Adding temp sensor to Mercury 40 2 stroke

[prariecreek]

I searched online for quite some time and can't figure out if this is possible or not. I have a 40 hp Merc 2 stroke, sn OT765740 that has a brass plug right above the two spark plugs. I haven't removed it to see if the hole goes into the water cooling channel or not. The manual lists the plug as ".125-27" but I don't know what this means. What I am hoping is that I can remove the plug and replace it with a standard temp sender, something like this http://faria-instruments.com/products.php?calledGauge=4_temp.

So, does anyone know if that is what this plug is designed for and if so, what size I temp sender I would need?

Thanks.

 

[wired247]

Yes, You can do that. Measure it . Its a pipe plug 3/8" or 1/2" NPT.

 

[merc850]

Do you already have a matched sender and gage? Try one from a dealer or maybe this site.

 

[prariecreek]

I'm looking at buying one from this site, the Faria in particular. Do I need to worry about the sender and gauge matching as long as I stick with the same brand? The Faria senders seem to only differ in threaded size.

Thanks for the replies guys, I really appreciate it.

 

[Silvertip]

You need to consider two things: 1) thread size for the sender, and 2) sender resistance. Sender resistance for an outboard temp gauge is 33 - 240 ohms. So yes, the gauge and the sender must match but they do not need to be from the same manufacturer.

 

[prariecreek]

OK, I pulled out the brass plug and it is tapered, about 3/8". I went to the auto parts store and it doesn't fit the fine thread 3/8" nut. The closest fit was the 10mm x 1.00 extra fine threaded nut which allowed it to go in about 2 turns.

I put the calipers on it and the narrow end is 0.365" and it widens to 0.408". It is a pretty fine thread, a bit finer than the 3/8" fine thread. Does this sound normal for a 3/8" temp sender? I haven't seen any metric ones, so I am guessing this will work but would like to know for sure before spending the money on the sender and gauge.

 

[Silvertip]

Why are you trying to make a tapered pipe thread fit on a standard fine or course threaded nut. The two are not compatible. A tapered pipe fitting (the temp sender) must fit into a tapered threaded hole (the engine). Straight threaded nuts are designed for standard threaded bolts. Tapered pipe fittings are tapered so they seal up when tightened. If you are worried about the size of the plug, go to Lowes, Home Depot or any other home improvement store and head for the plumbing dept. See if your plug fits a 3/8 tapered pipe coupling. Problem solved.

 

[achris]

That hole is for a water PRESSURE gauge, not a temp gauge. Outboard temp gauges are 'surface contact' senders, and does not go into the water jacket....

And there is no provision on these engine for a temp gauge. If there was, there would be a tan wire in the harness up to the dash, there isn't. There is a temp switch for overheat, and the wire is tan with a blue stripe... It runs to the warning horn in the control box.

 

[Silvertip]

If the plug is close to the spark plug, that hole can indeed be used for a water temperature sender. The telltale line can be "tee'd" into to add a water pressure gauge. As Achris mentioned, you will need to run a separate wire from the sender to the "S" terminal on the water temperature gauge. If there is a plug near each spark plug, use the uppermost plug as that will be the hottest part of the engine. Contact type senders can be used as well but are typically used when there is no access to the water jacket. As I mentioned, the sender has a pipe thread not a straight thread. You need the single, American sender shown in the Faria list you linked to. The only thing you need to verify is the plug size. The .125 - 27 likely refers to 1/8 inch and 27 threads per inch. I think you will find the size you need is 1/8 inch. 1/8 inch is not the outside diameter of the sender.but rather in pipe terminology would be the inside diameter of the particular fitting. Since a temp sender does not have a hole like a pipe fitting, the 1/8 inch may be confusing but it does represent the correct size. Verify this by taking the plug to a hardware store and mating with a female 1/8 inch pipe coupling.

 

[dancour]

That hole is for a water PRESSURE gauge, not a temp gauge. Outboard temp gauges are 'surface contact' senders, and does not go into the water jacket....

And there is no provision on these engine for a temp gauge. If there was, there would be a tan wire in the harness up to the dash, there isn't. There is a temp switch for overheat, and the wire is tan with a blue stripe... It runs to the warning horn in the control box.

I found a TAN wire on my 90 hp Merc EFI outboard, but where do I find/install a temperature sensor? Any help appreciated.