Balky coil?

[JimG]

While motoring between ports today, the engine was running fine, for about two hours, then stopped suddenly - no coughs, etc. just like someone hit the ignition switch.

There was no wind and the lee shore is not friendly so I was in a bit of a panic. I quickly found (by removing a plug wire and holding near a head bolt) that there was no spark. I then checked at the coil output (the same way) - and still no spark. I verified that the points were working (by watching the coil pin with a voltmeter). I wiggled the seemingly tight connections, re-plugged the coil to distributor wire - still no spark. I've only had the boat for a year, and a coil wasn't in my spares.

So we tried sailing for 30 minutes or so, then I re-checked everything - still nothing. I then made sure the points were open, then using a short piece of wire, tried grounding the points connection to the coil. After a few tries, I finally saw a little spark on my jumper wire (makes the capacitor look good), and the coil was now sparking to ground. I replaced the coil secondary, and it started right up and ran another three hours to my new berth.

Any ideas what this was? Anyone ever experience a coil that was temperature sensitive? I'm unable to examine my points since the distributor cap is held on with a totally corroded screw which I'll have to drill out or something.

I'm obviously going to expand my spares supply to include a coil and points, but I'd sure like to know what actually happened!

[Don Moyer]

Jim,

Old coils with faulty insulation (from prior overheating or mechanical damage) are known to open up when their internal temperature gets to a certain level, but in these cases the engine will usually behave very erratically - short periods of stopping and starting, backfiring, etc. Because coils are rather difficult to test, it's usually cost-effective (in time and money) to simply replace old coils.

Here's a Q&A that we prepared on this subject which addresses other (and usually more frequent cause(s) of your symptoms:

Q: What would cause my engine to intermittently shut down as though some one accidentally turned off the ignition key?

A: If your engine intermittently shuts down as though someone accidentally turned off the ignition key, a loose connection within the primary ignition circuit is usually indicated.

The primary ignition circuit starts at the big battery cable on the starter solenoid, and then runs to the ignition switch. When the ignition switch is turned to "On" or "Ign", the circuit continues back to the positive terminal of the coil. A faulty connection within this circuit that opens up now and then will seem to the engine exactly as though some one really did turn off the ignition switch.

Good places to check for a breakdown in this circuit include:

1) Quick disconnects.

2) Terminals on the coil and ignition switch.

3) The ignition switch itself.

4) The short black lead from the negative terminal of the coil to the distributor. This short black wire is actually the negative leg of the primary circuit, but if it develops a faulty connection, it will shut the engine down in exactly the same way as a break in the positive leg will.

Don

[Triton#335]

Jim,

Both myself and a marina neighbor have had a distributor cap held on with a corroded screw. On the back side of course. Not a very high grade of metal used on distro caps and they corrode easily. A good soaking in PB blaster and I was able to work it out with some needle nose vice grips.

My neighbor had to break/sacrifice his cap. Once gone he could get some large vice grips on the screw.

From now on we put a dab of white grease on the screws. No corrosion.

Sincerely, Dave

[JimG]

Thanks all.

I know the original question is pretty much unanswerable, but I was able to confirm I was getting 12v at the positive coil pin (and there's a firmly secured crimped lug) and that the points pin was 'operating'. The coil is firmly clamped to ground, and the high voltage leads seemed well seated. All the connections are a little rusty, so I guess I must have wiggled something enough to get through the crud...or drawing the arc on the points pin restored the connection to the coil primary.

My next boat will have a removable hood over the engine so everything doesn't require a contortionist!

[Kelly]

Jim,

I saw this post a little late but just wanted to add one thing: I, too, had an intermittent engine shutting down as if someone had turned off the key suddenly. The engine would start again, but only after having cooled down for about an hour.

I suspected the coil but the problem turned out to be an untrustworthy capacitor. This is an easy and inexpensive part to replace, thankfully (as long as you can get into your distributor).

My two cents...

Kelly

[Don Moyer]

Kelly,

Thank you so much for reminding us that a condenser failure can be another cause of sudden shutdowns. I'll be sure to add this note to our list of potential causes of sudden shutdowns.

Don