Coolant Level light

[BigMikeO]

My coolant level light will come on if I am going down hill or if i stop suddenly and then it will go off and on until I turn off the truck then back on. It will then be fine until the next hill or quick stop. I just bought this truck about 4 months ago and I noticed that the previous owner made their own mark on the overflow tank about an inch above the full line. It is staying at that point and I have not had to fill it up since I bought it. Any ideas. The truck specs are in my signature below.

[catmandoo]

when i bought mine the guy said that on occasion the coolant lite would come on and it did and it was full and one day it stayed on so got to checkin and the wire that goes on the sender in the rad was loose i had to get in there with a pick and push the looped contact in there closer together and it has'nt come on since.