My fan belt was quite loose, so i replaced it, tightened the new belt up to about a 1/2" of play. After about 10 minutes of driving the alternator red light popped on, the tach, signals, and gas gauge all stopped working. Stopped the car thinking the belt had fallen off but it is still on with the same tension. Why should I have an issue with the alt after just changing the belt?

Turn on the car and find out if the belt is moving.
make sure the alternator is spinning.
See if the alternator is charging using a voltmeter.

There are lots of possibilities here - I just went thouh this. You have to start crossing things off the list.

Zach

It sounds like a fuse blew. The gauges should not stop working if the alternator is not working.

Thanks for the responses. The alternator is spinning, the belt is still on. Will now check fuses, but why would the alternator light go on for a blown fuse? Upon starting the car neither the green oil or red alt light were lit. once started the red alt light came on.

I think it's fuse #8 that has popped. A few other things won't work either, you'll find. Turn signals, possibly some other lights...

Find the short and fix it.

--DD

How did you know that?

Replaced fuse 8 and it popped immediately upon starting the car.

Tach drops to zero, alt light comes on, some other stuff doesn't work? They're all powered by fuse #8. (The alt light is sort of powered by that fuse, it's a bit complicated.)

Mine used to short one of the connectors on the back of the fuse to the bracket holding the fuse box. I was missing one of the screws that held it in, and sometimes the whole thing would shift just far enough to short. POP! There goes the tach and other stuff.

I think the short in the license plate lights also popped that one? A body shop wired up the lights backwards, and the power lead was shorting to the metal body of the bumper... That was a fun one to track down...

--DD

It is likely the secondary voltage from the alternator was trying to drive the components on the blown fuse circuit which is why it came on when the engine was running.

with the license plate lights. Had that happen once. IIRC, isn't the coil on that circuit also? Maybe it has shifted?

Is this just a coincidence happening now, after replacing the fan belt? Or could I have hit/moved something on that relay board while adjusting the belt?

Unlikely, but not impossible. Worth checking anything you could have bumped in the area...

--DD

Found the short, it was the lighter! Back on the road. Thanks for the help.