Mercury 850 Switchbox Melting

[1960 Starflite]

S/N 4294375 a 1975 I think. Background, all the wiring was replaced four years ago, by me, due to insulation problem. It has been running very well ever since on my Harris Flote Boat 24ft Toon.

Last year while looking for a fuel leak (hose connector) I notice that the back of the switchbox has somewhat melted and dripped a little (may have been this way for awhile and never noticed). I just uncovered the pontoon and was prepping it for summer and said to myself "I need to ask about that". It is tacky to the touch cold.

Is this normal? Switchbox internal heat or heat from engine? Should I add an insulator (exhaust type material)?

The engine meets my needs and it started from it's winter rest within 5 seconds. I searched the forum without effect for an answer.

Thanks for any replies in advance.

 

[Chris1956]

The switchbox should be an aluminum box with epoxy potting. It should not melt. Make sure the casing is well grounded. Make sure the wiring is clean and tight.

On a 1975 850, you will have a distributor/battery powered CDI ignition. The battery is connected directly to the switchbox, with no fuses. You could have a fire is the battery wire gets grounded.

 

[Texasmark]

On my engines, I use Thermally conductive(white) paste, to ensure a good heat sink between something like that or the rectifier-regulator to whatever surface to which it's mounted. The electronics industry used it for years, may still, for heat sinking power transistors to their aluminum heat sinks.

https://www.homedepot.com/p/Super-Lube-3-oz-Tube-Heat-Sink-Compound-98003/202932736

 

[Faztbullet]

On a BDI ignition system over charging can cause switchbox to overheat...its only designed for about 15 volts max..

 

[1960 Starflite]

Thanks for the replies. I purchased this engine knowing that the wiring was in need of replacement. The switchbox could have been in this condition then. Every wire touched would result in insulation falling off.

I have a muti-meter and will check ohms/continuity to ground of switchbox. I'll also check DC voltage from the rectifier to battery. Good point on the dialectic grease. I did have the switchbox removed but don't remember if I cleaned the mounting surfaces.

Now if Mr. Weatherman is correct, I should be able to work on this tomorrow.

 

[1960 Starflite]

Update: I checked the switchbox base to battery ground for continuity and all was well. I started it up with my amp probe clamped over the positive battery cable and my volt meter attached to battery. Showing 6 amp charge at 1500 RPM. Voltage started at 12.6 volts and rose to 14 volts. (Battery was a little low upon startup).

I ran it for around 10 minutes on muffs/hose at 1500 RPM. Using my infrared temp gun, the temp on the rear of the switchbox stayed at 75 degrees (air temp 72). Of course the engine cover was off.

I'm now wondering if the melting problem was there and I didn't notice when I replaced ALL the wiring. I also had the switchbox off the engine during rewire.

Maybe I should run the engine with all covers in place and for a longer period of time, then recheck temperature on switchbox, Thoughts?

 

[JohnnySeals]

I am experiencing the same problems with my 1993 Force 40 the epoxy can’t take the heat of the engine 165-170 plus outside sunlight on a black cowl and outside temp at 100 degrees, well in short 2 switchboxes in 2 weeks and the epoxy is getting soft and distorting causing failure to the board. But I found made by jet ski plus that looks like it’s aluminum for less than half the price of factory so I’m headed that direction. Will post an update.

 

[racerone]

A 1975 model 85 HP Mercury will not have an overhear warning / shutdown system.----A new water pump every 3 years is the proper thing to do.