Mercedes: Mercedes 1999 E300 Turbo Diesel, mercedes 1999, petroleum jelly

Question
My car will not start in very hot weather.  All panel lights are on, but it will not turn over.

Answer
Rodney,
Are you getting any effort out of the starter. Click,click, with no motion, Ggrr ---rrrr --rr to stop....?
The compression ratio on diesel is way higher that a gas engine. It requires a larger and stronger starter to squeeze the fuel to make it ignite, so diesel requires much more starting amperage that a gas engine.
Larger starter... greater cranking amps to get it to move against the torque.
If so it could be the following.
Poor battery (s). not holding full charge. Have the batteries checked with a load meter.
If battery (s) OK. Look for bad connection caused by dirty terminals. Remove the terminals (-)then (+). Clean terminals and post with brush and reinstall in reverse order. Cover with petroleum jelly or install the felt anti-corrosion washer before installing the terminals. Secure tightly.
Check grounding wire terminals to chassis; remove and clean if possible.
If, after each of the above items has checked OK,  you still have the same issues, then I expect that you will need to have the starter checked.
It could be the unit switch..(Bendix) or bad brushes within or even a bad ground at the unit.
A good independent starter/alternator rebuild shop should be able to rebuild it for less that $100.00.
Good Luck
Let me know what you discover.
Rob