DIY Thermal Fuse for Overheat Protection 9.9hp for $15

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Sandor27

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Feb 28, 2011
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After a nasty overheat fried my powerpack, blew my headgasket, melted my thermostat housing, made a mess of my motor paint, I decided to look into thermal protection for my 2000 Johnson 9.9hp motor. Just because the motor is peeing does not mean it is cooling. I hit mud, pee went to brown, then stopped, then went brown, then clear so I thought I was OK. Nope. Thermostat and cylinder head clogged with silt.

I found Thermal Cutoff switches available online from Digikey.com among others. These switches are either normally open (when they reach the rated temperature they switch on) or normally closed (when they reach the rated temperature they switch off).

Switch from digikey

$8.52

http://www.cantherm.com/products/thermal_cutouts/cantherm_f_type.pdf

I chose normally open since the kill switch on my motor grounds the pickup coil when you press the kill button. So I mounted the thermal switch to a convenient location on the cylinder head and spliced one connection into the kill switch wire and routed the other connection to ground. Now if the motor overheats, the thermal protection switch closes and grounds out the coil and stops the motor. I placed a connector in the switch line so I can easily disable the sensor if need be.

I chose 185? F since the service manual says my motor should operate between 125? and 155? F (140?+/-15?) and 185?F is not all that hot. When my motor overheated and the paint melted it was well above boiling temperature.

I tested the switch in hot water and it switched at 183?F on a candy thermometer, so I know it works. This is not a fuse, once the temp came back down to 145? it switched back open. I tested it two more times to make sure it worked.
I hope to never have to need this again, but the tidal marshes I hunt in get awfully skinny at low tide and this was my second overheat.

So there you go! $15 insurance : )
 

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nwcove

nwcove

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Re: DIY Thermal Fuse for Overheat Protection 9.9hp for $15

nice job!! but i would still consider an alarm over a complete shut down. ( just for personal safety in certain situations)
 
clanton

clanton

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Re: DIY Thermal Fuse for Overheat Protection 9.9hp for $15

I`m impressed. How is the heat conducted to the thermal switch, thru metal to metal contact? I did not see the mounted switch in the photos.
I`m working on Suzuki motorcycle that need overheat warning light. OEM system very pricey, and only works part time.
 
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Sandor27

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Re: DIY Thermal Fuse for Overheat Protection 9.9hp for $15

clanton - look at the first pic where I am tapping the block, then the second pic same location. That is the thermal switch. It has a metal plate with a mounting hole and is bolted directly to an ear on the block. Metal to metal contact. I might pick up some thermal grease compound like they use on CPUs for cooling and put it between the switch and block.
 
clanton

clanton

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Re: DIY Thermal Fuse for Overheat Protection 9.9hp for $15

I got it. This will make the lady that owns the Suzuki happy. Thanks again for posting this.
 
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minuteman62-64

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Re: DIY Thermal Fuse for Overheat Protection 9.9hp for $15

I'm doing something similar for my Mariner, except it will trigger an alarm. I purchased a thermostatic switch, but find the business (contact) portion is 1" diameter - difficult to fit to my head without using a big blob of epoxy.

What's the approximate diameter of the actual sensor on your switch (I looked at the on-line site but can't decipher the dimensions)?

Also, I couldn't tell from the on-line info - is the unit sealed for protection against moisture/corrosion?
 
achris

achris

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Re: DIY Thermal Fuse for Overheat Protection 9.9hp for $15

That's a good idea, and OMC tried that a few years back (quite a few in fact)... It was called S.L.O.W. (Speed Limiting Overheat Warning). It didn't kill the engine completely, just slowed it to 2,500rpm (from memory). They did end up with one issue... If the engine was running at high speed when SLOW activated, all the occupants and their gear ended up in the very front of the boat, very quickly. And that was only with a shut-down to 2,500rpm. They alleviated the problem by introducing a ramping down of the speed limit, rather than a hard cut.... Sort of a slow SLOW if you like.... Now your switch will have the engine cutting dead... I feel the occupants will end up in the front of the boat even faster.... I would hate to see that end up on funniest home videos... :eek: :facepalm:.... I think wired to an appropriate alarm might be safer.... :noidea:

Chris........
 
