Just had this problem on a Ford diesel truck. The problem turned out to be one of the batterys (these trucks have 2 batterys in parallel) was defective. I actually have seen this many times on other vehicles. The battery will not take a FULL charge but will take the amperage. This truck was charging at about 90 amps but could not get the voltage up. I unhooked the bad battery and the voltage came up as it should and the amperage droped as it should. This is the type problem that normal battery testing will not find. Alternators may be able to supply a certain maxium amperage but they can't supply it for long periods of time in extreme heat. And by the way, this truck had 2 new batterys installed at the same time the first new alternator was replaced and the alternator was a Bosch reman. The alternator had already been replaced again before I got the truck. You will need to find a meter that can read voltage and at least 100 amp. These tend to be expensive (the one I use now was about $1000). See if you can find a shop that has this type of equipment and someone that knows how to use it.

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84 500 SEL (307,xxx miles)