TOPIC: Mercury Orange, Green, and Blue Ignition Coils

F.Y.I. fellow old Mercury fans. I recently bought two Mercury outboards. One, a 1972 4HP single cylinder 2-stroke "maker point" "Thunderbolt" CD ignition, and a 1978 9.8HP two cylinder 2-stroke "Thunderbolt ignition", but without points (a "switch box" motor).

No spark on the 4 HP. I bought various applicable Mercury service manuals, including those from all the "major publishers". Well, they all were pretty much useless concerning Mercury CDI ignitions theory of operation/troubleshooting. However, a readily available online PDF from CDI was more informative.

The 4.0HP came with a "green coil", which "ohmed" within published specs (ca. 500 ohms on the secondary, less than an ohm on the primary). The "low-speed" and "high speed" capacitor charging coils also "ohmed" good. The ignition capacitor tested good (I have an ancient (1960s) EICO capacitor tester/bridge), and I then charged the ignition capacitor to 300VDC and jumped it (thus bypassing the points and points wiring) directly to the primary winding of the "green" ignition coil...still no spark. (the green coil didn't spark when put on my working 9.8HP either) So I borrowed an orange ignition coil from the 9.8HP (which is also a coil with isolated windings), and voila, a nice fat blue spark when jumped directly to the 300V charged capacitor. However, still no spark when I installed the orange coil on the outboard, so I dug out my "points file" and "cleaned" and adjusted the "maker points", and voila...SPARK!

The orange coils are still available new (P/N 339-5288T2), but at 85 bucks, that cost more than I paid for the motor...so...based on what an online retailer told me, I put on the thinking cap and lab coat.

Since the dawn of recorded history, most, if not all, ignition coils shared a common ground for both the primary and secondary windings. However, the "Maker point" (and some other Mercury)ignitions require a coil with "isolated" windings (i.e. separated grounds), because the ignition coil primary winding plus terminal is directly connected to the charge capacitor, and the ignition coil primary minus terminal is periodically grounded by the "maker points" which causes the timed ignition spark. A shared ground coil would normally, of course, constantly drain off the voltage from the ignition charge capacitor to ground, so that won't work. The orange coils are still available new for 85 bucks, and used from various outboard "scrappers" on Ebay for about half that. However, the newer Mercury "blue coils" are readily available (used) for about 10 bucks or so, and have been purported to me by an online retailer to work in maker-point motors, and WILL, indeed,in my experience, work on an old maker point Mercury (for a while, at least). I just removed and tossed out the chrome colored strap from the primary winding minus terminal on the blue coil, then removed the rubber coil pad on the blue coil and placed a six inch piece of de-soldering braid (folded in half) on the bottom side of the blue coil body. I put some electrical tape between the braid and the coil body, and then I replaced the rubber pad, leaving half or less of the braid inside the pad, and half or more of the braid outside the pad. (Solder braid is readily available at RS) The point is, about half the braid goes under the pad, and about half the braid goes outside the pad to make physical connection with the engine ground. Even though the primary and secondary windings on the blue coil still share a common internal connection (as evidenced by an ohmeter), the primary winding is isolated from the motor ground at primary voltages (hundreds of volts), but at secondary voltages (thousands of volts), the secondary winding is effectively "connected" to ground, so to speak, through the braid, perhaps, perhaps through the points, or perhaps some combination thereof. So, my maker points 4HP was indeed running quite well with a ten dollar "blue" "junker" coil. YMMV, of course! I haven't tried this on any "orange coil" outboards (like my 9.8HP), nor will I, as I recently bought four orange coils from an Ebay scrapper/seller for 40 bucks, but I did use the blue coil on my 4HP for about 10 hours this summer. And since I got the recommended orange coils so cheaply, I changed out the "blue coil" I put on my maker-points 4HP Merc, as I'm still wondering as to whether the common internal connection in a "blue coil" might have any long term residual ill effect. Again, I did not, and probably will never try to put a blue coil on any "orange coil" Mercury that has a "switch box" CDI ignition which grounds the coil primary through the switch box. (like my 9.8HP)

I also noticed that the orange coils have a secondary resistance of about 1000 ohms, vs. the ca. 500 or 600 ohms of the green coils, so to me, IMHO, all green coils are/were junk from the get go...

Does anybody else have any thoughts/experience in this area?