Wiring pertronics electronic ignition and coil on mercruiser3.0

Tacopete

Cadet
Joined
Oct 22, 2016
Messages
10
I have a 1975 mercruiser 4 cylinder. I have the pertronics igniter II (91146a) and flamethrower II coil (45111) that I will be installing this week. I need help with wiring. My current coil has two wires coming to the + lead. Both are tannish brown and both disappear into a taped up bundle, so it is hard to trace where they are coming from. I'm assuming that one may be a resistor wire, which I don't need anymore because I want a full 12v coming to the new coil. The customer support guy at pertronics said to just unhook and tape off the resistor wire to get full voltage to the coil. How can I figure out which wire is the resistor wire? I'm not good with wiring, I'm sure this is very simple to many people, and many people have to work around a resistor wire in this situation.

Any simplified suggestions would help. I'm thinking that if I just unhook one of the wires to the positive lead and see what I get for voltage, and then try the other one and see what I get for voltage, I might get my answer? If one of them gets me 12v and the motor turns over, and the other gets me nothing, then just unhook and tape off the one that gets me nothing?

Sorry. There is a lot on this topic, but I have been struggling to understand all of it.
 

bruceb58

Supreme Mariner
Joined
Mar 5, 2006
Messages
30,454
Take them off. One will have power with ignition on and the other one won't. The one that has power is the resistor wire.
 

stonyloam

Vice Admiral
Joined
Mar 13, 2009
Messages
5,827
Right, one comes from the Ignition, the other should come from the starter solenoid. Take a closer look at the wires, the ballast bypass from the starter should be tan in your "75" and the resistance wire will be solid Purple. Bruce is correct, tape off the one that HAS POWER with the key on. The other wire will only have power when the starter is turning. The older color code has White for 12v, so you will need to splice into a white wire, maybe follow the resistance wire if you can. Use a Stranded copper wire 18 ga should do it. Run it directly to the coil +. A solder connection is best, coat with Liquid Tape for a waterproof seal. I had to edit this since I was late seeing it was an older engine. Good luck.
 
Last edited:

mickyryan

Rear Admiral
Joined
Apr 18, 2016
Messages
4,210
I would use 16 gauge wire and use crimped connections with the heat shrinkable type
 

Tacopete

Cadet
Joined
Oct 22, 2016
Messages
10
Thanks for the input. This makes perfect sense. I will try tracing the resistance wire to its source and run a new copper wire....pertronics says to use 12g in the directions. Funny thing is both wires coming to + side of coil look the same color, I'll trace up under the taped up wire bunch to expose true color. But, like Bruceb says, check the power of each wire coming to coil. One should be zero(as long as only have the key to on and not start) and the other should be a reduced voltage. This all finally makes sense.

At sunrise, new igniter 2, flamethrower 2, cap, rotor, plugs. I'll let you know the results. Just bought the boat, it is running poorly (Boggs out and won't go above 2000 rpms under load. Was thinking it COULD be points, so just changing ignition setup all together. Could also be a carb issue...we will see.
 

bruceb58

Supreme Mariner
Joined
Mar 5, 2006
Messages
30,454
...the other should be a reduced voltage. This all finally makes sense.
It will be full battery voltage since there is no current going through the resistance wire hence no voltage drop.
 

Tacopete

Cadet
Joined
Oct 22, 2016
Messages
10
So, I have two wires coming off the pos. terminal of coil. A tan one and a light brown one. The tan gives me full voltage with the key in the on position. The light brown (I pulled back tape to identify try colors) gives me no voltage with the key in on position. Could I just leave the light brown one off and keep the tan hooked up since I want the full 12 volts to coil?
 

bruceb58

Supreme Mariner
Joined
Mar 5, 2006
Messages
30,454
No...you want the one with no voltage on the coil since its the one that comes from the starter solenoid. You need to add another wire which comes from a switched source of 12V from the ignition switch. Just so you know, the one from the starter solenoid is there to bypass the resistor wire while you are cranking the engine. You might as well leave it on since it doesn't hurt anything and you would need it if you ever went back to points when your Pertronix fails....yes they do fail.

The one you measured 12V on is coming through the resistor. The reason the one that comes through the resistor has full 12V is because there is no current going through it and is why there is no voltage drop.
 

Tacopete

Cadet
Joined
Oct 22, 2016
Messages
10
Also, the old coil that I am replacing says on the side "12v. No external resistor required". It doesn't have a name.
 

Tacopete

Cadet
Joined
Oct 22, 2016
Messages
10
So if I trace the resistor wire back to where it comes from, I can run a regular copper wire in its place ( still leaving the resistor wire "in place" but taped off at coil end in case I do go back to points?

Thanks for all the help bruceb58
I am a real rookie when it comes to this, but learning is better than not knowing.
 

Tacopete

Cadet
Joined
Oct 22, 2016
Messages
10
On my ignition, there are three options to run a wire. One says battery, one says ignition, one says solenoid. Does it sound correct if I just run a wire from the ignition terminal of the ignition to the coil then? Thanks again.
 

Tacopete

Cadet
Joined
Oct 22, 2016
Messages
10
Ok. I'll tap into the ignition wire, but closer to the coil instead of at the ignition to get my switched 12 v source.
 

bruceb58

Supreme Mariner
Joined
Mar 5, 2006
Messages
30,454
Also, the old coil that I am replacing says on the side "12v. No external resistor required". It doesn't have a name.
Interesting! Wonder if they left the resistor wire in there when they put in the coil. That could be most of your current issues.

Real easy way to test his is to put the wires back on, turn on the ignition, close the points and then measure the voltage at the positive side of the coil to ground. With the resistor wire in there, you would have 9V or so.

Instead of closing the points, you can also run a jumper wire between negative side of the coil to ground for the test
 

stonyloam

Vice Admiral
Joined
Mar 13, 2009
Messages
5,827
Yeah, sounds like the coil has been replaced. The stock coil circuit should have a total of about 3 ohms resistance, 1.5 from the coil and 1.5-1.8 from the resistance wire. The original coil may have been replaced with a 3 ohm coil, giving about 4.5 total ohms which may result in a weak spark and poor ignition. That tan wire was probably purple at one time but has changed color over the years due to heating, I know mine was not very purple anymore.
 

Tacopete

Cadet
Joined
Oct 22, 2016
Messages
10
Just finished. Ended up just running a wire all the way from ignition and taping off resistor.

With the old wiring and coil, when I would ground negative side of coil and measure voltage to positive while turning over motor, I was only getting 6 volts to coil.
 

bruceb58

Supreme Mariner
Joined
Mar 5, 2006
Messages
30,454
Yeah...they left the resistor wire in there AND had an internally resisted coil....no bueno. No wonder it didn't work well.
 

Tacopete

Cadet
Joined
Oct 22, 2016
Messages
10
Boat was new to me but not starting or running well. Got the electronic ignition. Boat starts better and runs well. I'm sure it would have if I just replaced the points too, but...I cannot compare because it wasn't running well before.
 
Top