Diagnosing Your Charging System 

Imperial 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 charge.
     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 disconnecting terminals.
      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, and come join the Imperial Mailing List - Online Car Club