hey...my son replaced his alternator and drove to the local auto parts house. They tested his charging sytsem and said the battery was shot... we replaced the battery and cable clamps. Itstarted and drove 2 miles and sat overnight...next day it's deader than a door ****. We checked the connections and all the wires...I've never heaqrd of such a thing ..anyone have any ideas? It's a 76 F250 w/ a 360...how could a new battery go dead over night and there's no hot connections left on.

 

Disconnect the negative battery cable from the battery and connect a test light between the cable end and the battery post. Double check that there is nothing on, this includes interior lights cause the door is open and the under hood light if you got one. If the test light is illuminated, you have a constant drain that is killing your battery. Pull fuses one at a time till the test light goes out to locate the circuit that is causing the drain. Then trouble shoot that circuit.

 

so ...pull the ground wwire and connect a 12V test light between the two. If the light comes on it's a short and then the track the short pull one fuse at a time to track it down..wow sounds simple enought...thanks for the advise..

 

Check for parasitic drain, as hdgapeach suggested ( I was typing this as I’ve seen his posting), an amp meter is preferable if you have access to one (capable of at least 3 amps). This is what you are supposed to do to check your system;

1. Charge the battery(ies) to read about 12.6 V, with the truck off. (If you can’t get them there, they are dead).
2. Start the truck and measure the voltage again, this time you should get between 12.6 and 14.5 V, 13.8 is supposed to be good. Make sure that all loads are turned off, radio, heater, etc. Less than 12.6V possible bad alternator, more than 14.5V possible bad regulator, which in my case, are one unit. If the alternator is suspect, make sure that the drive belt is good, and the tensioner (if any) is working. If there is no slippage with the drive belt, look for voltage on the “Battery” wire out of the alternator, about 9-10 volts, but any voltage is a good sign. Also check the “B+” (big cable), should read the same voltage as you do at the battery. These tests have to be performed with the truck running.
3. With all loads off, and keys off the ignition, disconnect the negative battery cable and place an Amp meter between the cable and the battery terminal. You should read not more than 500 mA, with less 300 mA ideal (at least for my truck, 2001 X). If you get a higher reading is time to let the games start. You turned off the hood light, right? Start by pulling fuses until you find the one that brings the load reading down to acceptable levels. This is the circuit that you would need to test for shorts, defective equipment, etc.

If you have performed all of these tests and still can’t find the problem, take the alternator back to the store and make sure that they perform a load test. This should show if there’s a shorted diode.

Hope this helps.

Rico

 

I've not only never owned a meter but I've never even used one...am I to test the amps's going accross the two poles, on the battery?

 

Vehicle of that age probably has an external voltage regulator. They are notorious for internal shorting and draining a battery over night. I would charge the battery and unplug the regulator and see if it stays charged up over night if you dont have a test lamp to observe a draw.

 

Here's how to do it. If you want to use a meter, hook it up the same way as the test light, not across the battery.