can i check my alt.without removing it?why is battery light on?

back yard trick,disconnect neg battery cable while car is running if it dies good chance your alt is bad

Yes that works too,if the car doesn't die,kill the truck/car and remove the positive cable and leave it off for a few minutes,this will reset the computer.If the light comes back on then you likely have a problem.If the car runs normal,then it is probably a sensor,depending on the age of the vehicle,the O2 sensor will cause this problem,if so check your gas mileage and if it has gone down substantially,replace that and see what happens!

For future reference on this subject,DO NOT disconnect either battery terminal cables from the battery or anywhere else with the vehicle RUNNING,or damage could/will occur to the ECM/PCM due to a voltage spike! Using a volt meter,set to the 20 volt scale on DC Volts or if using a digital volt meter,just set to DC Volts. With the engine running,test at the battery terminals,red lead to positive terminal,black lead to negative terminal.The reading should be 13.5---14.5 VDC,14.2 is ideal. If you're tempted to test an alternator by disconnecting the negative battery cable, don't do it. A good alternator may indeed keep the engine running, but it was never a good test. In the pre-computer days, you could pull it off without damaging anything. Today, you risk frying every electrical device in your vehicle.