1970 7.5 hp Mercury Coil Compatibility

Cessna150

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Mar 23, 2017
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May I use a 339-832757a4 coil instead of a 336-4409a4 coil on a 1970 7.5 hp Mercury outboard? The coil I would like to use is newer and half the cost. The only obvious difference in appearance is the lack of the braided grounding wire that would ground to the mount through the coil cover. The newer one is apparently grounded to one of the terminals. If I had to, could I just clip the grounding strap and bend it up to ground it to the cover like on the old one? Or could I simply use it as is? Thanks for any help.
 

Cessna150

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Mar 23, 2017
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How are the newer coils designed differently from the old ones? Is the only difference to where it is grounded?
 

flyingscott

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It's more about the difference in the ignition systems the 2 different coils are designed to run on.
 

flyingscott

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Do what you want it's your motor. Wait till you see what the stator costs if your damage it.
 

Cessna150

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Mar 23, 2017
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Now we are getting somewhere. That is the kind of information I am looking for. Will the stator actually be damaged if I use the newer coil? Would changing where it is grounded to keep that from happening? Why would it be damaged?
 

flyingscott

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I have worked on several Phasemaker ignitions. Here is the three things I have seen on them.

#1 is the wires disintegrate from the stator. That is the most common.
#2 is people use the kill switch while it is on muffs which destroyed the stator.
#3 is they come in with 2 different coils on them and no spark needing a stator. I can't say for sure it was the problem but I don't think it's a coincidence either.
Over 1/2 of the ones I see never get fixed because they cost to much.

If you don't have spark on one cylinder have you checked the points.
 

Cessna150

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Mar 23, 2017
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I just bought it a while back. Yes, I should have insisted on trying it first, but I didn't. Wires and everything else look pretty good. When we did try it, my son said he saw a spark in one plug, but now neither spark. The points were a bit dirty, but I cleaned them. Resistance on the coils is bad though. However, I read somewhere else about a guy who said his coils failed the test, but when he replaced the stator, they worked. I will test the stator for voltage today, to be sure.
 

Cessna150

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Mar 23, 2017
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Can anyone tell me about the actual differences in design/output between the two coils in my original post?
 

flyingscott

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Get a manual you need to understand the ignition you are working on. Good luck with your motor
 

Cessna150

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Mar 23, 2017
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O'Reilly Auto Parts can order the newer of the two for less than half the price of the old kind. It was around $45.
 

Cessna150

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Mar 23, 2017
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I ended up finding some good deals on the correct parts on ebay. I am still interested in knowing the difference in coils if anyone has that knowledge.
 

schematic

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Jan 12, 2008
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1,102
the difference is where the secondary winding gets its ground. your gets its ground from engine ground (bracket). The other cheaper coils get their ground from the + side of the primary winding. You cannot interchange. It will fry the switch box if you try...
 
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