#1. Are the connections all "tight and clean".
AutoZone ,Advance,Etc will all test the Alternator for free!
(it may just be the regulator/brushes)
#2 Forget driving in the dark with a dead alternator...The lights might kill a
"New" battery before he gets there.(Who knows what else is wrong!)
#3 NOTHING else will work...But the SDL's engine will run without electricity...
the EDS (Electronic Diesel System[engine computer]) will not function...which
may not allow the engine to "drag him home".

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'84 300SD sold
124.128