1978 merc inline 6 165 - ignition resistance problem!

Hi Guys,

I have been having some ignition issues. My motor runs for around 40minutes and then the coil starts breaking down, I am not sure if the coil is actually the right one so I have a new mercury sierra one on order. I figured its either the coil breaking down or its getting too much voltage. I measured the voltage to the coil when its running and when I rev the motor it gets above 11volts and it is my understanding it is suppose to be 7-10v supplied to the coil. I also have noticed when cranking I cant seem to measure the 12v from the starter motor, I see about 4v - but it starts right up after 1 or 2 cranks.

I did order a new resistance wire and put that on and it still gives above 11v. I then attached the two resistance wires together (the old and new) and now it gets just a hair above 10v when i really rev it.

Why am I getting excess voltage with my resistance wire?
Why does it seem to give me what I need with two resistance wires attached?

I do have a "ONE WIRE Delco Marine Alternator Mercruiser Volvo 100A" from ebay, paid about 75$ for it.

Thanks a lot

If the alternator is delivering its rated voltage, I'd say your coil is the wrong one. I'd also bet the resistance wire is just fine...now you have an on board spare...just in case.

but why would it be getting more than 11v when it should be under 10?

the real purpose of the resistance wire is to limit the current flowing thru the ignition....this is most easily measured by reading the voltage at the coil when its running. when you added the second resistance link, you doubled its resistance (nominally) and reduced the measured voltage.

yes, but why am I needing to add 2 resistance wires to get the voltage down to where it should be while its running? I am getting 11+ with one wire too the coil (+ side) when its running.

There could be several reasons for a coil to fail. Over current would make the coil very hot and it would fail.
If the coil is not getting hot then you have another issue.

What is your running voltage as measured at the battery or alt output at 1500 - 2000 rpms?

If it is higher that 14 volts then that is your answer.....

the numbers you have found 7-10 volts are based on 12.5-13.5 volts, running voltage.

Today most alt output 13.5 to 14.5 volts and sometimes a bit higher at higher rpms. so when you are on the high side of the voltage scale you see higher voltage at the coil.

Having two resistor wires in series will not affect the coil in a bad way but what it will do is lessen the output of the coil so the spark will be weaker.

Did you hook them up in series or parallel?

Series = r1 +r2 so 2 ohms + 2 ohms = 4 ohms
parallel = r1/r2 so 2 ohms divided by 2 ohms = 1 ohm this would allow more current and hotter spark.

Example: instead of 20,000 to 30,000 volts to the spark plugs it may now be 15,000 to 20,000 so the spark is weaker (more yellowish/orange vs blue in color) with 4 ohms of resistance

What a point ignition needs is
1. externally resister coil
2. correct ignition components especially the condenser
2 ~ 1.5 to 2 ohm resistance wire or ballast resister from ignition on (purple wire) to coil + (GM style ignition)

Ohms law

E=IR

E= volts
I = amps
R = resistances

thanks kghost,

I will measure my running voltage at battery today.
Are you saying if I am to run two resistor wires to counter an overcharging batter, I should wire the resistor lines in parallel ? If I am getting the proper output to the coil with the two wires in series l, I should still be getting a good strong blue spark - right?

No

One resistor wire only.............~ 2 ohms is all that is needed.

Regardless of what you think you may need.

Check the running voltage first.

Maybe you have high resistance spark plug wires? Have you checked those? ~ 5000 ohms per foot.

Also when was the last time you tuned it up? WHat parts did you use?

Any other issues to speak about with the motor running? Maybe a little history would help.