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Mercruiser ignition coil with resistance wire

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Imafisher47

New member
Anyone have a wiring diagram for the ignition coil on 4 cylinder 3.0 I have a feeling the purple resistance wire was not on the right side of the coil, I keep burning them up, my manual doesn't show this. I put everything the way I got it and the resistance wire is on the negative side and I feel like that isn't correct
 
The resistance wire is the power to the coil, it goes to + (plus).
There may be a second wire to plus + and that should be coming from the on starter solenoid. ( this is to supply voltage when cranking only).
There should be Two wires to negative -
If modern wire colors, they should both be grey i believe.
 
kghost said:
The resistance wire is the power to the coil, it goes to + (plus).
There may be a second wire to plus + and that should be coming from the on starter solenoid. ( this is to supply voltage when cranking only).
There should be Two wires to negative -
If modern wire colors, they should both be grey i believe.
Click to expand...
I thought so, purple and white to positive and brown and black to negative, so I'm guessing not only the coil is burnt up but the condenser in the distributor since that wire was on the positive side too which is black
 
Imafisher47 said:
Anyone have a wiring diagram for the ignition coil on 4 cylinder 3.0 I have a feeling the purple resistance wire was not on the right side of the coil, I keep burning them up, my manual doesn't show this. I put everything the way I got it and the resistance wire is on the negative side and I feel like that isn't correct
Click to expand...
I'm having the same co cert with mine but it has been switched to electronic ignition and I'm not sure how it should be wired up either I'm getting no spark from new coil marina said that it should have a coil with internal resistor however the one I removed when it was running didn't have an internal resistor in it
 
Imafisher47 said:
Anyone have a wiring diagram for the ignition coil on 4 cylinder 3.0 I have a feeling the purple resistance wire was not on the right side of the coil, I keep burning them up, my manual doesn't show this. I put everything the way I got it and the resistance wire is on the negative side and I feel like that isn't correct
Click to expand...
 
Purple wire goes to coil +

Any purple wire is Ignition on power.

Grey (tach) and wire to point ignition (distributor) is Coil -

What ignition is it you have now?

Point ignition uses a resistor wire and a external resistor coil.

Electronic ignitions typically use a internal resistor coil.
 
I think a sticky note is needed for all new threads- there's nothing to use in order to make a good recommendation in far too many of these.

Engine make, model, year of production, Serial Number........
 
Imafisher47 said:
Anyone have a wiring diagram for the ignition coil on 4 cylinder 3.0 I have a feeling the purple resistance wire was not on the right side of the coil, I keep burning them up, my manual doesn't show this. I put everything the way I got it and the resistance wire is on the negative side and I feel like that isn't correct
Click to expand...
Are you actually using the purple wire from the ignition switch to energize the coil, or another one that may have 12V on it constantly?

Purple is ignition, resistance wire is for saving the points, which burn if the voltage is too high. While cranking, the battery voltage drops to about 10.5VDC and the ignition switch has a separate wire for cranking that doesn't pass through a resistor, then the purple wire has voltage after cranking, at the typical voltage while running/charging. The resistor then functions to drop the voltage so the points will survive.

Do you have and know how to use a multimeter? Use that to find out if it's wired correctly, using the correct wiring diagram for your engine and boat.
 
Retiredno1 said:
I'm having the same co cert with mine but it has been switched to electronic ignition and I'm not sure how it should be wired up either I'm getting no spark from new coil marina said that it should have a coil with internal resistor however the one I removed when it was running didn't have an internal resistor in it
Click to expand...
wire the ignition according to the directions for the electronic module used...
 
I've just found a partially scorched section on the Purple Wire (that's hanging by the starter motor area), which I think goes from the alternator to the coil, or possibly the ignition - I need to open the harness to trace it.

I found this post while beginning to look in to the issue, and I see that there is mention of two wires to the positive side of the coil. Mine only has one Purple Wire to the positive (12v), and one Black Wire to negative (earth). I was going to simply replace the Purple Wire once fully traced, but i've found reference that it's a "resistance wire" and should be a specific length :unsure:

I've tried looking up wiring diagrams for the 4.3L, but I can only seem to find ones for the 3.0L (see attached PDF), none explain the requirements of the Purple Wire (length, AWG, etc).

