Home Page -> Repair -> Electrical
System ->Charging System
Problems in the charging system show up as either
an undercharged or an overcharged battery. The first
is easy to spot, with poor cranking or the need to jumpstart on a regular basis.
The latter usually manifests itself in loss of the electrolyte and the swift
degradation of the battery due to hardened plates and an inability to accept a
Before condemning the alternator,
though, especially with an undercharged battery, take a look at a few obvious
potential causes, such as slipping belts, loose or corroded battery terminals or
terminals separated from the battery. You might also check that there is not a
parasitic load that is always on, draining the batteries especially when your
Imperial is shut down over a weekend, for instance. Another cause of poor
cranking current is an internal short in a battery.
If your Imperials electrical system has been
worked on at some stage in its history, the size and capacity of the wiring
between the alternator and the battery should be verified to ensure there is not
a major voltage drop between the two electrical components.
If all this checks out, undercharging is most
likely a problem within the alternator. The diode pack may be defective or there
may be an open circuit or grounded rotor coil. If the alternator has just been
rebuilt and is not charging, it may be there is not sufficient residual
magnetism in the rotor to get the alternator to kick in. If so, place a
temporary jumper wire between the diode terminal and the output terminal to
restore the magnetism before running further tests.
To check the current output of the alternator you
can place an ammeter in series between the ground terminal on the alternator and
the ground battery lead. However, since running the alternator on open circuit
can destroy the diodes, you may be better to place an inductive clamp ammeter on
to the battery ground cable. Such ammeters are relatively poor at measuring low
currents, but since the alternator is tested with full load or a carbon pile
across the battery, the inductive ammeter is accurate enough and saves
With a voltmeter across the battery terminals and
a full load for the circuit, start and run the engine to get maximum alternator
output. This may be as much as 2,000 rpm.
As all accessories are turned on, or the carbon
pile load is applied, the battery voltage will drop. At 12.6 volts, the
alternator regulator should apply full system voltage to the field and the
alternator should be producing its rated current. If the ammeter shows within
10% of rated power, the alternator is okay.
As the alternator charges the battery and system
voltage rises, the charging rate should taper off. Somewhere between 13.8 and
14.5 volts, the output should be very small. If the system voltage rises above
15 volts, the batteries will overheat and might even spew out electrolyte
through vent holes. This condition indicates a faulty diode pack and the shop
can replace this part of the alternator for a few dollars or, more simply,
retrofit a rebuilt unit and send the faulty alternator out to for rebuilding.
To test for high resistance in either of the
changing wires, connect the voltmeter between the output terminal of the
alternator and the battery positive terminal. There should be less than 0.2-volt
difference. Check the ground circuit for the same voltage drop. High resistance
here will give a large voltage drop and account for undercharging.
This page last updated
September 1, 2001. Send us your feedback,
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