* P0700 (Transmission Control System (TCS) Malfunction): This is a generic code, meaning it indicates a problem within the transmission control system but doesn't pinpoint the exact cause. It's often a symptom of other, more specific codes like P1784 and P1789. It's the "catch-all" code for transmission problems.
* P1784 (Transmission Range Switch Circuit): This code indicates a problem with the transmission range sensor (also called the PRNDL sensor or shift position sensor). This sensor tells the transmission control module (TCM) what gear the transmission is in. Possible causes include:
* Faulty Range Sensor: The sensor itself might be malfunctioning or improperly installed during the rebuild.
* Wiring Issues: Damaged, loose, or corroded wiring between the sensor and the TCM.
* TCM Problem: In rarer cases, a faulty TCM can trigger this code, though less likely with the other codes present.
* P1789 (Transmission Output Speed Sensor Circuit): This code signifies a problem with the output speed sensor (OSS), which measures the rotational speed of the output shaft of the transmission. Causes include:
* Faulty Output Speed Sensor: The sensor itself might be defective or incorrectly installed.
* Wiring Problems: Similar to P1784, damaged wiring to the OSS can cause this code.
* Sensor Ring Damage: The sensor ring on the output shaft, which the sensor reads, could be damaged or misaligned. This is more likely given the transmission rebuild.
Reasons for these codes after a rebuild:
The most likely reasons for these codes appearing after a transmission rebuild are:
* Improper Installation of Sensors: The range sensor and output speed sensor were not properly installed during the rebuild, leading to incorrect readings or no readings at all.
* Damaged Wiring During Rebuild: Wiring harnesses were damaged or improperly reconnected during the rebuild process. This is a common issue during complex repairs.
* Faulty Sensors Used in Rebuild: The replacement sensors used in the rebuild are themselves defective.
* Low Transmission Fluid: Improper fluid level can lead to incorrect sensor readings.
* Electrical Connector Problems: Poor connection at the sensor or TCM connector can interrupt proper signal.
* Faulty TCM (less likely): Although less probable, a faulty TCM could be contributing. This would be diagnosed through further testing.
Troubleshooting Steps:
1. Check Transmission Fluid Level: Ensure the transmission fluid is at the correct level and is the correct type.
2. Inspect Wiring: Carefully examine the wiring harnesses for damage, loose connections, or corrosion, focusing on the connections to the range sensor and output speed sensor.
3. Test Sensors: A scan tool can often test the sensor circuits directly, verifying their output signals.
4. Visually Inspect Sensors: Check for physical damage to the sensors themselves.
5. Verify Sensor Installation: Ensure the range and output speed sensors are correctly installed and seated properly.
6. Check the TCM for any faults: Using a suitable scan tool you can check for any internal faults on the TCM.
If you're not comfortable performing these checks yourself, take the vehicle back to the transmission shop that performed the rebuild. These codes strongly suggest a problem related to the repair work. They should diagnose and rectify the issue under warranty (if applicable).
