1974-76 mercury 7.5 coil wiring

[Mason Sowell]

I was about to swap coil positions to see if the problem was the coil or the stator, if the problem followed the coil, when I noticed the wires were reversed" the top cylinder coil has the green with white stripe on the negative terminal, the solid green wire is on the positive terminal, (the top cylinder is the one it's running on) the lower cylinders coil has the green with white stripe on the positive terminal, and the solid green is on the negative terminal, is this the correct position for the wires to be in? ( the lower cylinder is the cylinder that's intermittent, is there a diagram out there for this motor, would just swap the wires around on the coil thats acting up but my luck it would toast the switch box/ what I would call the cdi, thanks for any info!

 

[Rmoore45]

Re: 1974-76 mercury 7.5 coil wiring

I think the wires are switched. I went out and looked at my '78 merc 9.8, the 7.5s big brother and the green w/ white stripe is on the negative side on both of the coils. The solid green wire is on the positive side of the top cylinder coil. The darker wire (brown?) connects to the + side of the bottom cylinder coil. My engine runs fine except when cold then it misses on 1 cylinder (not sure which).

 

[Texasmark]

Re: 1974-76 mercury 7.5 coil wiring

Look at the part numbers of the CDI/CDM's. If identical you can switch positions to troubleshoot. The reason that interconnect wires are different color codes is the way Merc develops the voltage necessary to get 40kv to the plugs. They essentially wire the primaries in series developing twice the primary voltage for the high voltage transformer (charging capacitor that stores the energy that smacks your plugs) in the CDI/M without having to have stators with windings that can withstand twice the voltage....cost thing.

Swap the CDL/M's and run your tests.

Mark

 

[Faztbullet]

Re: 1974-76 mercury 7.5 coil wiring

Wires are switched, solid green goes to +....

 

[Mason Sowell]

Re: 1974-76 mercury 7.5 coil wiring

Good morning," and thanks to all! I'll have to do my switching after work, local boat center, gets really confused when I tell them my serial # 3803381 same # that's on the transom clamp as well as stamped on the lower cylinder. He started mumbling something about $75 a coil, y'all know of another place to get them cheaper? I'll pay it , but dangit, didn't give but $ 150 for the whole motor, if I knew it then, I woulda went with a Honda 8 hp fourstroke on CL for $600, oh well" I'm learning. "life", thanks again! Y'all have a good one!

 

[Rmoore45]

Re: 1974-76 mercury 7.5 coil wiring

I have a couple spare coils for my 9.8, I'd be willing to part with one for about $30.

 

[Texasmark]

Re: 1974-76 mercury 7.5 coil wiring

I have a couple spare coils for my 9.8, I'd be willing to part with one for about $30.

FYI, some folks like to replace all the coils because their reasoning is that if one fails the others are sure to follow. I have a triple cylinder and a couple of years ago I had a coil that didn't operate when cold. I replaced it and have had no further problem with coils....soooo maybe one of these is the answer.....if the coil is the problem, but you need to pay attention to what Fatzbullet said about the wiring error. Course the question is, how did that come about in the first place and when did it happen?

HTH,

Mark

 

[Mason Sowell]

Re: 1974-76 mercury 7.5 coil wiring

Dangit rmoore45 where was you at yesterday? Or last night, I ordered two coils along with stator and charging system off of eBay for $99.95 I did check with local boat shop prior to ordering. He said that the coils were interchangeable from 1974-1986. I'm just hoping that I can jump from the regulator straight to my trolling motor battery. My little motor has no charging system, pull rope only. And Texasmark I feel the same way you do about the wiring, I've been wondering why and when, judging by the way the previous owner and his son striped out the reverse cam, when reinstalling the lower unit along with a few other knuckle head repairs they attempted like the main jet in the carb it holds some type of plastic sleeve in the center passage of the carb between the choke butterfly and throttle, well the plastic sleeve was laid side ways only allowing throttle to open maybe an 1/8th", I'm thinking Coor's light beer might have been a factor in there cloudy repair adventure, Thanks again fellow boaters. Have a good one!

 

[Rmoore45]

Re: 1974-76 mercury 7.5 coil wiring

Its better that you got new coils. I found out yesterday that one of the coils was causing the engine to lose spark on the top cylinder when cold and idling, the other coil was good though. I'll just keep them in my parts box in the boat.

 

[Texasmark]

Re: 1974-76 mercury 7.5 coil wiring

Its better that you got new coils. I found out yesterday that one of the coils was causing the engine to lose spark on the top cylinder when cold and idling, the other coil was good though. I'll just keep them in my parts box in the boat.

Interesting you had the same type problem I had. I picked up a few parcels of info recently and seems a company who's name or initials is/are CDI either provides design assistance to Mercury or manufacturer's them for Merc. Having worked high voltage my entire career, 40kv open circuit voltage is a lot to control, especially in a marine environment. Handling this kind of voltage successfully requires someone who knows what they are doing and apparently CDI (whoever they are) has that expertise.

Mark

 

[Mason Sowell]

Re: 1974-76 mercury 7.5 coil wiring

Been trying to reply back on cdi and my progress on coils/ running well! Don't won't to write to much. Perhaps iboats thinks I'm to long winded, see if this will go through"

 

[Mason Sowell]

Re: 1974-76 mercury 7.5 coil wiring

It did" yay" company called cdi? New to me. Only CDI I know is, (capacitor discharge ignition) got coils in mail Saturday, idles now, good spark! Did reverse the wires, both greens to hot, green/white stripe to negative. Redoing transom wood now. Gonna keep it short. Makes my temples pulse when I type three paragraphs and it don't send. Thanks everyone! In case it's three more days before it will let me post, happy thanksgiving Christmas and new years.

 

[Mason Sowell]

Re: 1974-76 mercury 7.5 coil wiring

I'm back, been awhile hope everyone has missed me! Lol" 7.5 merc's been on a saw horse, swapping the coil wires helped for about thirty minutes of run time, ran it back to shore after fishing, on one cylinder burnt the switch box out or at least #2 side, got another switch box, and it's firing on both cylinders again. Scared to crank it due to the wiring of the coils, don't wanna fry another box! (costly) Went to local library found a manual, and sure enough they talk about everything but what your looking for. Was just wondering how Rmoore45's motors doing or should I say running? I remember it was missing on a cylinder and changed out his coil hows it doing now?

 

[Rmoore45]

Re: 1974-76 mercury 7.5 coil wiring

Changing out the coil helped my problem but it sometimes misses on 1 cylinder when cold; once its warmed up it runs fine.

 

[Texasmark]

Re: 1974-76 mercury 7.5 coil wiring

Changing out the coil helped my problem but it sometimes misses on 1 cylinder when cold; once its warmed up it runs fine.

Sneak your wife's hair dryer out of the house and before you start your engine, heat up the coil that is slow to fire if you know it, if not do one one day and the other another day after things have cooled back down. Don't go nuts with the heat before you start it. If you can put your finger on the plastic and hold it for about 3 seconds you are Ok.

Mark