Which Coil - External or Internal Resistor?

zealer

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I've got an 2001 Mercruiser 4.3, I think it is Thunderbolt IV ignition system (only ignition control module on exhaust manifold, no knock control module). Can someone confirm the coil that I need? I believe it is a 12V internal resistance type coil that I need. My friend gave me a 12V external, which I believe is not for my application. Thanks
 

sti1471

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The resistor being internal or external does not matter. If you use the coil with the external resistor just make sure you have an external resistor. It can either be a ballast resistor or a resistor wire. The external resistor coil most likely has 1.5 ohms resistance, so you need a 1.5 ohm external resistor to have 3.0 ohms. The internal resistor coil already has 3 ohms resistance so nobwire or ballast is needed. Internal or external does not matter as resistance is resistance and long as the coil gets the same amount internal or external. However your ignition is electronic (I assume). I have seen coils that have (for use with electronic ignitions only) so you might need to locate one of those. To be sure look in your service manual and it will have the could ratings. Look at the resistance and use a multimeter to see how much resistance the new coil has. If it needs more you can add an external resistor to get it where it needs to be. If it has too much resistance then you can't use it as you can't take resistance away.
 

achris

More fish than mountain goat
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May 19, 2004
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I've got an 2001 Mercruiser 4.3, I think it is Thunderbolt IV ignition system (only ignition control module on exhaust manifold, no knock control module). Can someone confirm the coil that I need? I believe it is a 12V internal resistance type coil that I need. My friend gave me a 12V external, which I believe is not for my application. Thanks

If it's 2001, it will be Thunderbolt V, not IV.... And yes, the V6s with TB-V did not have a knock sensor...
 

zealer

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Ya it is TB-V. This stuff is hard to identify. The service manual has a knock sensor listed as one of the ways to ID a TB-V ignition system. I suppose adding an external resistance isn't too difficult, but a plug and play coil would be easiest. Thanks for the tips.
 

zealer

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Can anyone confirm Mercruiser coil specs? (18-5834) I can't seem to locate it. I did read 0.4-0.7 ohm primary, 9-11k secondary resistance, but it was in context for a 5.7 mercruiser (TB V still). The coil I have now is 1.5ohm primary, 9000 secondary. The thing is, I can't find a single aftermarket coil that has such little primary resistance and such high secondary. Typically the aftermarket coils are 0.6 primary and 4500 secondary. Unfortunately, I don't know enough about how these ignition systems work to understand what difference these resistances can cause. For example, the coil I have now is only 1 ohm off of the specified 0.6 primary, but mathematically that is a difference of more than 200%. What role does resistance play in these HEI systems?
 

achris

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Coil for the TB-IV and TB-V will be the same. And also no difference between a V8 (big and small block) and a V6.

Merc spec for the coil are. Primary 0.6 to 0.8. Secondary 9.4K to 11.7K...

Resistance will ultimately determine the current flow through the coil. The field strength is determined by current flow. So if the resistance is too high, then the field will be weaker, as will the resulting spark.
 

zealer

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Coil for the TB-IV and TB-V will be the same. And also no difference between a V8 (big and small block) and a V6. Merc spec for the coil are. Primary 0.6 to 0.8. Secondary 9.4K to 11.7K... Resistance will ultimately determine the current flow through the coil. The field strength is determined by current flow. So if the resistance is too high, then the field will be weaker, as will the resulting spark.
Thanks for the explanation. Theoretically, wouldn't it be better to have a lower resistance on the secondary, 4.5k as opposed to 9k+ for the reason you mentioned above? I will have to look further into coil functionality. I did read briefly about something for starting and running the engine, but don't recall what and if it bears any relation to the separate resistance specs. Don't get me wrong, I'm going to pick up the properly spec'd coil - I'm just playing devils advocate. It's always very interesting to hear this more technical side of things as opposed to a simple 'what part do I need' question. I understand the effect it can have on a point-condenser type ignition (too high voltage will burn the points), but I don't know the effect on a pointless system like TB V (scenario where less resistance = more voltage reaching distributor and cap).
 
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achris

More fish than mountain goat
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The secondary resistance has no effect on spark strength. It's to do with 'impedance matching' with the spark leads and the spark plugs. Much like impedance matching radio antennas. If the impedances don't match too much energy is reflected back to the source, less is available where required (in this case the spark plug) and can actually damage the source. The coil is just a simple transformer. The primary and secondary resistances don't effect each other, just the turns ratio. Having a very low (primary) resistance does increase the field, but it's a comprise between the field strength and the current being passed by the switching device in the ignition module. The output driver, usually a Darlington transistor or MOSFET, will have a maximum rating, exceed that and you pop the back end out of it (literally).

HTH,

Chris.......
 
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