mach4 sorry for the misinformation I ment to say the battery light comes on during initial key on before starting then goes out after the engine is running so if the light does not come on during or when there is trouble with the charging system or the battery then how do I know how reliable is it since there is no volt or ammeter to go by?

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'85 F250 6.9L diesel 2wd XL XC LB Centurion, Gone
'95 Mercedes E 300 Diesel W124.131 with a OM606.910 and just over 200k on the clock, needs top end engine work and a transmission.
2000 S500 W220 with a 5.0L V8 gas power.