1. Verify TCM Power and Ground:
* Power: Locate the TCM (usually under the dash or in the center console; consult your repair manual for the exact location). Check for the correct voltage at the TCM's power supply terminals. Use a multimeter to test for battery voltage (around 12V) with the ignition ON. A lack of power here points to a blown fuse, faulty relay, or wiring problem.
* Ground: Similarly, check the ground connection to the TCM. A poor ground can prevent the TCM from functioning correctly. Clean the ground connection and ensure it has good contact.
2. Check Transmission-Related Fuses and Relays:
* Your owner's manual or a repair manual will list the fuses and relays related to the transmission. Inspect them carefully for any signs of damage (blown fuses, burnt relays). Replace any faulty components.
3. Inspect Wiring Harness:
* Carefully examine the wiring harness connecting the TCM to the transmission and other components. Look for any broken, frayed, or corroded wires. Pay close attention to the connectors themselves; ensure they are securely plugged in and free of corrosion.
4. TCM Diagnostics:
* OBD-II Scanner: Connect an OBD-II scanner to the car's diagnostic port. This may reveal trouble codes related to the transmission or TCM. These codes are crucial for pinpointing the exact problem.
* Advanced Diagnostics: If the OBD-II scan doesn't yield helpful information, you'll likely need to consult a professional mechanic with access to more advanced diagnostic equipment that can directly test the TCM and its communication with the transmission. This could involve a scan tool that can communicate directly with the TCM or even bench testing the TCM itself.
5. Consider the Used Engine Installation:
While unlikely to be the direct cause, something may have been damaged or disconnected during the engine swap. Double-check all connections related to the transmission, particularly the wiring harness, after the engine replacement.
Important Considerations:
* Repair Manual: A repair manual specific to your 1996 Dodge Intrepid 3.5L is essential for accurate troubleshooting. It will provide detailed diagrams and specifications.
* Professional Help: If you're not comfortable working with automotive electrical systems, it's best to seek professional help from a qualified mechanic specializing in automatic transmissions. Incorrect diagnosis or repair can lead to further damage.
The problem is almost certainly electrical, not mechanical (since the engine swap didn't fix it). Systematic checking of power, ground, fuses, relays, and wiring is the key to finding the fault. Don't jump to conclusions; follow this process methodically.
