Electrical Diagnosis

Systematic diagnosis of electrical system issues in the 1970 AMC Rebel Machine, including testing of charging system, starting system, lighting circuits, and accessory electrical components.

Warnings

Tools required

Parts

• Electrical contact cleaner spray × 1 — Use quality electronics cleaner
• Dielectric grease × 1 — Use OEM specification
• Replacement fuses (assorted) × 1 — AGC glass tube fuses, various amperage

Preparation

1. Park vehicle on level surface and apply parking brake
2. Turn off all electrical accessories and remove ignition key
3. Allow engine to cool completely if recently operated
4. Ensure interior lighting is off and doors are closed to prevent battery drain during testing
5. Gather wiring diagram specific to 1970 AMC Rebel Machine for reference
6. Clean battery terminals and cable connections before beginning diagnosis

Procedure

1. 1

   Initial visual inspection
   Perform thorough visual inspection of entire electrical system. Check battery for physical damage, corrosion, or leakage. Inspect all visible wiring for chafing, burns, melted insulation, or damage from rodents. Examine fuse panel under dash for blown fuses, corrosion, or signs of overheating. Check ground straps from engine to firewall and body to frame for tightness and corrosion. Inspect alternator belt for proper tension and condition.
2. 2

   Battery condition testing
   Test battery state of charge using DMM - should read 12.4-12.6 volts for fully charged battery (or 6.2-6.3V if 6-volt system). Perform hydrometer test on each cell to check specific gravity (should be 1.265 or higher and within 0.050 points between cells). Clean battery terminals and cable ends with terminal cleaner. Perform load test - battery should maintain at least 9.6 volts (or 4.8V for 6-volt) under load equal to half the CCA rating for 15 seconds. Record all voltage readings for baseline reference.
3. 3

   Charging system output test
   Reconnect battery if disconnected. Start engine and allow to reach normal idle. Connect DMM positive lead to battery positive terminal and negative lead to battery negative terminal. With engine at 1500-2000 RPM and all accessories off, charging voltage should be 13.8-14.8 volts (or 6.9-7.4V for 6-volt system). Turn on headlights, heater blower, and other accessories - voltage should remain above 13.5 volts. If voltage is low, check voltage at alternator output terminal (BAT terminal) to verify issue is alternator and not wiring resistance.
4. 4

   Voltage regulator testing
   With engine running at 1500 RPM, monitor battery voltage for 5 minutes to ensure regulator maintains steady voltage without fluctuation. Voltage should stabilize between 13.8-14.8 volts and not exceed 15 volts or drop below 13 volts. Check mechanical voltage regulator (typically mounted on firewall or inner fender) for proper ground connection and clean mounting surface. On external regulators, verify all terminals are tight and free of corrosion. Excessive voltage indicates faulty regulator; insufficient voltage indicates alternator or regulator failure.
5. 5

   Starting system circuit test
   Turn ignition to START position while assistant holds key. Use DMM to check voltage at starter solenoid battery terminal (large terminal) - should be within 0.5 volts of battery voltage. Check voltage at starter solenoid trigger terminal (small wire) during cranking - should show battery voltage. If no voltage present, trace back through neutral safety switch (automatic) or clutch switch (manual), ignition switch, and wiring. Check voltage drop across battery cables during cranking - positive cable should drop less than 0.5V, ground cable less than 0.3V. Excessive voltage drop indicates poor connections or undersized cables.
6. 6

   Ignition system primary circuit test
   Turn ignition switch to RUN position (do not start). Check voltage at coil positive terminal - should read full battery voltage. If no voltage, check ignition switch operation and wiring from switch to coil through any ballast resistor. With ignition on, check voltage at coil negative terminal - should read 0.5-1.5 volts less than battery voltage due to ballast resistor. Check continuity from coil negative to distributor points - should show continuity when points are closed, infinite resistance when open. Test ballast resistor if equipped - typical resistance 0.8-1.2 ohms.
7. 7

