Ignition Coil Failure 1997 Merc 5.7 Thunderbolt V

rpdwyer

Seaman Apprentice
Joined
Apr 22, 2012
Messages
38
Hi.
Went to take my boat out for the first time this season and my port engine refuses to start... not even close as if it's not getting gas or not getting spark. I look into the carb and it's getting gas so I assume it's not getting spark. I remove the main coil wire and use a screwdriver from the socket to a terminal to look for spark when I turn it over. There is none. So I swap out the coil (which is about 2 years old) with a backup coil I have that was taken out of service when I was having ignition issues last year on the other engine. Starts right up...accelerates great... everything appears good to go.

I come back a couple of hours later with the intent of taking it out... port engine won't start. Same symptom... not even coming close to firing. Again, I check for spark on the main terminal and there is none.

What would cause two coils to die within hours of each other? Granted, neither is new and one was suspect to begin with... but that's still unusual since it ran fine the other day on muffs.

Before I go out and put a brand new coil on it, what should I be looking for as culprits that can kill a coil? I have not test the resistance on the coils yet... but that will only tell me if they are bad... not what is causing them to die. Volts when running appear to be just under 14.5v. My tach behaves normally for this engine as well. This coil has a 2 grey wires on the negative side and 1 purple on the positive side. Both coils had electrical grease on them.

Any info is appreciated as I want to minimize my chances of killing a brand new coil when I go to the boat tomorrow.

Thanks,

--Rick
 

dubs283

Vice Admiral
Joined
Jul 27, 2005
Messages
5,332
Re: Ignition Coil Failure 1997 Merc 5.7 Thunderbolt V

rare that one coil would fail, pretty near impossible two coils fail within an hour or so but stranger things have happened

here is a troubleshooting guide for thunderbolt ignitions:
 

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NHGuy

Captain
Joined
May 21, 2009
Messages
3,631
Re: Ignition Coil Failure 1997 Merc 5.7 Thunderbolt V

That's the right approach to take. It sure seems as if the coils were the symptom. now what's the problem?
My thinking is the remaining ignition parts. How good are the wires, sensor, cap rotor and plugs? All grounds and connectors clean and tight?
Does your bilge get super wet? It's a boat so a little dampness can be expected. But if it's constantly coated in moisture, that can short out even newer parts.
 
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