Here's a breakdown of the likely culprits:
* Vehicle Speed Sensor (VSS): This is the most probable cause. The VSS sends signals to both the transmission control module (TCM) and the instrument cluster. If the VSS is faulty (malfunctioning sensor, wiring issues, or a bad connector), the TCM won't receive the necessary information for proper shifting, and the speedometer won't work. The VSS is usually located on the transmission itself.
* Transmission Control Module (TCM): While less likely than a bad VSS, a failing TCM could also cause these symptoms. The TCM uses the VSS signal to control shifting. A faulty TCM might misinterpret or ignore the VSS signal, leading to poor shifting. A faulty TCM would also likely throw trouble codes.
* Wiring: Problems with the wiring between the VSS, the TCM, and the instrument cluster are a possibility. A broken wire, a corroded connection, or a short circuit could disrupt the signal flow.
* Speedometer Cable (Less Likely): While less common in electronic systems like the 1996 Aerostar, a damaged speedometer cable could be responsible for the non-functioning speedometer, but it wouldn't directly affect transmission shifting.
* Low Transmission Fluid: While unlikely to cause the speedometer issue, low transmission fluid *can* cause poor shifting. Check the fluid level and condition.
Troubleshooting Steps:
1. Check the Transmission Fluid: Ensure the fluid level is correct and the fluid itself is clean and not burnt.
2. Check for Trouble Codes: Use an OBD-II scanner to read any diagnostic trouble codes (DTCs) stored in the TCM. These codes will point towards specific problems.
3. Inspect the VSS: Locate the VSS (usually on the transmission), visually inspect it for damage, and check the wiring harness connected to it for breaks, corrosion, or loose connections. You might even try cleaning the sensor's connector with electrical contact cleaner. Testing the VSS with a multimeter requires more technical knowledge.
4. Test the VSS Signal (Advanced): If you have the technical skills, you can test the voltage signal coming from the VSS using a multimeter. Consult a repair manual for specific testing procedures.
5. Check Wiring: Carefully inspect all wiring related to the VSS, TCM, and speedometer. Look for any obvious damage.
6. Replace the VSS (Most Likely Solution): If the VSS is suspected, replacing it is often the most cost-effective solution first.
Important Note: If you're not comfortable working on your vehicle's electrical system or transmission, it's best to take it to a qualified mechanic for diagnosis and repair. Improper repairs can lead to further damage.
