I got this in a visitor message and there's no way to respond to those so hopefully Steve will see this.. To test any electric gauge , you power it up then disconnect the sender wire and the gauge should go to one full extreme . Now ground the sender wire post and the gauge should driven to it's other extreme . It's important to be able to drive the gauge to both extremes else it'll never read accurately . The rocker box has either four perimeter screws or two acorn nuts on the top of it depending on what engine you're fooling with . Use a perimeter screw or run the ground wire from the cylinder head , the other end always goes to the cab , this is apart from any other grounding the engine might have already . -Nate Nate,on a post of yours about checking the temperature gauge on a 1958 GMC or Chevy you said to ground it from the rocker box screw to the firewall. Where is the rocker box screw? Is it just the threaded part that comes out of the top of the valve cover?Also, your test says it should peg one way and then the other. I have no needle movement at all when I turn the key on with the wire connected to the sender. It doesn't move at all when I disconnect the sender wire either, so does this qualify as pegged. It does peg out to High when I disconnect it and short it to the negative battery terminal. Thanks in advance for any help. Steve
Thanks Nate, Its an original 270. I'm still not clear if my situation qualifies because I do not get any reading or movement on the temp gauge when I turn the key on with the sender wire connected or when I disconnect the sender wire. It does peg out to High when I jump the sender wire to the negative battery cable. I am going to try to ground it as you describe and see if I get any results. Thank you. Steve
Gauge Testing You don't touch the battery when you're driving or testing the gauges . Use the dash or cab for grounding . When you ground the gauge's sender terminal then turn on the key , it should peg one extreme or the other . then disconnect the sender terminal and it should peg to the other extreme . This tests the gauge head apart from all other wiring .
Thanks again Nate. Gauge checked out. Also cleaned all wire connections and ran a new wire from gauge to sending unit. Seems to be operational now. Without actually testing what temps are supposed to be associated with the marks on the gauge. Its a 1958 GMC with a 270 if that helps. Also if operating fine where should gauge typically read once it reaches operating temp(i.e. 1/4 of the way,1/2 way. etc.). Thank you - Steve
Gauge Success ! Whew ~ I was beginning to doubt my ability to explain things correctly . In general , anything 1/2 or below is good . I bought an inexpen$ive infra - red thermometer @ Harbor Freight some years ago (yes it still works !) and used it to gauge the accuracy of my temperature gauges , most of them in all my various cars & trucks , show about 1/2 way up @ 185? F ~ a food place to be . Remember to occasioanally peer in the radiator when the engine is warm and running , so you can see if the coolant mix is cloudy or whatever .
