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Old 03-19-2002, 10:20 AM
dwight hinton dwight hinton is offline
Registered User
Join Date: Nov 1999
Location: Matthews, NC, USA
Posts: 133
Alternator Output?

Battery dead again on wife's car 300E. Seems to be a short circuit (in her brain and mine). Need Help.

So I pulled the new battery out of the 911Turbo and put it in the 300E.

1986 300E stock 70 amp alternator
at 2500 RPM and loading till Voltage drops to 12 volts yielded
alternator putting out 35 amps with HVAC system and radio turned off.

Replaced alternator (went to 80 amp as it was available)
Woops, also realized that wife had left rear window defroster
on and it was not timing out so maybe alternator replacement was not necessary but at 155,000 miles, I'll bite that one.

However, with new alternator, I note the following:

80 amp min. Gross Alternator output (per local alternator shop)
less +/- 10 amp draw from fuel pump (guess)
less +/- 10 amp draw from ignition system etc. (guess)
should yield > or = some 60 amps net at 2500 rpm
while loading the battery down to 12 volts.

Yet mine is only putting out 50 to 53 amps.

any suggestions?

She is bad about starting the car, turning on the defrost (both front & rear) and then going back in the house for 5-10 minutes.
I realize at idle that the alternator cannot put out its max. and for this time frame is likely pulling the batter down. She only drives 3 miles to work so recharge time is limited.

Is there a hidden problem that I am missing
is her procedure and circumstance inherently doomed?

Cold weather is nearly over so if its her method, it may solve itself until next winter.
If there is a hidden problem, I fear I am merely going to watch another battery die?


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