Ten Years Gone
New Member
Hey Guys,
Does anyone know precisely why an ignition system calls for a coil with a specific primary and secondary resistance? :huh:
As I was getting ready to warm up the 'ol Chevy 350 in the boat today, I noticed oil all over the top of the ignition coil. I pulled the coil wire, and the inside of the tower was full of oil. It was definitely the coil cooling oil. The coil appears to be original from 1986 based on the part number, so I am not upset that it finally sprung a leak...at 28 years old! I looked up the superseded part numbers and found the correct replacement for $40 on a boat parts website. No problem. My curiosity got the best of me while I was looking at the spec's for the coil resistance measurements. I had checked the numbers on the original to see if the windings had shorted causing the leak. That lead me to looking for the numbers and here's what I discovered. Apparently, my engine (1986 Merc 260, Thunderbolt IV) is supposed to have a coil fitted with a primary winding resistance of .08 ohms and a secondary of 8K to 11K ohms. Virtually all of the standard automotive coils I could source from places like AutoZone were 1.5 ohm primaries and higher. Most were 3.0 ohms. My "bad" coil was testing at 4 ohms and 9K ohms respectively. I have a Pertronix Flamethrower coil from another boat that I sold. It's a 3 ohm coil however, and I wonder if using a 3 ohm coil in place of the "proper" .08 ohm coil will cause reliability or performance issues with the engine? Someone please school me on the science of coil resistance and how it affects the engine operation. We're talking about a stock 350 with Vortec heads and stock Mercruiser TB IV ignition. Can I use the Flamethrower coil, or do I HAVE to replace the "Mercruiser" coil with another one?
Thanks very much! :smt001
Does anyone know precisely why an ignition system calls for a coil with a specific primary and secondary resistance? :huh:
As I was getting ready to warm up the 'ol Chevy 350 in the boat today, I noticed oil all over the top of the ignition coil. I pulled the coil wire, and the inside of the tower was full of oil. It was definitely the coil cooling oil. The coil appears to be original from 1986 based on the part number, so I am not upset that it finally sprung a leak...at 28 years old! I looked up the superseded part numbers and found the correct replacement for $40 on a boat parts website. No problem. My curiosity got the best of me while I was looking at the spec's for the coil resistance measurements. I had checked the numbers on the original to see if the windings had shorted causing the leak. That lead me to looking for the numbers and here's what I discovered. Apparently, my engine (1986 Merc 260, Thunderbolt IV) is supposed to have a coil fitted with a primary winding resistance of .08 ohms and a secondary of 8K to 11K ohms. Virtually all of the standard automotive coils I could source from places like AutoZone were 1.5 ohm primaries and higher. Most were 3.0 ohms. My "bad" coil was testing at 4 ohms and 9K ohms respectively. I have a Pertronix Flamethrower coil from another boat that I sold. It's a 3 ohm coil however, and I wonder if using a 3 ohm coil in place of the "proper" .08 ohm coil will cause reliability or performance issues with the engine? Someone please school me on the science of coil resistance and how it affects the engine operation. We're talking about a stock 350 with Vortec heads and stock Mercruiser TB IV ignition. Can I use the Flamethrower coil, or do I HAVE to replace the "Mercruiser" coil with another one?
Thanks very much! :smt001
