* Transmission Issues: This is the most likely culprit. A significant RPM drop like that points to problems within the automatic transmission itself:
* Low or Worn Clutch Packs: The clutch packs may not be engaging fully, leading to a significant slip and subsequent RPM drop. This is a common issue in older automatics.
* Torque Converter Problems: A failing torque converter won't properly multiply engine torque, resulting in a similar RPM drop. It might be slipping or not locking up correctly.
* Internal Transmission Damage: More severe internal damage could be causing the issue.
* Incorrectly adjusted linkage: Improperly adjusted linkage between the transmission and the engine can lead to incorrect engagement.
* Engine Issues (Less Likely but Possible): While less probable given the specific scenario, engine problems could contribute:
* Vacuum Leaks: Significant vacuum leaks can affect engine performance, but usually manifest in other ways too (rough idle, poor performance). However, it could exacerbate the transmission issues.
* Carburetor Problems: A faulty carburetor could be delivering insufficient fuel, particularly under load (when the transmission engages).
* Ignition Timing (Less Likely): While advanced timing can cause high idle speeds, a sudden and drastic RPM drop upon shifting usually isn't directly caused by *just* the timing. It's more likely a secondary effect if another problem is already present.
* Other possibilities: A worn or damaged driveshaft could also contribute to the problem.
Troubleshooting Steps:
1. Check the Transmission Fluid: Check the level and condition of the transmission fluid. Is it low, burnt, or discolored? This is the first and easiest thing to do.
2. Listen for Unusual Noises: When shifting, listen carefully for any unusual noises from the transmission (whining, clunking, grinding).
3. Observe the Torque Converter: If you can safely access the torque converter (often requires dropping the transmission pan) check for any obvious damage or signs of wear.
4. Check the Linkage: Inspect the linkage between the transmission and the shifter for proper adjustment and condition. A loose or broken linkage can cause shifts that aren't properly engaging.
5. Check Ignition Timing (After Transmission): If you have ruled out major transmission problems, then you can look into ignition timing. But this is a secondary step, as it's less likely to be the root cause of a sudden large RPM drop.
Recommendation: Given the severity of the RPM drop, it's highly recommended to have a mechanic experienced with older automatic transmissions diagnose the problem. Continuing to drive the car with this issue could cause further damage to the transmission.
