I have the 5.7 260hp mercruiser (1983) Temp gauge just sits on"C"
I used the recommended method
took wire off the sending unit and grounded that to the block and the gauge goes all the way up to HOT every one tells me this verifies that your wiring tothe gauge and the gauge itself is in good working order.
So I go out and get the new sending unit.
It will only scew about halfway in..I tried everthing to get it to scew in farther but, that is as far as it wil go the acutal sensor( inch long finger at the bottom of the unit) is certainly down in the water flow.
The old one was only half way screwed in also I used no dope or teflon.
Attached the wire lead back onto the top of the unit and IT DOES NOT WORK the guage just sits down on the letter "C" and as a matter of fact after I shut the key off , the indictor will climb up to the first hash mark.... but soon as you turn the key to the run position it goes down off the 1st hash mark to the letter "c" this must be the bottom most position.
So if that is all you can test : Guage / wiring /and sending unit ...and that
1st test validates the guage and wiring to the guage....and you put a brand new sensor...... what the heck else is left to troubleshoot ??
summer will be gone and I will have not used this boat once at this rate