Another battery/ABS/alternator 850 question..

  • Thread starter Thread starter Robert
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R

Robert

By the way,

what is the definitive method for determining whether you have a bad battery
or alternator?
Or both?

Thanks in advance.
 
what is the definitive method for determining whether you have a bad battery
or alternator?
Or both?
Click to expand...

I'm going to qualify this by saying I don't know if this is good
advice, but here's what I was told a long time ago. Take a voltage
reading of the battery with all accessories off (and the ignition
off). It should be very close to 12 volts (11.6 - 12.4). If you
don't, the battery is in poor condition.

Now with the engine on, accessories off, and alternator at about 1500
rpm take another reading. You should get about 14 volts (13.8 - 14.2).
If you don't, the alternator may be in poor condition.

Beverly
 
Robert said:
By the way,

what is the definitive method for determining whether you have a bad battery
or alternator?
Or both?

Thanks in advance.
Click to expand...

First charge the battery. There are 2 load tests, one using high
current for a short period (to see if it's going to work the starter
motor), and the other a much lower current for a longer period (to see
if it will run the car properly if the alternator fails). Usually
batteries fail the first test while still passing the second, but not
always. (There's a third test involving no current, and nothing but
time, some batteries have internal current leaks that cause them to
discharge over a 2-3 day period, but are otherwise fine if used every
day.)

The alternator test involves measuring voltage under load - start the
car, run at 2000 rpm, turn on the lights, heater fan, rear defogger, and
measure voltage at the battery. If it's at or above 14 then your entire
charging system (including alternator wiring) is probably OK.
 
One quick & dirty test I use for the alternator, is to disconnect the
battery, and see if the car stays running, ( This is while it's on.)
This could be bad for your car, I don't know. Any advice you take is
your responsibilty, and I take no responsibility for anything you do
based on what I say.
Good luck,
Ryan
 
I would not do that! In older cars, without computers, that is ok to do. Not
with any car with a computer. It could cause damage to some component that
costs a lot more than an alternator, and could be difficult to track down.

You should be able to pull it off and get it bench tested. You could also
try to take a reading off the connectors from the alternator itself.

Jeff
 
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