Issues with the differential fluid in a 1969 AMC AMX manifest as unusual noises, vibrations, or leaks from the rear of the vehicle. Left unchecked, low or contaminated differential fluid can lead to severe drivetrain damage, compromising the vehicle's performance and safety. To resolve this, you'll need to check and possibly refill the differential fluid. Here is how to solve this issue:

Tools and Supplies Required:

• Differential fluid (check your owner's manual for the recommended type)
• Socket wrench and suitable sockets
• Oil drain pan
• Clean rag
• Funnel with a long hose
• Jack and jack stands (if needed)
• Safety glasses and gloves

Step 1: Prepare Your Vehicle
Before you start, ensure your vehicle is parked on a level surface and the engine is off and cool. If necessary, use a jack and jack stands to lift the car to gain better access to the underside. Always follow proper safety procedures when lifting and securing your vehicle.

Step 2: Locate the Differential Fill Plug
The differential is found at the rear axle of the vehicle. Look for a metal casing (the differential housing) between the two rear wheels. On the differential housing, there will be a fill plug, which is typically a bolt located mid-way up the differential cover. It may look like a conventional bolt or may have a square head. Wipe away any dirt or grime around the plug to ensure no contaminants fall into the differential once opened.

Step 3: Remove the Fill Plug
Using a socket wrench with the appropriate socket, carefully remove the fill plug. Be prepared for some fluid to come out, indicating that the differential already has fluid. This can help in assessing the current fluid level.

Step 4: Check Fluid Level
Insert your finger into the fill hole. The fluid level should be just below the hole or at the bottom edge of the hole. If you can't feel any fluid, it’s definitely time to refill the differential.

Step 5: Drain the Old Fluid (if necessary)
If you suspect the fluid is contaminated, consider draining the old fluid completely before refilling. Position an oil drain pan under the differential, then locate the drain plug at the bottom of the differential or remove the differential cover entirely (consult your service manual for specifics). Once fully drained, clean the differential cover and apply a new gasket if needed before re-securing it.

Step 6: Refill the Differential Fluid
Using a funnel with a long hose, add the new differential fluid through the fill hole until it starts to drip out, signaling that the differential is full. Typical capacity for most differentials is 1 to 2 quarts, but refer to your owner’s manual for the precise specification for the 1969 AMC AMX.

Step 7: Reinstall the Fill Plug
Once the differential is filled to the appropriate level, reinstall the fill plug securely. Clean up any spilled fluid around the fill plug area with a clean rag to prevent any dirt accumulation.

Step 8: Lower the Vehicle and Test Drive
If you’ve lifted the vehicle, carefully lower it back to the ground. Conduct a short test drive, listening for any unusual noises or checking for any leaks around the differential area. Make necessary adjustments if anything seems amiss.

By following these steps systematically, you will ensure that your 1969 AMC AMX's differential is properly maintained, which is critical for its smooth operation and longevity.