Immediate Overheat per Gauge upon Start?

[Vykkylou]

Finally got her all back together and the test drive of about 10 miles was great. Rested engine for a day, started her up and Boom! The temp gauge goes straight to past hot.
Long story made a bit shorter: Pedal tree sheared in 2 spots a month ago. Had to pull dash, knee bump, gauge cluster, etc. to replace pedal tree. Yes, double and triple checked plugs to all gauges before and after re-assembly. Was also under the hood and pulled out rain shield and tray to see about drilling mounting bolt through. All was pulled carefully and replaced cleaner than removal (LOL)
Since this latest trouble began have: verified lower hose was cold so stat not opening, replaced thermostat, replaced coolant, checked all hoses and crossed a lot of fingers.
BUT NO...as soon as you start the engine, the temp gauge goes right up past hot.
Also found a shorter smaller black wire which, when touched to the block, starts the fan. Fan will run as long as wire is held against block.
Yes, car has well over 500,000 miles on it. And yes, I'm old and still do most of the work on her myself. But most importantly, Yes...I want to learn more and get her running again! Thanks in advance for any help or advice offered! Vykky

[Vykkylou]

After all these miles, I should have known better. Thermoswitch replacement was no good so tried another and Viola! She's running cooler and the fan stays on longer than before but these are little things I can adjust to. For now, I'm just a Happy Camper as all seems good again!