Could anyone help clear this up please?

1. Should my coil have more than one wire on the Positive side?
2. Does the Purple Wire need to be a certain length (for resistance)?
3. What is the AWG of the Purple Wire - 12 AWG?
4. Anyone have a wiring diagram for the 2004 Mercruiser 4.3L Carb version, or are they all the same?

Thunderbolt V Ignition and my serial number is: 0W034153

Thank you
 
1. Coil negative is NOT earth ground. It is simply coil negative. The wire attached goes to igntion control module.

2. You have an electronic ignition so the coil only gets One Purple wire.Old point ignitions had two and one was a resistance wire (point ignitions only) Some of your year may have two wires on coil + and one purple wire may feed the electric choke. Or there may be a wire coming out of harness near choke.Either is OK.
 
kghost said:
1. Coil negative is NOT earth ground. It is simply coil negative. The wire attached goes to igntion control module.

2. You have an electronic ignition so the coil only gets One Purple wire.Old point ignitions had two and one was a resistance wire (point ignitions only) Some of your year may have two wires on coil + and one purple wire may feed the electric choke. Or there may be a wire coming out of harness near choke.Either is OK.
Click to expand...
Thanks @kghost, I thought I might be missing a load of wires :oops: ... few! lol. I presumed the Black was an earth, i'll have to double check that next time. I haven't moved it, but thought it went to the earth, maybe it went into the harness wrapper.

kghost said:
Where is this damaged wire attached?
Click to expand...
That I still need to find out, I'll be working on the boat next week and will open up the harness and trace the wire both ends. Weather turned today so only got so far.
 
This is what I have. I m not 100% sure it is for your year engine but it may be close enough for reference purposes..


Screenshot 2025-10-04 123130.png
 
kghost said:
Where is this damaged wire attached?
Click to expand...
Hi @kghost, I can confirm the slightly damaged Purple wire goes from the alternator (through the DELPHI plug) and then hangs by the starter motor, then in to the harness, (which I can't get to easily at the moment to open), but based on the wiring diagram it should go to the coil. My coil has one Purple wire + and one Grey wire - attached only. Thanks for helping me check that.

IMG_2706-min.JPG


The damage to the wire looks like external damage, perhaps by being to close the exhaust's at some point in time. It's slightly blackened and bubbled, It's a bit hard to see in the photo (below).

IMG_2707-min.JPG


I will look to replace it, does anyone know what AWG size this Purple wire should be? I need to find the equivalent in metric (or larger) as AWG size is quite hard to find in the UK.
 
_jp said:
Hi @kghost, I can confirm the slightly damaged Purple wire goes from the alternator (through the DELPHI plug) and then hangs by the starter motor, then in to the harness, (which I can't get to easily at the moment to open), but based on the wiring diagram it should go to the coil. My coil has one Purple wire + and one Grey wire - attached only. Thanks for helping me check that.

View attachment 36312

The damage to the wire looks like external damage, perhaps by being to close the exhaust's at some point in time. It's slightly blackened and bubbled, It's a bit hard to see in the photo (below).

View attachment 36313

I will look to replace it, does anyone know what AWG size this Purple wire should be? I need to find the equivalent in metric (or larger) as AWG size is quite hard to find in the UK.
Click to expand...

Just for clarity, the wires go TO the alternator- the purple one is used to create (excite) the field winding inside. Without that, it won't generate voltage/current.

I wouldn't worry as much about the color of the insulation but if it can be routed farther from a heat source, do that and use some kind of wrap for protecting it from high temperatures. If it's brittle, you can splice it, but that needs to be done in a way that will add no resistance, so solder/heat shrink with cement and the wrap for high temprature.

Conversion chart for wire size-

AWG to mm2 | Gauge to mm conversion

American wire gauge (AWG) to mm and mm2 conversion calculator, chart and how to convert.
www.rapidtables.com www.rapidtables.com
 
Thanks @jimn, I'm ok with soldering, etc. But, I'll leave "as is" for now then and just wrap it up and reroute it a little, if it's not considered that bad.

