Hey, my temp gauge was pegged last night so freakin out I pulled over and checked things out. I recently changed the alt. and thought I screwed up with the belt but all was fine. As I drove home the temp. gauge was pegged again but when the motor was under load (accelerating and sometimes decelerating) it would momentarily drop back to normal just under 190. Did I screw something up when I changed the alt. or is some sending unit going bad.

 

Sounds like the sender, what year is your truck?

 

its a 97 with 185K on it

 

Yep. If the temps were high enough to peg the guage, it should have boiled and sent coolant out the over flow bottle. Also if an engine truly has that high of a temperature, you should hear funny noises too.

Check the connection and wiring from the temp sensor located on the thermostat housing (points straight up if I remember right). You may have bumped it, or crushed the wired. I believe if the wires are shorted or the signal wire (there is a signal wire and a ground wire for this sensor) is shorted then the guage will peg. This seems like the case since the guage acted normal sometimes, this indicates to me a loose wire.

Hope this helps you.

J.R.
USCG Electronics tech 2nd class