* Torque Converter Issues: This is a high probability in older automatics. A failing torque converter can't properly transfer power from the engine to the transmission. It might spin freely, or not engage properly under load. This is often heard as a whining or shuddering noise before stalling.
* Transmission Problems: Worn clutches, low transmission fluid, a failing valve body, or other internal transmission issues can prevent the transmission from engaging smoothly or at all. Check the transmission fluid level and condition – it should be bright red and not burnt smelling or dark brown.
* Low or Dirty Transmission Fluid: As mentioned above, low fluid is a major problem. Dirty fluid can cause various problems within the transmission, preventing it from functioning correctly.
* Faulty Neutral Safety Switch: This switch prevents the car from starting unless it's in Park or Neutral. If it's malfunctioning, it might not tell the engine it's safe to engage the transmission, leading to a stall.
* Failing Engine Components: While less likely to cause an immediate stall *only* when shifting into gear, weak ignition components (coil, wires, distributor cap/rotor), a failing fuel pump (not delivering enough fuel under load), or a clogged fuel filter could contribute to the problem. The engine may be struggling to provide enough power.
* Vacuum Leaks: A significant vacuum leak can affect engine performance and cause stalling under load (like when shifting into gear).
* Computer Issues (PCM): While less common, a problem with the Powertrain Control Module (PCM) or its related sensors could be sending incorrect signals, leading to the stalling. This is a more advanced troubleshooting step.
Troubleshooting Steps (in order of easiest to hardest):
1. Check Transmission Fluid: This is the absolute first thing to do. Check the level and condition. If low or dirty, this is a very likely culprit.
2. Inspect for obvious leaks: Look under the car for any signs of transmission fluid leaks.
3. Neutral Safety Switch Check (if applicable): See if the car starts reliably in neutral, but stalls when put in any other gear.
4. Test Drive (with caution): Try putting the car in gear *slowly* and gently to see if the stall is immediate or delayed, and if there are any accompanying sounds.
5. Professional Mechanic: If you're not comfortable diagnosing these issues yourself, take it to a qualified mechanic for diagnosis and repair. A transmission issue is potentially expensive.
Without further information or diagnostics, it's impossible to pinpoint the exact cause. The transmission fluid check is the most crucial first step.
