350 MAG MPI Ignition Coil

Mark V

Member
Oct 12, 2007
60
Lake Travis, Texas
Boat Info
2006 240 Sundancer
Engines
350 MAG MPI, Bravo III
I have a 2006 240 Sundancer with the 350 MAG MPI/Bravo III. It has 800 hours on it and it has never died or failed to start until now. I am the original owner.

While running fine on plane for 5 minutes, about 4000 rpm, it just died like you turned off the key. No Smartcraft beeps or fault message on the display when it died. Now It will not start. Starter cranks it OK, but it will not start, on rare occasion a single back-fire happens (boat is now on trailer, backfire seems to be out the exhaust, not out the throttle body).

A long list of troubleshooting has been done, no smoking gun. Smartcraft seems to be working as it beeps and displays fault messags as I disconnect sensors, etc during troubleshooting. Currenty focused on the ignition coil. Would like to rule it in or out before moving on.

The first day I did 2 quickie visual ignition tests: one holding the distributor High Voltage cable near ground while cranking the engine, and then with a spark plug grounded to engine. Both times sparks jumped as expected, but they did not look robust; kinda yellow, with some blue.

It's a lot to ask, but if someone has an ohm meter and a known good coil like mine, please take a look at the pdf I attached, and check resistances as I did and compare readings to mine. My local Mercruiser parts place does not have a new or used one to compare. Unfortunately, Mercruiser Tech Support does not have any check out values for this coil. Obviously ohm values on a coil are not a tell-all test, but they still may be helpful.

-Thanks in advance!
 

Attachments

  • 350 MAG MPI ignition coil.pdf
    172.4 KB · Views: 171
Was not a Mag but had the same symptoms occurre in mine. It turned out to be the ignition sensor in the distributor that had failed. It basically tells the coil when to fire.
 
Thanks Thornton. I'll see if I can bench test that one, and if so, will post the result.
I was able to do a bench check on the crank sensor and it passed.
 
Same thing happened to my mpi 6.2 right after i bought it. Turned out to be the ignition module as well. They are external on the mag mpi’s, not in the distributor
6DB1BD3D-8D34-46B9-84CB-5FBE4DB93B90.jpeg
 
Same thing happened to my mpi 6.2 right after i bought it. Turned out to be the ignition module as well. They are external on the mag mpi’s, not in the distributor
View attachment 107338
And in this case if being in a pinch and rid yourself of any doubt, 'for a lower cost way of throwing parts at it, you can find these at local auto parts stores as these are GM specific vs Marine....Try using a 1998 GMC 1500 5.7l engine at an auto store for a match on both/all 3 mentioned.

Do you see the RPM gauge moving to around 300 RPM while cranking?

The one sensor in the distributor if plugged in will be the CAM sensor and could do this as well.
 
Thanks Shenanigans. By coincidence, I just got back from Autozone. Used the same model GM engine as you suggested to compare my coil to a new one they had for the 98 GM 5.7 truck engine. All resistance values on the new coil matched the values I listed earlier for my coil, so for now I am considering the coil OK at room temp/under no stress.

Then I had AZone test my ignition module sd if it were from the same GM truck engine, and it passed.
So I'll press on assuming the coil and module are OK to at least start the engine.

RPM value on the LCD display reacts kinda slow, but I am seeing 150 then 250 max RPM when cranking with all the spark plugs in it. Voltage on the helm meter holds about 10.5 vdc during cranking. I think it sounds the same as before this problem, but maybe a new starter is in my future. Will check the power cable and engine ground.

Roger that about the lower cost of automotive items. Just wish there was a 100% trustworthy, published cross reference table - Hah ha, fat chance.

Planning to bench test the CAM sensor this evening, it is plugged in on mine, and another trip to Autozone if needed......
 
RPM value on the LCD display reacts kinda slow, but I am seeing 150 then 250 max RPM when cranking with all the spark plugs in it.
I've seen a low RPM signal being mixed with an back-fire type sound as being the CAM sensor in the passed... Hope you find the same...Good luck...
 
Thanks Shenanigans. By coincidence, I just got back from Autozone. Used the same model GM engine as you suggested to compare my coil to a new one they had for the 98 GM 5.7 truck engine. All resistance values on the new coil matched the values I listed earlier for my coil, so for now I am considering the coil OK at room temp/under no stress.

Then I had AZone test my ignition module sd if it were from the same GM truck engine, and it passed.
So I'll press on assuming the coil and module are OK to at least start the engine.

RPM value on the LCD display reacts kinda slow, but I am seeing 150 then 250 max RPM when cranking with all the spark plugs in it. Voltage on the helm meter holds about 10.5 vdc during cranking. I think it sounds the same as before this problem, but maybe a new starter is in my future. Will check the power cable and engine ground.

Roger that about the lower cost of automotive items. Just wish there was a 100% trustworthy, published cross reference table - Hah ha, fat chance.

Planning to bench test the CAM sensor this evening, it is plugged in on mine, and another trip to Autozone if needed......

Were you able to watch Auto Zone test your ignition module or did they take it in the back, then come back and say they tested it? Never heard of an auto parts store being able to test an ignition module.
 
Yes I was surprised, believe or not, Autozone had a nice professional looking electronic box made for testing ignition modules. It had various cables/connectors and a book and part number tables that they used to set it up. I wish I had gotten a closer look, (technical curiousity) but I did watch them hook it up and test my module, and all the indicators on the device lit green for "Pass". That much I saw. I may ask to see more details up close, as I will probably go back and buy a CAM sensor. I could not figure out how to test a CAM sensor at home and Autozone does not has a tester for that, nor for coils, nor for crank sensors.
So far I am hearing CAM sensor as a recurring theme from folks here.... inexpensive, worth a try.
 

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