Here's a breakdown of the likely culprits:
* Shift linkage: This is the mechanical connection between the shifter in the car and the transmission. Over time, this linkage can become worn, bent, broken, or disconnected. This would prevent the transmission from receiving the correct instructions for reverse or forward gears.
* Shift cable (if applicable): Some automatics use cables instead of a linkage. These can stretch, break, or become dislodged, causing similar problems.
* Transmission control unit (TCM) or valve body issues (less likely): While less probable given the symptom, problems with the TCM (electronic control unit) or the valve body (internal hydraulic control) *could* cause this. A faulty TCM might not be sending the correct signals, or a problem in the valve body could prevent the right clutches and bands from engaging. This is less likely than a linkage problem because it would usually manifest as more erratic shifting, not just a complete inability to engage certain gears.
* Park/Neutral safety switch: This switch prevents the car from starting unless it's in Park or Neutral. A malfunctioning switch could be interfering with gear selection, preventing reverse or forward engagement.
Troubleshooting Steps (in order of likelihood):
1. Check the linkage/cables: Visually inspect the linkage or cables for any obvious damage, looseness, or disconnections. This is the easiest and most common fix.
2. Check the transmission fluid: Low fluid level or contaminated fluid could also contribute to shifting problems. However, it's less likely to cause a complete inability to engage certain gears.
3. Test the Park/Neutral safety switch: This often requires a multimeter for testing.
4. Consult a mechanic: If the above steps don't reveal the problem, a mechanic with experience in older automatics should diagnose the issue. They have specialized tools and knowledge to test the TCM and valve body if necessary.
It's crucial to diagnose the problem accurately before attempting any repairs, especially if you're not mechanically inclined. Improper repairs can worsen the situation.
