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Mercury Switch Box Failures

L

leachy

Contributing Member
Hi All,

I really appreciate the input from members with the issues I have come across with my new to me 75HP 1986 2 stroke 4 cylinder Merc. Faztbullet you were on the money I appreciate you advice. After looking at the forum and doing a bit of a search it seems that switch boxes are a bit of a weak point in the electrical arear of these older motors. I haven't had a lot of experience with them, up until now its been newish (mid 90's and younger) Yamaha's and Honda's that never experienced i single fault. Just wondering am I being overly concerned or is it a real issue that one day Ill be out in the water and for no apparent reason the switch box will die. Just wondering as I bought the boat the engine started perfectly from cold ran dead smooth, started from warm no issues, towed it home and didn't start is for about 3 months due to flooding here in Australia, but in that time the switch box had died.

Do others carry spare electrical components on board?

Regards
Leachy
 
If the battery is weak or the starter is bad , you might not get spark.---If motor cranks slow there will not be any spark.
 
Ha! Anything can happen with electronics, it's funny you should mention carrying spare parts. By the way, hello there in Australia. I'm gonna come and visit again someday. Love Queensland, especially around Cairns.
I carry a spare MOTOR because we have camps in remote NW Ontario Canada. My 86 Merc 115 has never stranded me, but it has come close. We always run 2 motors, doesn't work for everyone, but up here it can mean your life. We have temperatures well beyond normal parameters. I have learned not to trust electronics with my life, especially deep back in the bush, alone....with a snowmobile, at -50 actual temperature. My snowmobile is a completely rebuilt 1969 Ski Doo Olympic. Magneto ignition. THAT, I can trust with my life....and I HAVE! This is a marine forum, but I have run into so many motor problems related to temperatures, fuels, and precipitation. The simpler and more basic the engineering, the more reliable it will be in extreme conditions. I DO carry a spare power pack for my Johnson/Evinrude in my tackle box, however. They can fail with absolutely no warning. It's happened to me before. Take care, Tim
 
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Thanks Tim for your input, I too would be concerned in those environments and having a backup plan all the time. Its more about inconvenience for me, its not like you can get out and push a boat and more of an issue when the current is pushing in the wrong direction.

I was just surprised how the switch box failed, maybe there was something else happening but didnt show up in the purchase / testing at the time. Im a little sus on the battery or rectifier, the charge voltage doesn't seem high enough to me. Im wondering if that contributed to it failing. Anyway I now have two spare stators by accident and will replace the voltage regulator to be 100% sure.
 
Right, you need to check your voltage change from battery at rest to running, from idle up to full power. A fully charged battery is about 12.65 volts. Also any voltage that might enter the switch box through the kill circuit could damage the box. So make sure that the wiring and switches are all "golden". Mercury for many years had a very brittle insulation on their wiring which would crack and then allowing oxidation on the conductor, not to mention the possibility of a short. This of course is exaggerated in saline environments.
 
Yeah I have been going through everything and checking but the surprise was how the switch box just died without any real signs. the battery does slowly charge when the motor is running but I was surprised that the voltage was not sitting at a much higher rate well above idle.
 
90% of failures is due to grounding issues. The SCR in Sbox is grounded to Sbox case and case is mounted to ignition plate and plate is isolated from block thus the use of grounding straps
 
faztbullet said:
90% of failures is due to grounding issues. The SCR in Sbox is grounded to Sbox case and case is mounted to ignition plate and plate is isolated from block thus the use of grounding straps
Click to expand...

Is there any need or point in adding an additional ground wire to help prevent this.
 
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