53 Mercury temperature gauge issue

[dean333]

With the ignition off, the needle sets at H, which it’s supposed to do. After startup the needle sits just above half and within five minutes goes up way past the H. I’ve already replaced both sending units to no avail. Any advice is appreciated.

 

[Oldgearz]

I assume your engine is not hot/hot after the five minutes? Why two sending units? Disconnect one at a time and see where things are after 5 minutes.

 

[miker98038]

I’m guessing here, but I think oldgez is onto something. The somewhat being a dim remembrance that on at least some flatheads those senders were side specific. One only “worked” at high temp, the other gave the normal gauge reading. Can’t remember where I saw that.

 

[dmsfrr]

Something miker98038 said made me remember I had these images.
Don't know where these came from but if they help, here they are.
Notice the two sensor units are Not the same.
.

 

[miker98038]

Ones’s a sender, the second an over temp switch. So two senders going to ground, dropping resistance (?) would result in the gauge reading high. Not sure my electrical logic is right. Isolate it to 1 and see what it does. Like oldgeaz said. Cheap and easy.