The problem I have is that (in the UK) I can't seem to get hold of an AWG gauge to check the Purple wire's thickness in order to find the mm2 equivalent. Wondered if anyone knew/know's the AWG size off the top of their head? I've read conflicting sizes it's 12AWG and 14AWG
 
Absent of having your wire in my hands, it has to be either 14 or 16 gauge wire.
So if you are only replacing a small section go with 14 gauge (mm2 equivalent) the thicker of the two and you will have nothing to worry about. If you are replacing a full length of say 3-6 feet of wire, I would still use the 14 ga and you will be fine.
Now, you should get a good quality specified automotive grade wire at a minimum. If you can easily find marine grade then get that.
 
Thanks @kghost. Getting marine grade wire isn't actually too bad here. but getting the correct colour is a challenge - marine grade Red or Black is in abundance. I was going to special order the right colour if I could find it and knew the size. I'd rather go bigger at 14 AWG if that's the case
 
I was just rereading the post showing the purple and red wire at Altenater.

Red is battery voltage sense and purple is excite. I believe the purple wire goes to a mechanical junction burried in the engine harness where other purple wires go to other locations on engine, like Coil + and electric choke.

I posted a schematic, look close to see what I speak of.

The larger 10 ga orange wire seen on the Altenator is the DC volt output to charge battery and I believe it connects at starter at the same point as the battery cable connects
 
_jp said:
Thanks @kghost. Getting marine grade wire isn't actually too bad here. but getting the correct colour is a challenge - marine grade Red or Black is in abundance. I was going to special order the right colour if I could find it and knew the size. I'd rather go bigger at 14 AWG if that's the case
Click to expand...
If there is a junk yard with boats go salvage some wire of the colors needed
 
From what I can find, Marine grage 14 ga stranded wire, the diameter of just the stranded wire (not including the outer coating) is .0641" = 1.628 mm

So 16 ga will be smaller inner wire diameter
 
kghost said:
I was just rereading the post showing the purple and red wire at Altenater.

Red is battery voltage sense and purple is excite. I believe the purple wire goes to a mechanical junction burried in the engine harOhness where other purple wires go to other locations on engine, like Coil + and electric choke.

I posted a schematic, look close to see what I speak of.

The larger 10 ga orange wire seen on the Altenator is the DC volt output to charge battery and I believe it connects at starter at the same point as the battery cable connects
Click to expand...

Oh right, I didn't realise there was a box within the harness (haven't opened it up yet)! I simply read the schematic as the Purple going to the coil. The orange wire does go between the Alternator and Starter, but that one looks ok, it's only the Purple one that looks scorched on mine.

kghost said:
If there is a junk yard with boats go salvage some wire of the colors needed
Click to expand...

Sadly, no boat junk yards here (nothing you can just walk in to). You would be hard to find automotive junk yards / scrap yards in the UK these days - Health & Safety 🙄. I did look up the cost of a replacement harness, which wasn't too bad, but only if I found more damaged. The delivery charge made it more than double the cost though.

kghost said:
From what I can find, Marine grage 14 ga stranded wire, the diameter of just the stranded wire (not including the outer coating) is .0641" = 1.628 mm

So 16 ga will be smaller inner wire diameter
Click to expand...

Thanks, I'll see what I can find.
 
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_jp said:
Thanks @jimn, I'm ok with soldering, etc. But, I'll leave "as is" for now then and just wrap it up and reroute it a little, if it's not considered that bad.

The problem I have is that (in the UK) I can't seem to get hold of an AWG gauge to check the Purple wire's thickness in order to find the mm2 equivalent. Wondered if anyone knew/know's the AWG size off the top of their head? I've read conflicting sizes it's 12AWG and 14AWG
Click to expand...

IIRC, it's 16 or 14 gauge but if you can get the equivalent to 14 gauge, it won't be a problem. Since it will need to be spliced in two spots anyway, you might as well do that in a convenient place since the damage is limited and not the full length of the wire.
 
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