My taurus wont start

the first thing to do is to check things like the trunk/boot light is not staying on then if you are a technical then use a multimeter on amps setting in series with a disconnected battery terminal neg is best. then start to remove fuses from the fuse boxes one at a time until you find the current drain the drain should only be a few milliamps ie,100 or less not then you have a clue to the problem area make sure all doors are latched before you test

I can't imagine how changing the belt would have any effect on the electrical system if the alternator is working.
The way you check for a drain is to disconnect the battery ground cable and use a test light to make the connection between the battery and ground. If there is something drawing power from the battery the light will glow.
Then you pull each fuse from the fuse boxes until the light goes out. You also disconnect the starter and alternator if necessary.

Did you check for B+ voltage at the heavier gage wire on the back of the alternator ? When yor turn the key to run , not start , does the charge indicator light ? When the vehicle is running is that charge light lit or off ? There is a fuse that supplies B+ voltage to it . Fuse # 30 located in the central junction box , it's a 30 amp fuse ! Plus one more fuse located in the battery junction block # 11 20amp . With out B+ voltage at these connections on the alternator it will not charge .

Check for a battery drain in the vehicles fuse box circuit.

BATTERY DRAINS ARE TRICKY. TURN OFF ALL SWITCHES, CLOSE GLOVE BOX. MAKE SURE GLOVE BOX LIGHT IS OUT. OPEN THE HOOD. DISABLE THE UNDER HOOD LIGHT. CLOSE ALL THE DOORS. MAKE SURE ALL COURTESY LIGHTS ARE OUT. DISCONNECT NEG. BATTERY CABLE. USE 12 VOLT TEST LIGHT, CONNECT ONE END TO A METAL PIECE ON THE MOTOR FOR GROUND. FASTEN THE OTHER END TO THE NEGATIVE BATTERY POST. IF THE LIGHT IS ON, THAT MEANS THERE IS A BATTERY DRAIN. LIGHT NOT ON, NO DRAIN. IF THE LIGHT IS ON, FIND THE POWER BOX NEAR THE BATTERY. PULL OUT ONE FUSE, IF THE LIGHT STAYS ON PUT HE FUSE BACK, PULL OUT THE NEXT FUSE. IF THE LIGHT STAYS ON PUT THE FUSE BACK . REPEAT THIS UNTIL THE LIGHT GOES OUT, OR GETS VERY DIM. OUT IS BETTER. WHEN THE LIGHT GOES OUT, THAT IS THE CIRCUIT THAT IS DRAINING THE BATTERY. REFER TO OWNERS MANUAL TO IDENTIFY THE CIRCUIT. IF THE LIGHT NEVER GOES OUT, REMOVE THE WIRES FROM THE ALTERNATOR. IF THE LIGHT GOES OUT, ALTERNATOR NEEDS REPLACING.

START THE VEHICLE, PUT A VOLTMETER ON THE BATTERY. THE VOLTAGE SHOULD READ 13.90 TO 14.80. IF THAT IS OK, THERE MAY BE A BATTERY DRAIN. TURN OFF EVERY SWITCH IN THE CAR.CLOSE ALL THE DOORS. MAKE SURE ALL THE COURTESY LIGHTS ARE GOING OUT. & THE GLOVE COMPARTMENT LIGHT. RAISE THE HOOD. DISABLE ANY UNDER HOOD LIGHTS. DISCONNECT THE NEG. CABLE FROM THE BATTERY. PUT A 12 VOLT TEST LIGHT, ONE END ON THE NEG BATTERY POST & THE OTHER END ON A GOOD GROUND { METAL }. IF THE LIGHT COMES ON THERE IS A BATTERY DRAIN. NO LIGHT , NO DRAIN. IF IT'S ON, ONE AT A TIME PULL THE FUSES OUT OF THE POWER DISTRIBUTION BOX. IF THE LIGHT STAYS LIT, PUT THE FUSE BACK. DO THIS WITH ALL THE FUSES BIG & SMALL. IF THE LIGHT STAYS LIT, DISCONNECT THE ALTERNATOR. THEY CAN CHARGE THE BATTERY WHILE RUNNING, & DRAIN WHEN STOPPED.

Check ground wire from engine to frame and battery to body

She has whats call a parasitic drain. Something is remaining on or activated that shouldn't be. You can get get a parasitic drain test to verify this but i wont find the problem. Unless its like a light bulb or something you can visually see that's still on you will have to get it hooked up to a scanner to see what's staying on. You could always disconnect the negative side of the battery when you leave it for awhile if you don't wish to spend the money.

have you checked to see if the alternator is working? If it isn't the battery is being drained. You may have lights and all but the cranking power is drained therefore you don't have enough juice to start it. So check the alternator and if possible do an obd 2 scan at autozone or advance auto

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