Help with Ignition Coils

[Rstorrie]

Mercruiser Thunderbolt IV ignition system. I have two coils and could they both be bad even though new? they are different manufacturers. Ohm checking the secondary windings, both read around 9.5 on the 20k setting, but on the primary windings at 200, one reads 0.7 at the center post and the second one reads 3.8. These both have primary winding issues? Thinking they are supposed to be around 1.4~1.6 on primary? Looks like the secondaries are good.

 

[wellcraft-classic210]

The typical ranges are listed in the stickies for both primary and secondary windings . The experts my have more to add.

https://forums.iboats.com/forum/eng...-mercruiser-thunderbolt-ignition-systems-work

 

[Rstorrie]

Well it looks like the one coil is good then. I am chasing a really weird issue on the TB IV. All new wires and parts, constant 12.6v to the + side of the coil. New distributor sensor installed also. With the purple wire connected to the + side of the coil, I get 12.5v and starts. Other times and intermittent, It shows 10.8v at + side of coil (no start). Remove the amplifier (purple) wire from the + side, back to 12.6v, touch the amp wire back to the + side, drops to 10.8v and can hear a spark inside the distributor cap. This comes and goes. Guessing the amp is bad, but there is no way to test them?

 

[wellcraft-classic210]

2 questions

When its at 10.8 volts at the coil ( as in not 12.6 volts ) whats the voltage INPUT at the key switch?

Reason ( verify supply connections )

Is their voltage present when the starter motor is engaged during no start conditions // If so what is it ?

Reason ( verify spark while cranking during no start condition)


The experts may have more to add here

 

[Rstorrie]

When at 10.8v, I am getting 12.6v at the key, and before and after the engine harness plug.

The weird part is when at 10.8, just touching the Amp wire to the + terminal makes it spark inside the cap. You can hear it. No spark inside cap when it is back showing 12.5v and touching the wire to the +. Thinking the Amp has a short or something.

I ended up ordering an aftermarket HEI system to replace the Thunderbolt IV, been chasing this issue for 2 weeks and nothing changes.

 

[wellcraft-classic210]

Are you sure its not distributor position making the change your seeing ? If you get spark at 10.8 V it seems you should have it at 12.6

TO VERIFY THAT -- Are you 100% sure its getting voltage when cranking the starter / And have you checked for spark WHILE CRANKING at the plug wires using a spare plug that's grounded at its housing or similar?

 

[Rstorrie]

I am thinking the Amp must be faulty or drawing power, with just the key in run, getting 7.7v now at the coil, was 10.8v previous, key is dropping to 11.8 from 12.6v and get a nice spark when I touched the amp purple wire to the + side of the coil, the amp shouldn't be throwing a spark not running should it? Just tried a second coil, still at 7,7v. Disconnected the amp purple wire and key volts went back up to 12.6v

 

[Rstorrie]

As for the distributor, new cap, rotor, wires, and sensor.

 

[Rstorrie]

Was doing this before changing those parts out in the distributor, so its not my doing or the new parts. Still the same.

 

[NHGuy]

I hope you bought the DUI, I have one and it's stupendous. I can idle in gear at 500 rpm.

 

[wellcraft-classic210]

Using OHM Law

With a .7 ohm coil resistance and an expected .5 ohm or so in the wiring its not surprising the voltage is dropping at the coil.

Sorry-- I really do not have enough experience with the individual component designs and typical measured values to provide a quick answer for this.


A few thoughts

Were both coils designed / intended for an electronic ignition system VS a points system which differs ?

And

If your checking for spark from the coil (s) itself you may want to try grounding the high voltage coil wire to the cap and leave the distributor out of it as depending on what rotational position its in it will affect your tests.