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car I've seen, DO NOT run the car with the battery off-line. Now I've
never investigated the electrical reason for this (though I imagine it
either has to do with internal resistance (i.e. load) on the system, or
the natural capacitance of the battery in line. Haven't even investigated
which component(s) would buy the farm if you do this, all I know is,
DON'T DO IT.

There are other ways to test the alternator, the easiest being the load
voltage test. Voltmeter across the battery leads, engine off, accessories
off. Should read between 12.5 to 13 V. If so, battery good.

Engine on, accessories off. Read voltage, it should be between 13.5 to
14.5V. If so, alternator shows promise. IF higher than battery, system
will, in this condition, apply a charge to the battery. IF higher than
14.5 V, the voltage regulator may be fried. Problems with this are,
though it'll charge the battery, it will OVERCHARGE it, causing lots of
hydrogen venting, frequent need for water (to replace the hydrogen),
shortened battery life, and, depending upon some ECU electronics (ones
without internal regulation) potential risk of ECU failure due to
overvoltage/current.

Engine on, all accessories on (load test). Voltage should STILL read
between 13.5 to 14.5 V. If so, alternator okay, your overnight battery
problem (which it most likely is) is probably an unknown accessory that
isn't turning off. See * to check out this problem. If the voltage is
lower or even the same as the battery voltage, engine off, the alternator
probably has worn brushes/commutator or failed coils. Time for rebuild.

* tracing power drains., see post of a day or two ago from Huw Powell.
Well done there.



Hi All,

I just acquired a 1988 90 Quarto without getting any
history.  If driven every day it is fine.  If left un
driven for 48+ hours it will not start without a jump.
 I was told to disconnect one of the battery cables
while the car is running; if the engine does not shut
off then the alternator is OK and the problem is the
battery.  Is this true?  And Is this a common
electrical problem with the 90?

Thanks.
Alex