Temp gauge problem

[Johnny Atomic]

I was driving my 83 pickup (305 bone stock) on the 4th and the temp gauge is pegging out to maximum, but the truck shouldn't even be warmed up yet. The coolant is cool to the touch. It stays pegged out even if the key is off, and even if the battery is unhooked. That would have to be the gauge, correct? Not the sending unit, ???????

 

[Matt69olds]

Find the green wire that goes to the sending unit in the head or intake. Disconnect it, see what the gauge does. If it reads cold replace the sender. If it still reads hot follow the wire towards the firewall. Make sure the wire isn’t pinched under a valve cover or grounded somewhere.

 

[75gmck25]

It runs through a protective sleeve where it’s next to the exhaust manifold and the sending units in the head. If the sleeve is missing it may burn through the insulation and ground on the manifold.

 

[Johnny Atomic]

Update after some testing. after sitting a couple days with no battery the gauge still shows 260 degrees (was initially maxed out), no change unhooking sensor wire on engine with power and key on, no changes wiggling wire around and pulling it back towards firewall. I used an alligator test lead and connected the white connector (on the green wire going to sensor) to the chassis. One time it maxed it out, but most of the time, no change stayed at 260.

 

[Johnny Atomic]

It runs through a protective sleeve where it’s next to the exhaust manifold and the sending units in the head. If the sleeve is missing it may burn through the insulation and ground on the manifold.

so it does normally run behind the exhaust manifold exit. between the manifold and the block.

 

[Johnny Atomic]

Installing a new temp guage fixed it.

 

[Big red square]

What if the truck stays on cold?
Changed sending unit.
Tested a different gauge.
Use alligator clips to wire it myself still reads cold.