TOPIC: 1968 160 HP Mercruiser 6 Cyl Ignition Coil ????

I was of the belief that a 12 volt coil was a 12 volt coil, some with internal resistors and some without. I have a NEW Mercruiser Coil #392-805570-1 (the price was right) and wondering if it will work on my 1968 160 HP straight 6 Cyl Mercruiser I.O. (The internet says use this coil without an external resistor) but I can not find why there are so many different part numbers for Mercruiser coils of this tall round style. Electronic engine coils are a different breed understood. Is there a MERCRUISER COIL EXPERT OUT THERE? Thanks for any info. - Rich

I'm no Mercruiser expert on coils, or anything else for that matter, but I'm with you on this question, Rich. Possibly one of the resident gurus can enlighten us further, but the only difference I know of other than the resistor deal you already covered, would be the output voltage. You've got a 12V power source ,the ground , and the big primary terminal sending spark to the distributor cap, right? Assuming there is sufficient voltage leaving to fire the plugs properly............should work!

Robert Bosch used to offer two high output coils that worked on almost any breaker point ignition system, The RC12 was the resistor style, with several different resistors to fit different applications, and the BC12 for those without resistors. Hopefully someone will educate us if we're wrong here, I'm always glad to learn more.

I am at home, parts manuals are at work & will try to answer from memory (I owned a 1968 120 many years ago & currently have a 1970 140.)

I believe your engine (and the 120/140) uses the external ballast resistor in 1968. I know by 1970 the external resistor is gone.

Some engines (other makes & models) used a resistor wire in place of the white ballast resistor that should be bolted on your coil mounting bracket (or right near by). Either system would technically be an external resistor system. The purpose of the external system was to prolong point life. They used a bypass system taking power off the starter solenoid thru a parallel wire to the coil. This gave a hotter spark for engine starting.

If the coil had an internal resistor, then the spark could not be made hotter.

I worked in a parts store back in the '70s and remember that GM used 2 common coils. One was for a 6 cyl (and maybe 4 cyl, I don't remember), and the other a V8. I believe the difference has to do with how fast the coil can recharge before it needs to fire again. Remember, that coil has to fire a 4 cyl twice per revolution, a 6 cyl 3 times & a V8 4 times. Obviously at low RPM, this isn't an issue, but at high RPM it is.

AS for other ignition parts, the point/cond/cap & rotor were all standard issue GM for the Chevy 2 153 CID 4 cyl or the 194/230/250/292 inline 6. The distributors were not automotive because they did not use a vacuum advance like a car. I believe the mechanical advance portion of an auto distributor would have different weights and so, a different advance curve. The 181 CID 140HP was never used in a car in the US, maybe elsewhere.

Jim

While surfing the web came across some info on the coil question. The GM points style ignition systems have two wires to the + side of the ignition coil. One, a 'resistor' wire supplies a reduced voltage for running off the ignition switch, the second wire comes off a small terminal of the starter solenoid to supply a full 12 volts during cranking/starting only.


Failure of the second wire makes engines hard to start.

Failure of the ignition side will fire/start and immediately die.

Mercruiser coil part #392-806529A1 (use with external resistor)

Mercruiser coil part #392-805570A2 (use with out external resistor)