dead batteries

about a week ago when i went to start my truck the starter made a weird noise. it did it once more after that another time. but when iwent to start my truck last nite it had no power. when i put the key in and turn it the first click all the guages shoot up to the max then go back to nothing. ( speedo goes to 100 mph then goes back to 0) then when i try to start it there is no power. im assuming its my alternator but when the key is in the ignition there is a black module on the passenger side firerwall about the tire that is makin a rattling noise like something is vibratig inside it. the only way my truck will start is when i hook the booster cables up and let is gain a charge. im just curious if anyone had any ideas of what might be my problem. and what that black module is vibrating. thanks. WW

You NEED a PAIR of batteries. They should be replaced as a pair. What you have is one battery, in two places. Then check your alternator output, it should be about 14.5 volts. If it is, then it was your batteries; if the voltage is not up to 14.5 volts, you need an alternator.

Dave

By what you describe, I'd say Batteries, Battery connections or Both.

Static and Load test the batteries >Independently< and clean up all the connections.. see what you get.

Not preach'n, or try'n to be the GURU here boyz'n girls but.. Good healthy Batteries and Good clean cables'n connections are OMNI important. With all the Electronic Wizardry of today.. Volt and Amp values.. or the OFF SPEC of them, can make us try'n to fix Anomalies (NASA speak)

What DaveBen sez too.

stupid question but where abouts on the alternator do i put the volt meter to get a proper reading.

theoretically, as long as you dont have extremely bad connection between alternator and ground/positive, you could check he voltage at the batteries with the truck running, to see if the alternator is charging. after you start it you should see it spike up to at least 13, and it should continue to rise from there.

Check your batterys one at a time. I had the same noise under the dash & I had 1 battery going bad, changed it & problem - gone. I know!!! I am going to replace the other battery soon.

I had about the same problem, it was the batteries like everyone else has said. Just changed one in my car too. They don't make 'em like they used to.