Mercury Temp Switch (alarm) alternatives

[Schtoopid]

Can someone give me some alternatives on my temp switch?
1991 mercury 40 hp
The temp switch by the spark plug.
Every listing I see is $300 for this little sender..

Can I get another option please?

 

[Silvertip]

What makes you think you need a new one? Install a water temp gauge with the appropriate sender. Much less than $300.

 

[Schtoopid]

I don't believe its a water temp.. its an engine temp. No thermostat in this engine, so the water always is pretty cool.
I know its bad, because the wire broke off when I was fuxing with it. Prior to that, the alarm would kick on, and then never shut down... Now the engine is doing weird things, so I believe that it is indeed overheating.

 

[Schtoopid]

Found the part number. 16775a3
crazy money for the part.

 

[Texasmark]

No stat so you don't have to worry about that. I assume your impeller in your water pump is new(ish). That engine has a tell tale that should be squirting a good solid stream at idle, like if on the boat at the dock, at idle, penetrating the water at least an inch. Further assumption that you are not running in salt water where scale could be clogging your cooling loop, or you haven't run through a lot of water borne vegetation recently.

If that engine had a tstat it would be between 125 and 143F for the opening temp. For a reference as to how hot that is, a US domestic hot water heater in the home, in Normal position is 140F. So when you say hot, how hot? Feel the top rear of the engine block. If you can leave your hand on it for several seconds, you aren't too hot. Look for your current engine problems elsewhere.

 

[Schtoopid]

Thanks Texas.
I replaced the impeller when I got it. Last guy ran it dry (no muffs)
started pre-mixing the gas, and the engine runs a LOT better.

 

[Texasmark]

Thanks Texas.
I replaced the impeller when I got it. Last guy ran it dry (no muffs)
started pre-mixing the gas, and the engine runs a LOT better.

Are you good to go or is this thread still open ended?

 

[Schtoopid]

Well, I was hoping to find an alternative to the $300 piece. but if nobody has answered yet, I don't imagine they will.

 

[Texasmark]

You can get on the www and look up temperature sensors and find them in all sizes and shapes. The last time I did that I found one for $15 + SH that was about the size of a tubular pharmaceutical capsule. Idea was to get some JB weld since it had metal imbedded in it and a big blob (completely covering the unit) of that would help the epoxy to conduct heat to the sensor. Just glob it onto the water jacket cover where the OEM was imbedded for a spot to mount it.

The specks for the switch would be:

Normally open contact (abbreviated NO SPST for single pole, single throw normally open) or words to the effect, CLOSES on rising temperature. You want it to close the circuit when the temp gets too hot and in doing that you put ground on the low side of the horn and make it blow.

The WRONG type would be CLOSES ON FALLING TEMPERATURE, or NC designation which would be Normally Closed at room temp. With only 2 terminals all you get are SPST switches but to have that info available on the switch listing it may or may not be present.

Something around 180F +/- 10 is ok for the Closing Temperature. The OEM unit imbedded in the water jacket cover is set to 195 F but with yours mounted externally, I'd derate it a tad.

The ones I saw were rated at around 15 amperes, but you are only talking about ? of an amp when closed to activate the alarm if that much, so current rating would be a no brainer I'd think.

If you get one with 2 terminals, which is which doesn't matter......wire one to something that is electrically connected to the battery "-"....a lug under a mounting screw would work fine as long as it has a metallic path back to where the batt cable is secured inside the engine. The other one gets your tan wire that was connected to the one that failed.

That's it.

 

[Schtoopid]

Thanks. Exactly the info I needed. Except this is mounted in the head. Not water jacket. Do you think that's the right temp range?

 

[Texasmark]

Possibly semanics. My 90 triple was cast as most Mercs were cast and that was with the head cast as an integral part of the block. The Cover you are looking at, where your current sensor may (if like my 90) be embedded is the water jacket. Easy way to tell. Look at the rear of the block from the side. If you have one outer plate and only one gasket you have the integral. If you have 2 plates and 2 gaskets then your engine is lost foam cast and would have the separate head plate like the/some of the V engines.

If the larger engines (75 and up) can tolerate 195F for the alarm to sound the smaller ones surely ought to also as they are made from the same substances and all that rot.

Your Tstat may be a 125 or 143. Chatter on here says that the small Mercs use 125 while I know for a fact the 90 series (75 and 90 3 cyl 115 and 125 4 cyl) and 150 V6 use 143F. You want a sensor that protects you but doesn't false alarm. For what it's worth, my 2011 Silverado sits on 210 all day long so 195 isn't that big of a deal. The auto industry has used thermostats over 190 way back into the 70's at least.

 

[Schtoopid]

Yes, there is a cover. But the temp switch I'm talking about is immediately beside the top spark plug. Not in the jacket cover, but in the cylinder head.
It most certainly doesn't measure water temperature, as it is dry. But measures the temperature of the metal of the cylinder head.
I'd guess it would run 20-30 degrees hotter than the water at least.

 

[Texasmark]

Okaye, how about a clear close-in picture and while you are at it, a view of the top of the engine block that includes the rear cover(s).