1. Linkage Issues:
* Bent or Broken Linkage: The linkage connecting the shifter to the transmission can be bent, broken, or simply out of adjustment. Inspect the entire linkage for any damage, looseness, or binding. Look for bent rods, loose bolts, or worn bushings. Adjustment is often possible; consult a repair manual for specifics.
* Shifter Cable (if applicable): Some 67 Mustangs may have used cables instead of a rod linkage. Check for fraying, breakage, or binding within the cable housing.
2. Transmission Problems:
* Worn or Damaged Synchronizers: The synchronizers in the transmission help match the speeds of the gears before they engage. Worn synchronizers are a common cause of difficulty shifting into a specific gear, especially 3rd. This usually requires transmission repair or rebuild.
* Low Transmission Fluid: Insufficient fluid will make shifting difficult or impossible. Check your transmission fluid level (with the engine running and warmed up) using the dipstick. Top it off with the correct type of automatic transmission fluid if needed. (Note: If you have a manual transmission, check the fluid level when the car is parked on level ground and the transmission is completely cold. This will typically be done with a drain plug and a measuring device).
* Dirty or Low-Quality Transmission Fluid: Old, dirty fluid can cause poor shifting. A fluid change and filter replacement (if applicable) might resolve the issue.
* Internal Transmission Problems: More serious internal problems within the transmission (broken gears, worn bearings, etc.) can prevent shifting into 3rd gear. This often requires professional transmission repair or replacement.
* Clutch (Manual Transmission Only): A worn or improperly adjusted clutch can cause shifting problems in manual transmissions. A slipping clutch might also lead to issues shifting.
3. Other Possibilities:
* Shifting Technique: While less likely, ensure you are properly operating the shifter. Are you lifting the shifter completely out of 2nd before attempting 3rd? Is there any unusual resistance?
Troubleshooting Steps:
1. Check Transmission Fluid: This is the easiest and cheapest thing to check first.
2. Inspect Linkage: Carefully examine the linkage for any visible damage or looseness.
3. Try Shifting Slowly and Deliberately: See if gentle, controlled shifting improves the situation.
4. Consult a Repair Manual: A shop manual specific to your 1967 Mustang will provide detailed diagrams and instructions for inspecting and adjusting the linkage and transmission.
5. Seek Professional Help: If you're not comfortable working on the transmission yourself, take it to a mechanic specializing in classic cars. They have the experience and tools to diagnose the problem accurately.
Important Note: Attempting transmission repairs without the proper knowledge and tools can cause further damage. If you are unsure about any step, it's always best to seek professional help.
