Activation temperature for Mercury sender # 78799 2

[HerbieJ]

Hello all,

Help please! The overheat alarm on my 1994 60 HP 3 cyl. keeps sounding above 3800 RPMS. I've explored everything I know of, t-stat, poppet, water pump -- even pulled the powerhead and changed the upper and lower exhaust plate gaskets and large rubber seal at the bottom of the exhaust passage. Also replaced the (undamaged) water tube while I was in there. Interior passages of the head appear to be completely clear, (as far as I could see). Running out of things to check, so now I'm looking at the temperasture sender, (which reads head metal temp, not water temp). I pulled mine and did the water test on the kitchen stove. The circuit completes / alarm sounds at 170 degrees. (did this twice to be sure.) This seems low for an engine block -type sensor. I'm told the replacement Mercury part is # 78799 2. Before I spend more $$$, does anybody know at what temperature this part is supposed to activate?

Thanks,
Herb

[fastback4]

You mean to say the alarm will not sound if you run at 3,000 rpm's? You replaced all of those items huh? You're a good mechanic then. That's no easy task. Have you performed any actual water temp tests. What is the water temp at the exhaust. I've seen readings of 180+ deg. @ 3,600 rpm. Nothing to get alarmed about here. The sensor may check head temp but aluminum head temperature is probably 10deg. +/- to the waters temperature. Check the alarms ground. Reground if you can.

[JUST-IN-TIME]

get a temp gun ad gun the head

sounds like bad sensor