Ignition coil and Thunderbolt IV

[Boilers2000]

Ignition coil has gone bad and needs replaced. Current ignition coil says external resistor required. I tested to confirm a resistor was installed and got 10 volts at the coil with the key on versus 12.8 volts directly at the battery. Picked up new coil and it says external resistor required except if using a TBIV. My engine plate says I have a TBIV, but if that's true why do I have a resistor also. Any insight would be appreciated? I don't seem to see a TBIV plate in the engine compartment so maybe it went bad and wasn't replaced and a resistor was installed instead.

 

[alldodge]

If you have a TB-4 then it doesn't matter if its internal or external resistance required.

I have a TBIV, but if that's true why do I have a resistor also.

Could be you have some corrosion and the reason for the voltage drop, there should be no resistance

 

[Scott Danforth]

How did you come to the conclusion that the coil is bad? Coils rarely ever go bad. Only about 3 out of a million changed coils is actually bad

 

[Boilers2000]

Couple items brought me to this conclusion in no particular order. 1. Its leaking oil. 2. In a dark room you can see it arcing from the + to the housing. 3. It heats up to where you can barely touch it. I think the coil itself is fine primary ohms out around 2.3 ohms secondary at 9000 ohms. When the coil overheats boat loses power and spark, swap in a new coil and boat runs fine.

 

[Boilers2000]

I am also aware that a distributor or plug wire issue could be restricting voltage from the coil and that could be the cause of the burned up coil. I am addressing those item simultaneously.

 

[Boilers2000]

AllDodge, the reason I thought there was an inline resistor was because the coil that was pre piously in there had a note saying must be used with an external resistor. New coil I bought 18-5438 as listed in manuals only Ohms out at 6800 on the secondary side, which seems low but I cant find any specs for this damn things.

 

[alldodge]

A points distributor needs a resistor wire, and when folks install electronic conversion many don't change the coil. It really doesn't need to be changed, but most conversions indicate to install a new wire to the coil without resistance.

Which conversion is in the distributor?

 

[Boilers2000]

I believe it's an electronic conversion. I also retested connected directly to the purple that goes to the positive lead on the coil and a ground and got 12v aligned with the battery...so I must of had a bad connection yesterday. Started up on Muffs and Tach was all jumpy but finally settled down to expected idle RPMs. Getting ready to drop it in the water this afternoon to see if that fixed the problem.

 

[Boilers2000]

Ran the boat for 6 hours this evening after replacing coil and ran great. Don't know if the coil was ultimately the issue or the dozens of electrical connections I have disassembled and cleaned. Either way I finally made some positive progress on tuning this 26 yo babe up! Thanks for all the advice from the centuries of experience on this site!