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Alternator/charging system tests for AMC vehicles

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Category: The Garage
Forum Name: Electrical - non engine
Forum Description: Charging systems, lights, non-ignition system, it goes here.
Printed Date: Feb/27/2021 at 3:21am
Software Version: Web Wiz Forums 12.03 -

Topic: Alternator/charging system tests for AMC vehicles
Posted By: billd
Subject: Alternator/charging system tests for AMC vehicles
Date Posted: Oct/20/2015 at 1:20pm
Troubleshooting AMC charging systems.
This will eventually include ALL AMC charging systems, from generator to Motorola, to Delco, Motorcraft, Bosch, etc. alternators and regulators.

For now this is a work in progress. I will try to get it finished up as much as possible as time allows, meaning breaks, lunch, etc.
But until my shop is back in order (which is a top priority right now), it likely won't be complete to the final goal - which includes pictures.
I will get the pieces in place, likely out of order, as they come to mind or I can think of them and be sure I'm correct in the processes and specs.

For most of this you will need these basic tools:

* Crocus cloth for cleaning connectors/connections found to be dirty or faulty. (see Note 1 below)

* Wrenches as needed to disconnect battery cables or other connections such as ground straps between engine and chassis, or connections on the back of the alternator.

* Decent functional volt meter that can read 0-20 VDC (volts DC) - not the volt meter in the car!

* Jumper wire or two with small aligator clips on each end. Helpful if the clips are insulated but not necessary if you are careful.

* Decent hydrometer for checking the battery's state of charge.

The car's battery presents a constant load (although it varies, it is always there) to the charging system. If that load is removed, say, a cable is disconnected, a circuit broken, wire removed, while the charging system is operating it will cause the voltage to rise to levels unsafe to the charging system, resulting in damage.
NEVER EVER disconnect either battery cable while the engine is running and/or alternator charging! Doing so typically results in more damage than you start out trying to find.

Before you even think about testing the charging system, aside from the tools listed above, you MUST do the following -
1. The battery needs to be at at least 75% of full charge for these tests to have real meaning. Using a good battery charger, charge the battery until at least 75% or even better, over night.
Since a FULLY charged battery should read 12.6-12.65 volts, a battery at 75% charge should read 12.4-12.45 volts without a load. Specific gravity should be at least 1.230 (Motorola states some batteries should read even higher, 1.250), indicating 75% charge.(note chart below)

2. Once the battery is at least 75% charged, make sure all connections are clean and tight. This includes the battery cable to battery, negative cable to engine block, engine block to chassis ground, battery positive cable to solenoid or other junction point, output terminal on the alternator to the output wire connection, alternator output wire to solenoid or other junction point, etc. You cannot always tell by LOOKING, a thin layer of copper oxide, lead oxide (at the battery) and so on can be between the parts of the connection, hidden from view. Tight physically does not necessarily mean good electrically.

Below I will list the conditions, and then the tests to perform based on the existing conditions. Be mindful of the order of things - whether or not the engine is running, jumpers or wires connected, etc.

On to the testing - I'll start with the later Motorola systems used by AMC during model years 1971 and later.

Problem - The battery is under-charged.
  Indicator light is ON: (engine running at idle)
    Perform open diode-trio test (Test 1)
  Indicator light is OFF: (with engine NOT running but ignition ON)
    Perform regulator test (Test 2) tests 2A and 2B.
Problem - The battery is OVER-charged.
  Check connection and condition of black wire between regulator and alternator frame or case.
    Black wire connection loose or dirty, clean, tighten, check again.
    Black wire connection clean, tight, regulator is likely bad or shorted inside. Replace regulator.

Test 1, open diode-trio test.
  Test Conditions - connect volt meter as described below, connect jumper as described below,
  start engine and run at idle during test.
     Connect volt meter black or negative lead to battery negative terminal.
     Connect volt meter red or positive lead to alternator output terminal.
     Connect a jumper wire from the regulator terminal on the back of the alternator and the output      terminal on the alternator.
     Start engine and allow engine to idle.
        * If volt meter now reads 13.5 to 15.0 volts the diode-trio is open and needs to be replaced
           (Internal part)
        * If the volt meter does NOT read 13.5 to 15.0 volts, shut off engine, remove jumper and
           perform regulator test 2B.

