I had the lower alternator bolt shear off while driving causing a loose belt. I was able to get the car to a garage and replace the bolt and re-tension the belt. Now the check engine light is on, tachometer and coolant gauges don't move, transmission is very clunky when shifting out of park and car seems to be running rich. I had someone try to retrieve the codes, but their reader couldn't connect. They checked the battery and said it's low, but the alternator is charging properly. They said the battery may be too low to pull the code. I pulled the battery and put it on a trickle charger for several hours. When I started the car, the rpms bounced from 1k to 2k and there was a click behind the glovebox each time it bounced. After a couple of bounces the tach went to zero. I shut it off immediately. Has the computer fried or is an O2 sensor gone bad? The last time it ran rough like this I had to replace the crank position sensor (after head gasket replacement).