Is there a definitive test for a bad coil?

[kaliona]

1978, 165HP Mercruiser I6. Fired up the boat at the launch ramp, no problem. Slow idled out passed the no wake zone, no problem. Hit the throttle, boat starts to get up on plane, then....Klunk, the engine just dies. Acted as if the coil wire to the distributor just popped off. So I lift the engine cover to check but everything looks ok. Try to start again but engine just cranks. Fuel is getting to carb, so I check for spark from the coil....nothing! Take the distributor cap off to look inside (for what? I don't know ). Had my buddy crank the engine and I can see the points opening and closing and sparking. Is that possible? With no spark from coil?
So I put the cap back on, check for spark from coil...and surprisingly this time I have spark. Button everything back up, go to start the engine...cranks, but no start. So I go back check for spark from coil, and again NO SPARK.
I take my pliers and give the coil a few whacks (out of frustration), check for spark...and surprisingly I GET SPARK. I quickly reconnect coil wire to distributor, turn the key, and the engine starts.
I ended up going back to the docks, running to the parts store for a new coil to have in the boat...just in case. Got back, fired her up went fishing all day, shutting her off and starting her up several times, no problems.
So my question is, do I have a bad coil? Can they go bad intermittently like that? Should I have gotten spark at the points with a bad coil? Did I move some wires or connectors that finally let the boat start? Would this problem have anything to do with my ignition cut-off switch? I really don't want to get stuck out on the water again.

 

[81 Checkmate]

Re: Is there a definitive test for a bad coil?

Id say you have a loose or corided wire somewhere. By the wack of the hammer you made contact again

Check all your wire conections...As Bono say's you have two ends to every wire.

Good Luck!

 

[kaliona]

Re: Is there a definitive test for a bad coil?

Would there be a spark at the points if the coil is bad? Obviously, I had the cap off in order to see the points which is why I'm confused about how the coil and points interact.

 

[Maclin]

Re: Is there a definitive test for a bad coil?

You should not have treated your coil like that as it only does what the points tell it to.

The points control the amount of spark the coil generates and also when it is available. The coil does not spark thru them. There are some good posts in the stickies topics about points ignition.

The coil is really a step-up transformer, has 2 sets of windings. When the points are closed voltage is sent to the primary part of the coil. Voltage goes thru the primary which creates a magnetic field across the secondary windings. When the points break the magnetic field collapses and that induces a higher voltage with less current in the secondary winding. The wire from the coil tower carries the high voltage from the secondary windings to the distributor.

When was last full electrical tuneup with new points, condenser, cap, rotor, point gap setting then dwell checked and points regapped as needed, and timing set? Oh yeah, new sparkplugs and maybe sparkplug wires?

 

[dingdongs]

Re: Is there a definitive test for a bad coil?

check your shift interuptor switch is good as sometimes the internals can play up which will short your coil as if you was changing out of gear and back to neutral.

 

[Maclin]

Re: Is there a definitive test for a bad coil?

You can test the coil fairly easily while all is hooked up, all you need is a jumper wire that can reach from the coil to a good engine ground.

Be sure there is not a gasoline spark hazard first on your engine. Read all of the below steps before proceeding so the procedure is understood. Also check for voltage at the coil positive terminal when the key is on, should be at least 8 volts. If no voltage is shown then this procedure will not work as it assumes the coil has good power when the key is on.

Remove the wire(s) from the coil negative terminal.

Unplug the big coil wire on the center of the distributor cap that goes to the coil tower, leave it connected to the coil tower.

Put the end of the big coil wire close to a ground.

Connect the jumper wire to the coil negative terminal.

Turn on the ignition, then touch the other end of the jumper wire to a good gound for half a second then remove, when you remove it from ground a spark should be seen coming from the big coil wire.

 

[Howard Sterndrive]

Re: Is there a definitive test for a bad coil?

my take on coils:
if in doubt, throw it out. They tend to act their worst when hot (i.e. farthest from shore), and then can test fine cold.

 

[Maclin]

Re: Is there a definitive test for a bad coil?

Replacing it works too. Now that it has been abused like that it is probably ready to go to a new home

 

[kaliona]

Re: Is there a definitive test for a bad coil?

Thanks for all the tips guys. I'll test the coil as Maclin says. Is the shift interrupter switch easy to get to and to test? That will be my next step.

 

[04fxdwgi]

Re: Is there a definitive test for a bad coil?

Connect a dwell tester to the neg side of the coil. If the coil has power to positive side, you crank the engine over, dwell meter moves and shows a dwell, and there is spark, chances are, the coil is good.

 

[stonyloam]

Re: Is there a definitive test for a bad coil?

do I have a bad coil? .

Probably not. You have a bad connection, or bad points.

 

[carguy001]

Re: Is there a definitive test for a bad coil?

I had some of the same issues with 470 @ the begaining of this year after i tuned it up with new points,wires,cap, ect had the boat out one time and the boat started missing over 3k rpm or if i used the trim. got it home re checked everything looked good. replaced the coil took it out 4-5 more times and had no issues.