then re-check. If you still have no spark, then remove the wire that goes to the points and put a short length of 'scrap' wire on the negative of the coil.... Now strike that against a good ground. If that produces a spark, the problem is the points or condenser, or the breaker plate not grounded properly....

Chris......

Thanks Chris, I have removed the tacho and shift interrupt switch leaving only the wire going to the points with no spark.

I will try the grounding of the coil tonight, haven't tried that yet.
I was thinking the breaker plate grounding could be an issue too, how is it supposed to be grounded? I dont have any wires from the distributor other than than the ground for the condeser / points. I was assuming the breaker plate just grounded to the distributor wich in turn grounds to the engine block.
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Re: mercruiser 140 hp no spark from coil

...I was assuming the breaker plate just grounded to the distributor wich in turn grounds to the engine block.

That's the theory.

By doing the wire strike on the block you'll know for sure. If you still don't have spark with that test, the only thing in the circuit is the resistance wire and the coil itself.

Chris....
 

stonyloam

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Re: mercruiser 140 hp no spark from coil

This is the way I check (similar to Chris). Pull the center wire from the distributor and place it near bear metal on the block. Put a piece of insulating material between the points. Run a wire directly from the bat. + to the coil +. With a piece of insulated wire briefly jumper from one side of the points to the other. You should get a spark every time you remove the jumper. If you do you have bad points. If no spark jumper directly to ground. If you get a spark there then you have bad ground.
 

sti1471

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Messages
312
Re: mercruiser 140 hp no spark from coil

I am betting the coil is bad. Since it was working sometimes and not others on the lake it sounds that maybe the coil is getting too hot and not working correctly, and now it is just gone. I know it was just replaced, but I have had faulty parts right out of the box.

However other things you can check:

make sure that the red barrel looking item labeled "A" is there. This is a piece of rubber that is used to prevent the wire from grounding out. this item is easy to slip off, or be left out when putting the coil back together.

dist.gif
 
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Princeofrogs

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May 28, 2010
Messages
1
Check the main wiring Harness connector (usually behind the engine) and make sure it is not beginning to corrode. If the ignition wire is corroded it won't get fire to the coil.
 

Fishermark

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Priceofrogs - welcome aboard! This post is over three years old... I suspect he has the problem fixed by now. ;)
 
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