Temperature gauge issue
 

Temp gauge goes straight to the top with the turn of the key.
It will occasionally float back down briefly only to go back up and stay there. Very erratic behavior. Coolant temp is normal when at operating temp.
Should I start with the sender, gauge or other source?
Thanks,
Pete
'84 HE 5.3

Weeeeell.

1) Remove the single wire, see if the gauge stays at the bottom, if so, you are a winner, the sender is flaky.

2) Just to be sure, earth that sender wire, and note the gauge going full scale.

3) Now, with someone watching that gauge, and with it unplugged as in (1) wiggle the wire around. This will often show up an errant earth in that wire, and that earth will give that gauge a full scale reading.

Sometimes, and the fact its a V12, the wire to that sender can short to earth anywhere from the sender on the RH thermostat backing, to the loom at the firewall. Age is now messing with any wire insulation under a V12 bonnet.

Thanks Grant. Looks like the sender is at fault. 👍