* Alternator Failure: This is the most likely culprit. A dead battery *could* be from simply not driving enough, but if the alternator wasn't charging it, that's a serious problem. A failing alternator can cause a voltage drop, leading to erratic behavior in various electronic systems, including the transmission, braking system (if it has electronic components), and the speedometer. The transmission control module (TCM) relies on consistent power, and a low voltage can cause it to behave erratically or even enter a limp mode, mimicking a locked-up transmission. Similarly, electronic brake systems could be affected.
* Battery Issues (Beyond Just Dead): While you jumped it, the battery itself might be severely damaged (internal short, etc.) and not holding a charge properly, even after a jump start. This would lead to the same voltage drop problems as a failing alternator.
* PCM/TCM Problems: The Powertrain Control Module (PCM) and Transmission Control Module (TCM) – often combined in newer vehicles, but potentially separate in a 1999 Expedition – are computer brains controlling the engine and transmission. A voltage fluctuation or even a slight power surge from the jump start could have damaged them. This would explain many of the symptoms.
* Sensor Failures: Several sensors communicate with the PCM/TCM. A faulty sensor (speed sensor is a prime suspect given the speedometer issue) might provide incorrect data, leading to the engine, transmission, or other systems acting oddly.
* Wiring Harness Issues: Age and corrosion can damage wiring harnesses, causing intermittent connections or short circuits. This is less likely the *primary* cause, but could contribute to the problems.
What to Do:
1. Have the alternator tested immediately: This is the quickest and most cost-effective test. A simple alternator test at an auto parts store or mechanic will confirm if it's charging properly.
2. Battery voltage check: While the car is running, check the voltage at the battery terminals with a multimeter. It should be around 13.5-14.5 volts. Anything significantly lower indicates charging problems.
3. Have the PCM/TCM scanned for diagnostic trouble codes (DTCs): A professional mechanic can use a scan tool to read codes from the PCM and TCM. These codes can pinpoint the exact problem. This is crucial.
4. Check all fuses and relays: While less likely to be the sole cause, a blown fuse or failing relay can cause problems.
Important Safety Note: If the transmission or brakes feel "locked up," don't force the vehicle. Towing it to a mechanic is the safest option to avoid further damage or injury. The symptoms you're describing are not trivial. Ignoring them could lead to expensive repairs or accidents.
