Mercruiser 1986 mcm 170 coil arc is weak

[Sizzlin]

Bought a new "old style" coil for this boat and had some inquiry after doing a test on the coil. So, for anyone familiar with this, obviously, you know that the ignition coil is cylindrical in shape and has positive and negative terminals on it with a spot for a wire coming out of the top. This wire coming from the top center of the coil goes to the distributor.

So, I manually fired it with a bench test jumper cable method. From the battery, I connected a wire from negative battery terminal to the ignition coils negative terminal. Then, I took the wire coming out of the top of the coil and put it in my left hand and hovered it next to a ground source (a shiny bolt on the engine). Next, I picked up a cable which I had prior to all this connected to the positive battery terminal. I then executed the manual fire test. As I touched the positive cable coming from the battery to the ignition coil positive post I transferred spark throughout the coil and up through the top cable in my left hand. The result was a spark passing though this and creating a tiny arc going to the ground bolt, simulating a spark that is supposed to go to the distributor.

So, the ignition coil does work. However at the end of this line the arc or spark is supposed to be much brighter and stronger.

I tried using a different wire coming out of the top of the coil and I still had the same result.

Why is the spark or arc of the spark so tiny and weak??? it's supposed to be wicked bright.

 

[alldodge]

Not real sure it will make much difference but the switch lead should be on the Negative side, not the Positive side.

 

[achris]

Not real sure it will make much difference but the switch lead should be on the Negative side, not the Positive side.

It does make a considerable difference. A coil is an 'auto-transofrmer' (as opposed to an 'isolating transformer'), and requires the positive to still be attached when the negative is opened. The windings of wire inside the coil charge up, and if it's the positive that is opened, the spark produced travels the wrong path.

Sizzlin, repeat your test, but use all the exisiting wiring on the engine. Just take the lead out of the centre post of the distributor cap, plug an old spark plug in, lay it on a clean part of the engine (good ground) and crank the engine normally. You should see a strong blue spark across the plug gap. If you do, then your coil, points and the associated wiring are good to go. Next plug the lead back into the distributor cap and do the same test with one of the cylinder spark plug leads. Same result (strong blue spark) and that verifies the cap and rotor.

Chris.......

 

[stonyloam]

Yeah Chris is right. The coil (a transformer) produces a high voltage pulse (spark) when the lines of magnetic force collapse and generate voltage in the secondary windings. Those lines of force are generated by the current passing through the primary windings. When you remove the negative (ground) the current stops instantly and the field collapses instantly generating maximum voltage in the secondary. If you remove the positive with ground still connected you get feedback into the primary and changes in current flow that affect coil efficiency.