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Sandor27

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Re: DIY Thermal Fuse for Overheat Protection 9.9hp for $15

Minuteman -

The switch is totally sealed. Look at the pic of me boiling it in water : )

The dims are approx 3/8 wide x 3/4 long x 3/16 thick - see pic.
 

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Sandor27

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Re: DIY Thermal Fuse for Overheat Protection 9.9hp for $15

achris -

My motor is only 9.9 hp, so even on plane I am only going 15mph. When I cut the motor using the throttle I slow abruptly, but not dangerously. I'd love to ramp down the speed, but that is not an option. My motor does not have a battery or alternator so I would need an external power source for an alarm. I'll take the catapult risk and I'll be sure to keep my goCam on while running so I can get my face on FHV. :laugh:
 
achris

achris

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Re: DIY Thermal Fuse for Overheat Protection 9.9hp for $15

Sandor27 said:
achris -

My motor is only 9.9 hp, so even on plane I am only going 15mph. When I cut the motor using the throttle I slow abruptly, but not dangerously. I'd love to ramp down the speed, but that is not an option. My motor does not have a battery or alternator so I would need an external power source for an alarm. I'll take the catapult risk and I'll be sure to keep my goCam on while running so I can get my face on FHV. :laugh:
Click to expand...

Fair call... :lol: (I'll be watching FHV. ;))
 
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seahorse5

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Re: DIY Thermal Fuse for Overheat Protection 9.9hp for $15

How are you going to wire the switch to the ignition system?

If you plan on tapping into the black/yellow kill wire be advised that 300-400 volts runs through it when the engine is at high speed. Is the switch rated for those types of voltages for continuous use? Any internal arcing or voltage leakage to ground will affect the engine running as the "kill wire" shorts out the main capacitor to ground when you hit the stop button.
 
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Sandor27

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Re: DIY Thermal Fuse for Overheat Protection 9.9hp for $15

Yes, tapped directly to the kill wire. Switch is solid state, so no chance of internal arcing and is rated to 2kV insulation. 2A at 250Vac load. 250V is a standard industry rating. I don't know the amperage at the upper limit, but I am going to assume that it can handle grounding the cap. And if it fries I'll at least get some sputtering and know something is up. The whole point is to alert me to an overheat before it causes harm. I'm good with a $9 sensor frying if is saves me $150 (power pack + head gasket + T stat housing + T stat)
 
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schematic

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Re: DIY Thermal Fuse for Overheat Protection 9.9hp for $15

Sandor27 said:
Yes, tapped directly to the kill wire. Switch is solid state, so no chance of internal arcing and is rated to 2kV insulation. 2A at 250Vac load. 250V is a standard industry rating. I don't know the amperage at the upper limit, but I am going to assume that it can handle grounding the cap. And if it fries I'll at least get some sputtering and know something is up. The whole point is to alert me to an overheat before it causes harm. I'm good with a $9 sensor frying if is saves me $150 (power pack + head gasket + T stat housing + T stat)
Click to expand...

Use the N.C. version and wire it in series with the trigger coil. Much safer voltage to deal with. The current spike when discharging the cap will likely kill the switch.
 
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minuteman62-64

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Re: DIY Thermal Fuse for Overheat Protection 9.9hp for $15

Sandor27 said:
Minuteman -

The switch is totally sealed. Look at the pic of me boiling it in water : )

The dims are approx 3/8 wide x 3/4 long x 3/16 thick - see pic.
Click to expand...

Ahh. So those numbers on the on-line site must be in millimeters. OK, I've found my thermostatic switch. That one should fit nicely. Thanks.
 
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minuteman62-64

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Re: DIY Thermal Fuse for Overheat Protection 9.9hp for $15

OK, went ahead and ordered. Shipping is more than the price of the unit - so ordered two so I'll have a spare if one turns to toast.
 
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