Diagnostic Scan

Perform diagnostic assessment of engine and emission control systems on a 1970 AMC Rebel Machine using period-appropriate manual diagnostic techniques, as this vehicle predates OBD systems.

Warnings

Tools required

Preparation

1. Ensure vehicle is parked on level surface with parking brake engaged
2. Allow engine to cool to ambient temperature for accurate baseline readings
3. Verify battery is fully charged (12.6+ volts at rest) for accurate electrical testing
4. Locate emission control information decal under hood for factory specifications
5. Have notepad ready to record all readings and observations

Procedure

1. 1

   Visual underhood inspection
   Inspect all vacuum lines for cracks, splits, or disconnections. Check ignition wires for damage, carbon tracking, or improper routing. Examine distributor cap and rotor for cracks, corrosion, or carbon tracking. Verify air cleaner assembly is properly seated and not restricted. Check all electrical connections at ignition coil, distributor, and alternator for looseness or corrosion.
2. 2

   Test battery and charging system
   Using multimeter, measure battery voltage at rest (should be 12.4-12.8V). Start engine and measure voltage at battery terminals; should read 13.8-14.5V at 1500 RPM indicating proper alternator function. Check for voltage drop across battery cable connections (should be less than 0.2V under load).
3. 3

   Set and verify ignition timing
   Connect timing light to number one cylinder spark plug wire. Locate timing marks on harmonic balancer and timing tab. Start engine and allow to reach normal operating temperature. Disconnect and plug vacuum advance line at distributor. With engine at specified idle speed, aim timing light at marks. Factory specification for 390ci with 4-barrel is typically 5 degrees BTDC; verify against underhood emission decal. Adjust if necessary by loosening distributor hold-down and rotating distributor body.
4. 4

   Check point dwell and condition
   Connect dwell meter to distributor side of ignition coil (negative terminal) and ground. Start engine and observe dwell reading at idle. Specification for 8-cylinder Prestolite distributor is typically 28-32 degrees. Remove distributor cap and inspect point contacts for pitting, burning, or excessive wear. Check point gap with feeler gauge if dwell is out of specification (typically 0.016-0.018 inches).
5. 5

   Perform vacuum gauge diagnosis
   Connect vacuum gauge to manifold vacuum source (not ported vacuum). Start engine and observe reading at idle with transmission in neutral. Normal reading should be steady 17-21 inches Hg at sea level. A steady low reading indicates retarded timing or valve timing issues. Fluctuating needle indicates ignition miss or valve leakage. Gradual drop indicates exhaust restriction. Snap throttle open and release; vacuum should drop to near zero then bounce back 2-5 inches above normal.
6. 6

   Test distributor advance mechanisms
   With vacuum gauge still connected, increase engine speed to 2000 RPM and observe vacuum reading increase slightly. Reconnect vacuum advance line to distributor; vacuum reading should increase 2-4 inches indicating advance is functioning. Using timing light at 2000 RPM, verify mechanical advance is occurring (timing mark should advance beyond base setting). If no advance observed, distributor advance mechanisms require service.
7. 7

   Assess carburetor function
   Observe idle quality and stability. Engine should maintain steady idle without hunting or loading up. With engine at operating temperature, check choke plate is fully open. Inspect choke pulloff operation by observing choke movement when engine is started cold (if possible). Look for fuel leaks at carburetor base gasket, fuel bowl, or fuel inlet. Check throttle linkage for binding or excessive wear.
8. 8

   Inspect emission control systems
   Verify PCV valve operation by removing from valve cover and shaking; should rattle freely. With engine running, place finger over PCV valve opening in valve cover; should feel strong vacuum. Check air injection system (if equipped) for proper belt tension and pump bearing condition. Inspect all emission control hoses and connections for deterioration.
9. 9

   Optional compression test
   If engine performance issues are evident, perform cylinder compression test. Remove all spark plugs and block throttle and choke fully open. Disable ignition system. Install compression tester in number one cylinder and crank engine through at least 4-6 compression strokes. Record reading and repeat for all cylinders. Typical compression should be 140-160 PSI with no cylinder varying more than 20% from highest reading. Low compression indicates ring or valve issues.
10. 10

    Document findings and recommendations
    Record all measurements taken including timing setting, dwell angle, vacuum readings, compression values (if tested), and charging system voltage. Note any abnormal conditions observed such as vacuum leaks, worn components, or systems operating outside specification. Compile list of required repairs or adjustments with priority ranking based on impact to drivability and emissions compliance.

Reassembly

1. Reconnect all vacuum lines that were disconnected during testing
2. Reinstall any components removed for access such as air cleaner assembly
3. Verify all test equipment has been disconnected from vehicle
4. Double-check that distributor hold-down is properly tightened if timing was adjusted

Verification

• Start engine and verify it idles smoothly with no unusual noises or misfires
• Confirm ignition timing remains at specification after reassembly
• Check that all vacuum lines are properly connected with no audible leaks
• Verify charging system voltage remains within 13.8-14.5V range at idle
• Review all documented readings to ensure diagnostic assessment is complete

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