I bought a used 1997 Starcraft Stardeck pontoon with a 1996 40hp Mariner Magnum outboard motor. The volt meter read low under any throttle condition and the Tachometer never worked. Despite the low voltage reading the charging system must have been working to some degree, since we were never stranded with a dead battery. Doing some on line research I found that these were symptoms of a bad voltage regulator.
It would be very difficult to replace the voltage regulator with the boat in the water and since I don't own a trailer I decided to replace it when the boat was pulled for the season.
My dealer quoted $192.33 for the voltage regulator, Quicksilver part #8M0084173. It is not inexpensive. Taking a chance, I bought it so it would be in hand when I had the time to work on the boat prior to launching in the spring. The voltage regulator is a potted electronic component in a square cast aluminum chassis shaped like an open box, somewhat like a sardine can. The new part has some modifications. They have added a fuse and a ground wire that were lacking on the original part. I think the original was designed to ground through the aluminum chassis. A ground wire probably provides a more secure ground connection. A fuse is an obvious safeguard, potentially preventing a catastrophic failure in the event of a problem in the charging circuit.
I contacted customer service at Mercury (The parent company of Mariner) wanting to know what other motors use this same part. I was told that my motor was produced in a joint venture between Mercury and Yamaha. They said it is used in many 30 to 125hp two-stroke and 30 to 60hp four-stroke motors. They said that could not be specific about what model years were included. While they did not say so, to me it is logical that any Mariner motor that was produced in the joint venture with Yamaha is potentially likely to have this same voltage regulator. If you suspect that you own one of these motors, I would recommend that you contact your dealer to see if this same part is in your motor. If you are experiencing these symptoms, I would waste no time in looking into it.
I disconnected the battery and removed the cover from the motor. I found a plastic cover on the Starboard (right side looking forward from the rear) covering the voltage regulator (Yours may be in a different location). I removed this cover. I was shocked to see its condition. It was badly burned. The potting material had melted and flowed out of the chassis onto other wiring. Two wires were completely burnt off their connections. It looked like it had come close to causing a fire that would have destroyed the motor and possibly the entire boat (see photo). Looking at the plastic cover that I had removed to expose the voltage regulator I could see the place that had been directly over it had a witness mark where the heat had melted a quarter sized circle.
Removing the old voltage regulator was easy. Two mounting screws and a few color coded wires to unplug. I mounted the new one, connected the wires to the correct colors. I connected the ground and mounted the fuse holder to an existing screw on the motor. I replaced the motor cover and reconnected the battery.
I launched the boat and stared the motor. Instantly the voltmeter read the correct voltage and the tachometer apparently read the correct RPM. I now have renewed confidence in the boat. I no longer worry about getting stranded with a dead battery, even after prolonged operation at night with the lights on. If you suspect you might own one of the motors with this part, I recommend checking with your dealer, soon. If they confirm you do indeed have this part, I recommend looking at it as soon as possible. If you find that you have the newer part with a fuse and ground wire I would not worry about it. If you have the older one without those modifications you might consider replacing it prior to failure, avoiding a potential motor fire.
I found while looking into this on the Internet that there were aftermarket voltage regulators available at much reduced prices. I would not buy one without the added ground wire and fuse even at a substantially reduced price. I opted to buy the genuine Quicksilver part.
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