How do I test my coil?

[Faerbairn]

1979 Mercuiser 140hp, Serial # 5332813

Mercruiser 140 Outdrive # 5337746

Standard UC-12 12v with External Resistor COIL

My problem: I turn over the engine and there isn't a spark at the plugs, nothing in the distributor, and nothing from the coil when the wirer is removed and checked within a 3/8" distance from ground. I show 12 volts at the + & - side of the coil. I tested the ohm reading from + to - and it reads 1.6. I tested the reading from + to the high tension center terminal and it reads 8.62.

Are these readings good? How can I see if I need a new coil? Who sells replacement coils for the Mercruiser 140hp engines?

Thank you for your help.

 

[Bondo]

Re: How do I test my coil?

What kind of condition are the Points in,..??..??....

To Test a Coil,........
With 12V on the (+) side,.....
Ground out the (-) side,......... That'll cause a Spark...........

 

[MercMonster]

Re: How do I test my coil?

As Bond-o said check your points for corrosion, or you may have a bad ground. Last engine I worked on, the points did have corrosion and on this engine, I had a bad ground connection at the block. So twirl that ratchet on a few bolts and wiggle some wires first before going out and replacing stuff. Of course, it wouldn't hurt to have a few odds and ends parts lying around while out on the water...

 

[Faerbairn]

Re: How do I test my coil?

With the ignition on I show 12 volts on the + side and on the - side. Is this how it should be?

 

[thrasher]

Re: How do I test my coil?

That's how it should be if the points are open. With the points in their current position, try connecting a wire connected to ground (negative) to the -ve terminal and see if you have a spark at the coil. You still need to make sure the points are operating correctly and bringing the -ve side of the coil down to zero volts when they close. If they don't then you have either bad points or a bad connection between the motor and the negative terminal of the battery.

Gary

 

[RWilson2526]

Re: How do I test my coil?

Dont know if yours is set up same as mine but my tachometer runs from the negative side of my coil. The tachometer had a short in it which gave me absolutely no spark out of my coil. I disconnected the tach wire and it sparked right up. If it should prove to be that, disconnect the tach at the gauge because when I disconnected at the coil I was also disconnecting the shift interupt switch which I found out on the water the next day.

Cant seem to find them now but there are some good trouble shooting diagrams for ignition systems here somewhere.

 

[Bondo]

Re: How do I test my coil?

The tachometer had a short in it which gave me absolutely no spark out of my coil.

Ayuh,........

When that's the case,.... There Isn't 12V at the (-) side of the Coil either.......

 

[Faerbairn]

Re: How do I test my coil?

Thank you everyone for your help. It is fixed.

 

[Bt Doctur]

Re: How do I test my coil?

So, tell us what the fix was .Others need to learn also.