* Low Transmission Fluid: This is the first and easiest thing to check. Low fluid can cause erratic shifting, slipping, and ultimately, failure to engage higher gears. Check the fluid level (with the engine running and warmed up) and its condition (color and smell – burnt fluid is a bad sign). If low, adding fluid might temporarily help, but a deeper problem likely exists.
* Transmission Internal Problems: This is the most likely culprit. Several internal components can cause these symptoms:
* Worn Clutch Packs: The clutch packs inside the transmission engage and disengage the gears. Wear and tear, often due to age or low fluid, can lead to slippage and inability to shift properly.
* Broken or Worn Gears: A broken or severely worn gear will prevent the transmission from shifting smoothly or at all.
* Valve Body Issues: The valve body controls fluid flow within the transmission, directing it to engage the correct gears. Problems here, such as sticking valves or internal leaks, will severely affect shifting.
* Torque Converter Issues: The torque converter connects the engine to the transmission. A faulty torque converter can cause slippage and an inability to maintain speed.
* Low/Stuck Shift Linkage: While less likely to cause *only* 30mph failure, a problem with the linkage connecting the shifter to the transmission could prevent it from selecting the correct gear. However, this would usually affect all gears, not just above 30 mph.
* Other Potential Issues (Less Likely but Possible):
* Speed Sensor Problems: A faulty speed sensor can provide incorrect information to the transmission control module (TCM), leading to improper shifting. However, this usually results in erratic shifting across all speeds, not a sudden cutoff at 30 mph.
* Transmission Control Module (TCM) Issues: Though less common, a malfunctioning TCM can cause various transmission problems, including the symptoms you describe. This requires more specialized diagnostic tools.
What to Do:
1. Check the transmission fluid level and condition immediately. This is the quickest and easiest check.
2. Do NOT continue driving the car. Further driving with a failing transmission can cause significant internal damage, leading to a much more expensive repair.
3. Have the vehicle professionally diagnosed. A mechanic with experience in automatic transmissions will need to inspect the transmission, likely using a scan tool to check for diagnostic trouble codes (DTCs), and possibly perform a road test to pinpoint the exact problem. Repairing a transmission is usually expensive; determining if a rebuild, replacement, or even a used transmission is the best option will depend on the diagnosis.
Ignoring the problem will only lead to more extensive damage and significantly higher repair costs. Get it checked by a mechanic as soon as possible.
