Ok my tachometer goes crazy. reads zero then jump up and down then will read normal again for a few seconds then go crazy again most of the time it just reads zero.
It all started with my friend hosing off the engine witch messed up my alternator it would peg the battery gauge but I didn't run it like that for long maybe a few seconds, so I changed the alternator but while I was changing my alternator my friend left the radio on so my battery died so I bought a battery charger from a yard sale for a dollar I figured it was worth a try but the people put the red and black clamps reversed but I noticed it right away so was only on the car for maybe 5 seconds got everything done but then the tach went crazy what could it be? I was thinking a ground somewhere

 

what year and engine?

 

Tachs are known to go nuts if the battery voltage is low, but from your story it is possible that by connecting the charger in reverse some electronic component got fried.

 

A funky ignition control module can cause an erratic tach bounce.

 

Its a 93 v6 automatic, now it seems my tach works for a while when cold but ounce it heats up it goes crazy again, how do I go about testing?

 

not sure if the 93 v6 has the tach module i see its the white wire coming outa the icm