I have a 2000 Chevy Blazer 4x4. Bought it at an auction. Had the oil and oil filter changed the next day, drove and ran fine until my tachometer started going crazy and the temperature gauge would bury at max would pull over turn it off wait a few minutes and restart and drive fine. Then a few days later drive maybe a mile or so and would act as if over heating. Checked coolant both in the radiator and the reserve reservoir both levels fine. Ended up changing the thermostat drove fine...for a week then tachometer started going crazy again. Was advised to change coolant hoses to and from engine / heater core and to flush heater core. So replaced hoses and flushed heater core. Still had tachometer going crazy and started shifting hard. Had the transmission fluid and filter changed after checking fluid and it was as black as motor oil. The next morning tachometer still going crazy, speedometer stopped working, and stuck in first gear. Can't afford to junk it too broke to take it to a highend garage. Was told it could be an electrical connector on the transmission loose but can't seem to find the name or location of connector to have it checked. Please any suggestions are appreciated

 

Welcome to the Blazer forum RF3,
The very dark ATF is, of course, bad news (as in the trans may, most likely, be in need of major repair/overhaul or, at the very least, a valve body replacement (not to costly IIRC).

As for the shifting issue: first, make sure that the ATF level is correct. These transmissions have a very specific fluid level check procedure that, if not followed exactly, will often result in an 'over-filled' condition that can and will cause erratic trans operation (improper shifting, iffy reverse, etc). So, make sure that you check the fluid level exactly as per the owner's manual procedure.

As for the crazy tach, etc, readings: I'd start with cleaning the intake manifold mounted electrical ground posts and Gnd wire terminals, along any other gauge and engine management system grounds (some of these vehicles have an instrument Gnd located on the side frame rail just below the driver's seat.

 

It could be an issue with your VSS (vehicle speed sensor). I had the same problem with mine after we replaced the engine. The wiring harness that goes to the VSS damaged and it wouldn't shift out of first gear. Your tachometer problem could either be a bad distributor. I had the same problem and replacing the distributor fixed it. It could also be the tach wire going bad. Are you having issues with it running or is it just the tach that doesn't work?