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Old 08-12-2019, 11:44 AM
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Guys,

New to the forum. I know these forums are a wealth of information, so I thought I'd try here first! Just bought a 98 Baja Hammer with a 7.4L (non mag)/bravo one setup. I recently had a coil failure and when I google the number on the coil (392-8055 70 1) the majority of what comes up is the number 392-8055 70A2??? Is this the same coil? For my own information, is the coil I have on my engine have an internal resistor, or external?

Thanks!!!
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Old 08-12-2019, 12:04 PM
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What ignition do you have ?
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Old 08-12-2019, 12:18 PM
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That would probably be helpful to know!
I will try to get over to the storage building in the next day or so to check.
So Mercury used more than one type in that year?
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Old 08-12-2019, 12:26 PM
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The part # is for a thunderbolt IV or V............ I've heard they have an internal resistor in them.

https://inboarengin.com/392-805570-1
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Old 08-12-2019, 12:34 PM
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If I am reading things correctly, the difference between the IV & V is the V's ability to advance/retard the ignition timing based on a knock sensor?
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Old 08-12-2019, 12:51 PM
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I was thinking about getting something with a bit hotter spark like the Pertronix 40511, but was unsure of the resistance required.
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Old 08-12-2019, 01:06 PM
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The TBIV and V ignitions (most carb engines) use a coil that , if I remember correctly, has .6ohm resistance. If you use a coil with different resistance you run a chance at frying the Thunderbolt timing module/amplifier. The OE coil does not use a resistor to step down the voltage. Just the old points type did.

98% of the computer controlled Merc engines used the ACDelco EST coil. Only a few used the Thunderbolt style. These will fire a SC engine with 7pounds boost and 5k+ rpms. It. also, is plenty fine for most marine engines that use this style coil.

The Thunderbolt V differentiates itself from the IV by using a knock sensor and a timing strategy (Mean Best Timing) in the module, vs a fixed 'curve' in the IV's module. Yes, a few V's did not use a knock sensor.

Last edited by SB; 08-12-2019 at 01:09 PM.
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Old 08-12-2019, 01:16 PM
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Interesting...
Pertronix does sell a .6 ohm coil.
I was initially concerned about the engines poor starting (it just kind of started by catching on a couple of cylinders then everything would start firing) but after my wife caught glimpse of the giant spark arching across the high output and the primary negative, I knew what part of the problem was. Of course not before I had to do the float of shame from the dock to the trailer because it decided it had had enough.
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Old 08-12-2019, 02:10 PM
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Put the OE coil on it, their cheap and dont put anything on it that says MSD ( may start deteriorating )
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Old 08-12-2019, 09:07 PM
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you have to use a coil that matches the primary and secondary resistance of the OEM coil no matter if it is T bolt IV or V or the est set up. The big block Merc s Black side 454 / 502 used a est ignition set up not the same T bolt set ups.. There is couple of sources to buy the exact coil or even better but its very few sources -- Sierra is NOT one of those sources. . Way to many poor quality copies from secondary China makers.

The EST coil is a NON oil filled coil. Pertronix does not have the coil your looking for unless they added it to their line up but I doubt it.

Many Merc efi engines used the T bolt set up as well. The small blocks including many 6.2 and the 4.3

Last edited by BUP; 08-12-2019 at 10:03 PM.
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