after compleating a voltage test on my 1999 explorer with new battery and a second altenator has ben instaled . why will it not charge the battery ?

There must be a bad wire in the charging system. Normally there is a fuse-able link in the big wire that attaches to the alternator, and possible broken wire a the plugin going into the alternator.

With the engine off and the key off check for voltage at the big wire on the alternator. It should read battery voltage even when everything is turned off. If it has no battery voltage then the problem is something in the wire such as the fuseable link that my friend Rowefast referred to. Normal voltage at the battery with the engine running should be 13.5 to 14.6 volts. If this voltage is below 13 volts the alternator is not charging the battery.

thank you thats whare I will start looking next .

What is the battery voltage with the engine off? If it is really low, below 11 or 10 volts, charge the battery with a charger until it reads over 12 volts. Now start the vehicle and test the voltage with the engine running and see what you get. Alternators today have electronic components and can be damaged if trying to charge a severely low battery. The purpose of an alternator is to maintain the charge, not charge a dead battery.

thanks OJ another thing is that when I have car running and disconnect positive it shows a full charge coming from alt but when going direct to bat it only shows half and wont maintane bat

I am not understanding what you are trying to say with your last post, so let me ask. With the engine not running and everything connected, what voltage are you getting on the DC volt meter? Is it over 12 volts or under. If it is under 12 volts, charge the battery with a charger. Once the battery is fully charged, start the vehicle. Now read the voltage at the battery. You should be getting somewhere around 14.5 volts with a new alternator as Bob indicated. What are the two voltage readings under the procedures outlined above?

Oh, and - DO NOT remove a terminal connector from the battery while the vehicle is running to test the alternator. This was a reliable test decades ago but bad today. Not only can the battery explode if a spark is present, but the spike in charging can actually blow out the pcm or other electronics components within the vehicle.

how do I locate a fuseable link with out dismantling the wireharness or removing other componets .

You would have to test the wire anyway, so it should run from the alternator to the battery positive cable. Now that end of the wire could be at the battery or at the fuse box that the positive cable from the battery runs to if thats the set-up you have. Then it is just a matter of checking continuity from one end of the wire to the other, at the alternator. You would need a digital multi meter set at the ohms Ω position.