I tried to jump car, had enough volts, but when I cranked, it would crank one time, and die, volt gauge would drop to 0. Then my tac would go nuts until I pulled the key out.

So we figured out its a bad connection at the negative terminal. I fiddle with it, get it to start, take it for a drive. Some slight misses, and tac is off 500 rpm. I get it home, go to start it to pull it in, same thing. Cranks one time, then dies. Fiddle with the connection, and it started. The connection is really poor.

So when I get this fixed, will my tac read ok again?

 

Clean up the battery post and cable terminal. You wont know anything until you do.

 

I need a new battery. When I got the car, the neg part of the battery was slightly stripped. Now it will bearly tighten.

 

When the engine is running you should be seeing adequate voltage to properly operate the tac, radio, ecm and etc. provided the voltage regulator in your alternator is functioning properly. If the engine is running and the battery is out of the system because of the bad terminal your system sees 14.7 volts from the alternator ( I think, working from memory). I have had one experience with voltage regulator failure to high voltage and cooking the water out of the battery and then with battery disconnected and engine running it blew every electrical item on the car that was energized, bulbs, points etc., long story that still chaps me a little. Point being if the engine is running all should be good but there is a possibility the bad connection could be driving the ecm and tac a little nuts. I would borrow a battery from the other car and see if it corrects the issues and if so get a battery rather than drive the car as is.