you must check the actual output -- bad diodes will give proper voltage up to when the load exceeds the output when voltage will drop.

You also need to check cranking voltage (must be higher than 9 V) and starter current draw.

Finally, you must be aware that you CAN run the battery down making large number of start and stop trips if the engine does not start instantly warm. If you have to crank 10 or 15 sec each time, it won't recharge in a mile or so, and eventually the battery will go flat.

Failure to engage the starter motor is either a bad solenoid or a tired neutral safetly switch -- this can also be missing viton bushings on the shift linkage so that the switch isn't always engaged in Park. That is an easy fix, anyway.

If it cranks slow and the headlights dim quite a bit while cranking, it's too much current draw and you need a new starter.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!