   Lighting circuit diagnosis
   Test headlight circuit by turning headlights to high beam and checking voltage at headlight connector - should be within 1 volt of battery voltage. Check dimmer switch operation by testing continuity in both high and low beam positions. Test tail lights, brake lights, and turn signals individually - check for voltage at each bulb socket and verify proper ground. For AMC-specific lighting: test instrument panel illumination through headlight switch rheostat. Check parking light circuit and verify backup light operation with transmission in reverse. Use wiring diagram to trace any circuits showing no power or improper operation.
8. 8

   Accessory circuit testing
   Test wiper motor by checking for battery voltage at motor connector with switch in ON position. Check heater blower motor operation in all speed positions - verify voltage at resistor block terminals matches expected values. Test horn circuit by grounding horn relay trigger wire - horn should sound indicating relay and horn function properly. If no sound, check voltage at horn terminal and verify ground. Test radio (if equipped) for proper voltage supply at radio connector. Check dome light operation through door jamb switches and manual override.
9. 9

   Ground circuit verification
   Check all major ground points specific to 1970 AMC Rebel: engine block to firewall ground strap, body to frame grounds, and battery negative cable to engine block. Use DMM in resistance mode (ohms) to verify less than 0.5 ohms resistance between engine block and battery negative terminal. Check ground integrity by measuring voltage drop during high current draw - place DMM negative lead on battery negative terminal and positive lead on component ground point, then operate component. Voltage reading should be less than 0.1V. Clean and tighten any grounds showing excessive resistance.
10. 10

    Fusible link inspection
    Locate fusible links in main power distribution wiring near starter solenoid or junction block. These are short sections of smaller gauge wire that protect major circuits. Gently pull on each fusible link - a burned link will stretch and feel different than good wire. Check continuity through each fusible link with DMM - should show less than 1 ohm resistance. Inspect insulation for signs of overheating (bubbled, discolored, or brittle covering). Common AMC fusible link locations include alternator output wire, ignition switch feed, and headlight circuit feed.
11. 11

    Gauge and sending unit testing
    With ignition on, verify instrument panel gauges respond properly. For fuel gauge, disconnect sending unit wire at tank and ground it - gauge should read full. For temperature gauge, disconnect sending unit at engine and ground the wire - gauge should read hot. Oil pressure gauge (if electric) can be tested similarly. If gauges respond to grounding, sending units are faulty. If gauges don't respond, check voltage regulator (IVR) behind instrument cluster - should output 5 volts to gauge circuits. Check voltage at gauge terminals and verify proper ground of gauge cluster.
12. 12

    Intermittent problem diagnosis
    For intermittent electrical issues, perform wiggle test on suspected circuits while operating affected component. Manipulate wire harnesses, connectors, and switches while monitoring with test light or DMM. Pay special attention to areas where wiring passes through firewall, around engine mounts, or near sharp metal edges. Check for loose or corroded connector pins by gently pulling on individual wires at connector bodies. For heat-related failures, use heat gun to warm suspected components and observe behavior. Document all findings including when problem occurs (hot/cold, over bumps, specific conditions).

Reassembly

1. Reconnect any disconnected electrical components and connectors
2. Apply dielectric grease to battery terminals and cable connections before reinstalling
3. Ensure all ground connections are clean, tight, and properly secured
4. Replace any blown fuses with correct amperage rating
5. Verify all test equipment is removed from vehicle
6. Reset clock and radio presets if battery was disconnected

Verification

• Start engine and verify charging system maintains 13.8-14.8 volts at idle and under load
• Test all lighting circuits including headlights, turn signals, brake lights, and interior lights for proper operation
• Verify all gauges read correctly and respond appropriately to engine conditions
• Test all accessory functions including wipers, horn, heater blower, and radio
• Road test vehicle to verify no intermittent issues occur during normal operation
• Confirm no warning lights are illuminated on instrument panel
• Check that no unusual electrical smells, smoke, or excessive heat are present at any components

More procedures for this vehicle