so i just installed a new ac compressor in my car....while installing it i had my stereo on in my car and after a lil while, my battery went dead, only maybe an hour. I did a few other misc stuff to my car, removed front bumper and a headlight to seal it up also. i jumped my car, let it run to operating temp and shut it off. i then continued with the front bumper stuff and went back to start it up, and it was dead. i jumped it again and noticed my CEL was on and that also when i put the car in reverse, my tach dropped down to 0 and wasn't working, but only in reverse. worked fine in other gears. would a bad batt cause this too happen, especially if it went dead after an hour and my sub wasn't even going...? 00 civic si

 

It could be, you should try driving it around for a little bit and let the alt charge up the battery.

If it doesnt start up after that, then its definitely your battery or alternator.

What do you mean when you put the car in reverse the tach wasnt working?

 

as soon as i put it in reverse, the tach dropped to 0 and wouldn't move when i would give it a lil gas

 

Take that ish to a local shop and have them throw an ED 18 on that bitch. It's just an Interstate Batteries machine. That little thing will give you descriptive information about what is going on with your starting and charging system. Have them check it out for you and don't worry about farting around guessing about what's wrong.

 

just went ahead and bought a new batt...works like a charm...as for the CEL, i think it was cuz i was messing wit my o2 sensor in my test pipe. thanks!