Finding the correct replacement coil?

[Finnerty]

My local Mercruiser mechanic suggested I replace my Blue Streak coil as it was not suited for my engine.
Problem is that we have tried two different coils (PerTronix 40,000v and another brand I can't remember) and they both produce the same result, only starts at high rpm and stalls below 1700 rpm.
1977 Mercruiser MIE 330
S/N 5551947
IGNITOR EIectronic Ignition ML-181
Blue Streak coil (MUST USE AN EXTERNAL RESISTOR)
What does an external resistor look like?
Not sure what to do next.

 

[Bt Doctur]

guessing its a points ignition?

 

[Fun Times]

Like mentioned by Bt Doctur, we'll/you'll need to know if you have point or electronic type ignition.

Here's your main parts catalog for this engine model, http://www.mercruiserparts.com/selectSerailRange.asp?doc_nbr=330+MIE+GM+454+V-8+1977-1980

Point, DISTRIBUTOR ASSEMBLY (STANDARD IGNITION)

And electronic type ignition, DISTRIBUTOR ASSEMBLY (THUNDERBOLT IGNITION) And WIRING HARNESS, SWITCH BOX AND IGNITION COIL

 

[Finnerty]

guessing its a points ignition?

The specs I gave in my original post are for the existing set-up.
That is it's an IGNITOR ML-181 electronic ignition, using a Blue Streak coil that says an external resistor must be used.

 

[Eric 3.7LX Alpha 1]

Looking up the Ignitor ML-181 electronic ignition... it comes up as a Pertronix conversion kit for a Points ignition.
https://www.google.com/webhp?source...safe=off&q=IGNITOR+ML-181+electronic+ignition

 

[Finnerty]

Looking up the Ignitor ML-181 electronic ignition... it comes up as a Pertronix conversion kit for a Points ignition.
https://www.google.com/webhp?source...safe=off&q=IGNITOR+ML-181+electronic+ignition

Exactly, but the PerTronix 40011 coil they recommend to use with it causes the problem I mentioned in my original post.

 

[Eric 3.7LX Alpha 1]

Have you checked if you have full 12-volts on the (+) terminal on the coil when the key is in the "run" position?

The points system has the coil get powered from 2-different wires..... during cranking the engine with the starter..... a wire off the starter solenoid powers the coil with full 12-volts ONLY when cranking the engine. When you release the key to "run" the coil gets powered thru a resistance wire (usually purple) which lowers the voltage down to help the points last longer.

With a Pertronix kit, you have to bypass/remove the purple wire and provide a (12-volt) source thru the ignition switch, usually off the electric choke on the carb.

Try a jumper wire from the (+) battery to the (+) on the coil and run it with the pertronix coil.

 

[Finnerty]

I'll try that Thanks!
Any suggestion where else I can run an ignition switched wire from (I've got a non-electric choke)

Have you checked if you have full 12-volts on the (+) terminal on the coil when the key is in the "run" position?

The points system has the coil get powered from 2-different wires..... during cranking the engine with the starter..... a wire off the starter solenoid powers the coil with full 12-volts ONLY when cranking the engine. When you release the key to "run" the coil gets powered thru a resistance wire (usually purple) which lowers the voltage down to help the points last longer.

With a Pertronix kit, you have to bypass/remove the purple wire and provide a (12-volt) source thru the ignition switch, usually off the electric choke on the carb.

Try a jumper wire from the (+) battery to the (+) on the coil and run it with the pertronix coil.

 

[Finnerty]

Well I solved the problem, thanks to the help here.
I took a reading off the positive side of the coil with the ignition in the run position and got 4 volts.
Although there was no sign of a ballast resistor, something was reducing the voltage so I started peeling the old wrap off the wire run coming off the alternator and discovered that the wire leading to the coil was actually a resistance wire spliced into the purple wire coming off the alternator. This was what the wiring diagram for my engine showed although I had been confused as I didn't realize the colors of the wires had faded outside the electrical tape wire wrapping.
I spliced in a new non-resistive wire off the purple wire from the alternator and the engine has never started and ran so well.
I installed the coil recommended for the Ignitor I electronic ignition, which is the 1.5 ohm 40,000 V Flamethrower coil.
Lessons learned:
The Mercruiser wiring diagram for my specific engine was correct.
The wire color had faded outside the electrical tape wrap on my wiring confusing initial diagnosis of the problem (eg the purple wire was now brown).
The installation instructions on the coil were correct.
These are simple facts that my licensed Mercruiser mechanic did not consider.

 

[Eric 3.7LX Alpha 1]

Glad to hear it fixed your issue! :encouragement:

 

[gm280]

When you come across a resistive wire, it is merely a "ballast resistor" circuit to control the current to the primary side of the coil. And it is there to limit the total current through the points to the primary side of the coil to reduce point and coil burn. While it may read less voltage, it really is limiting the current and therefore the voltage seems reduced when reading it at the coil positive side to ground. And that is because of the low resistance with the primary side of the spark coil. That resistance is usually less then an ohm. If it was one ohm at twelve volts, it would pass 12 AMP of current through both the points and the coil windings. So a resistive wire is needed to lower the amount of current through that primary winding to protect both the coil and the points from burning up as well. You can prove this to yourself by reading the voltage at the primary coil positive side to ground and then disconnect that wire from the coil and read it again. It will read 12 volts or what ever the battery voltage is supplying... And because of that point current and point burning up, they came up with Electronic ignition. They uses an electronic switch (point) circuit so the points are out of the picture then. And the technology gets even more complicated from there so I'll stop here...