Test 2, regulator test.

  2A Test Conditions - connect volt meter as described below, connect jumper as described below,
  engine OFF, key ON during test.
    Connect volt meter black or negative lead to battery negative terminal.
    Connect volt meter red or positive lead to alternator output terminal.
    Connect a jumper wire from the regulator terminal on the back of the alternator and the field          terminal on the alternator.
            *Indicator light is now on after jumper was connected.
              Regulator is open, replace regulator, Proceed to 2B
            *Indicator lamp is still not on, check dash bulb and light circuit in harness.
              (See Note 2)
            *Indicator lamp and circuit checks out but light still not on with jumper in place,
              there is likely an open in field circuit i.e. bad brushes, slip rings or open rotor windings.
              Remove alternator for repairs.

  2B As above in 2B, light is now on with key on and engine OFF.
    Start engine, run at about 1,000 RPM with no electrical loads.
    Volt meter should now read 15.0 volts or more.
            * Voltage less than 15.0 volts, alternator defective, remove for repairs.
             (faulty/bad stator,  diodes, etc.)
            * Voltage 15.0 or greater, proceed to Output Test to check for proper regulated voltage
            (COMING LATER!)

Note 1 -  Emery cloth or even fine sandpaper will work to clean electrical connections or contacts but you should be aware that the latter may leave bits of grit behind that can stick in or on connectors and actually make things worse and not better. If you use sandpaper or emery cloth DO ensure that the connectors/connections are blown off or wiped with a clean grease-free cloth before putting things back to ensure you don't have grit holding things apart.
DO NOT USE SILICON di-electric grease on automotive primary electrical system connections. DI-ELECTRIC means it insulates, any such grease or compound in the connection can cause a voltage drop - meaning bad connection, it not now down the road.

Note 2 - dash light bulb circuit is typically orange wire with black trace OR a blue wire running from cluster, out of cabin to regulator or alternator area. Make sure the bulb is GOOD and seated well in the cluster, you can also test the bulb and bulb circuit by grounding the outer end of the blue or orange with black trace wire. Check schematics for specific details for YOUR own car first!! Or ask......... someone on the forum will know or can look it up for you!

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Posted By: billd
Date Posted: Nov/27/2017 at 8:03pm
1971 and later AMC with Motorola alternator "glowing dash ALT light"

If all else has been checked, the alternator charges, passes a load test, regulated voltage tests fine and all connections external of the alternator have been disconnected, cleaned, and reconnected but the dash ALT light still glows dimly, try this test:

You will need a decent multi-range volt meter (the sort often referred to as VOM, etc.)
Start the vehicle and let it run at idle with ALL accessories and lights OFF.
Connect the negative (usually black) (-) lead of the volt meter to the aux or regulator terminal of the alternator (the orange wire connects here)
Connect the positive (usually red) (+) lead of the volt meter to the output terminal of the alternator.

The reading on the volt meter is the voltage drop across the diode trio in the alternator. Voltage drop is an indication of resistance - more voltage drop means more resistance.

If the volt meter reading is OVER .6 (six tenths) of a volt, replace the diode trio.
If the volt meter reading is under .6 volt the trio or field diode assembly is not the cause of the dash ALT bulb glow.

Check for loose, corroded or dirty connections in the dash bulb circuit at:
Alternator output terminal
Starter relay battery terminal
Ignition switch
Fuse panel
Instrument harness connector
Instrument panel printed circuit
ALT indicator bulb socket

Any connection issues at these points can lead to the bulb glow. 

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Posted By: billd
Date Posted: Nov/27/2017 at 8:15pm
1967 - 1969 Dash ALT light glow even though alternator and regulator operate correctly - 

(I deem this as an "if all else fails" fix - the alternator should be tested, especially voltage drop across the isolation diode, etc.)
According to AMC, replace the original equipment #158 bulb with their service replacement, bulb number 1445.
AMC deemed the 158 may be too sensitive. 

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