My alternator works is good when off the car when its on the car the alternator is bad. what is my problem

A bad alternator no longer charges the battery, the battery will die once it is drained. This will occur more rapidly when using equipment that runs off of the battery( lights, heater blower, windshield wipers etc.) even electronic ignition will run down a battery. Your battery just went over night the alternator may have stopped working long before. Go to an auto parts store that offers free testing to make sure it is your alternator before making repairs.
Good luck.

ok , i bet you didnt test all fuse. in dash and under hood, i bet
fuses that look good, can be bad (hair cracks)
use and ohmeter, 0ohms good, infinity bad.
if you have 12v and the Cig lighter and the dash is dead.
then the fuses are blown.
log in to alldata.com and look, all the drawings are there.

Your alternator cant be good if all those dash lights are on then the alternator is bad, reoplace it and make sure the battery is tested, a bad battery can and will destroy a new alternator, also make sure the connections at the rear of the alternator are good, the reason for car dieing in motion is due to the battery draining down and theres not enough voltage to keep ecletrical components going, you must have a bad connection at alternator or alternator is bad. and the last thing to check would be the alt fuse which should be located near the postive battery terminal. good luck.

Your alternator is likely bad. Start the car, disconnect the battery, if it stops you need a new one - or head down to the parts store to ask them to test it on the car for you.

Hope this helps.

IT COULD BE A BAD ALTERNATOR. JUMP START THE CAR, REMOVE THE JUMPER CABLES, PUT A VOLTMETER ON THE BATTERY. MOTOR RUNNING, VOLTAGE SHOULD READ 13.90 TO14.80. IF IT READS 13.00 OR LESS OR THE VOLTAGE STARTS DROPPING, THE ALTERNATOR IS NOT CHARGING. BEFORE BUYING AN ALTERNATOR, GET A GOOD FUSE TESTER, CHECK EVERY FUSE,, FUSEBOX UNDER THE HOOD & THE FUSEBOX INSIDE THE VEHICLE. IF ALL FUSES CHECK GOOD & BATTERY VOLTAGE IS LOW WITH MOTOR RUNNING, THE ALTERNATOR MUST BE BAD.

Sounds to me like the alternator is bad and not charging the battery sufficiently to start the car once stopped. Also, a bad alternator will show a 'charge' light that is red on the dashboard if the circuitry inside the alternator is working correctly and the bulb is not burnt out.

Check that the alternator's drive belt is present and tight on the pulley when the car is stationary and the engine is not running.

Good luck on repairing this charging problem.

bad battery?

bad alternator?

If you have a voltmeter, measure the battery with the car off. Voltage should be about 12.

Start the car, now measure the voltage, if the alternator is good the voltage should be over 13 volts.

If you don't have a voltmeter, go to Autozone, Checker......for free diagnosis.

Sounds like you may have bad battery make sure your battery is getting the charging voltage of 13,8 volts or higher check it with a volt meter and see if the alternator is working right I've gotten bad alternators one can never be to sure than elminate that problem and move onto the battery make sure its good. it should have 12.6 volts or higher after it has been fully charged or you think its charged . than last check your relays and your fuses make sure there all working good luck

Yes, It could affect the cooling fan. R&R the alternator. Good Luck, Ned

You have probably been getting 'rebuilt' units which is not bad in itself but if the rebuilder received a batch of bad diodes they may have these failures until they are exhausted.
But, buddy, you ain't broke my record yet; a lousy Saturn (LAST GM I will own!) bought new, ate SIX (6) alternators in 57,600 miles. That wasn't the worst problem; it ate the auto tranny at 31,000 and dropped a rod cap at the above final mileage.