I have a 1996 ford powerstroke, and im having a problem with the batteries both being completely drained overnight, i checked both batteries on a tester, they are both good. any one have any ideas?

 

Check out your glowplug relay. Must be something big to drain them down overnight.

Brian

 

im not really sure how to go about checking that, i used a test light on one post it lights up and on the other side it doesnt, so im assuming that its not the relay.

 

You can place a multimeter in series with the circuit and on the current setting can read how much current is being drawn from the batteries with the key off. There should be very little current, just enough to power the clock, computer, and other accessories.

The best way to put the meter in the circuit is to totally disconnect one battery, both terminals. On the other battery, remove only one cable and place one multimeter lead on the battery post, and the other on the cable. This will give you the current draw. To isolate the draw, start pulling fuses in the fuse box. When the current draw goes away, you know which circuit is causing it.

Hopefully this helps, you can also do this with a test light by judging the brightness of the light bulb, but a meter is more effective.

 

you can do the same thing by just pulling the fuses, and putting the multimeter in place of the fuse and read the draw on that particular circuit. Anything over an amp with the key off is probably where to start looking. If none of the circuits show an excessive draw then you need to start looking between the battery and the fuse box.

 

I have located a battery drain with this same method but I use a low voltage indicator lite and hook it in series with the positive battery terminal just like the mulitmeter method above. Start pulling fuses till the light goes out and this will indicate what circuit the drain.
GL

 

The methods mentioned above are accurate, but a litle different from my approach. If it was me i would use an inductive DC ammeter. Mke sure that it is a DC model. Just clamp it on the battery cable and you will know if current is flowing through the cable. If it is, then it's just a matter of disconnecting indiivual circuits until you notice that the amperage drops off. That will help you isolate the problem circuit. As mentioned before you can pull fuses until you find the affected circuit. Also keep n mind that some circuits run through fusible links instead of fuses so if pulling fuses does not find your problem you can remove fusible links until you find the culprit.
The ammeter methods does work but IMO the inductive Ammeter is safer , especially if you are not confident in your electrical skills.
Also keep in mind that if you have two batteries and one is defective, the bad one can drain the good one and make them both weak. You may be able to charge the batt's individually and see if they perforn the same. Keep us posted and good luck.