Here's why:
* The Input Speed Sensor's Role: The ISS tells the PCM (Powertrain Control Module) how fast the transmission's input shaft is spinning. This information is crucial for the PCM to determine the appropriate shift points, engine timing, and fuel delivery.
* The Symptom Connection: When the transmission is in gear and moving, the ISS sends a signal to the PCM. If the ISS is faulty (e.g., malfunctioning sensor, broken wire, bad connection), the PCM receives incorrect or no signal. This leads to the PCM not knowing the proper speed and load conditions, causing it to either over-fuel or mismanage other engine parameters. The engine may rev up excessively and eventually stall due to this mismanagement. When placed back in neutral, the PCM relies less on the ISS data, and the engine can idle normally because the load is drastically reduced.
Other less likely possibilities (but worth investigating if the ISS check proves negative):
* Throttle Position Sensor (TPS): A faulty TPS could provide incorrect information to the PCM about throttle position, potentially leading to similar symptoms.
* Mass Airflow Sensor (MAF): An inaccurate MAF reading can also disrupt fuel delivery and cause erratic engine behavior.
* Transmission Issues (less likely given the specific description): Although less probable given the symptoms, severe internal transmission problems *could* indirectly cause this, but it's less likely to stall instantly when put in neutral.
Troubleshooting Steps:
1. Check the ISS: This is the most probable culprit. You'll need to locate the sensor (usually on the transmission case near the input shaft), visually inspect it for damage, and possibly test its resistance using a multimeter (consult a repair manual for the specific resistance values). A bad connection at the sensor's wiring harness is also a common problem.
2. Scan for Diagnostic Trouble Codes (DTCs): Use an OBD-II scanner to read any codes stored in the PCM. These codes will offer clues to the underlying problem. The code is very likely to relate to the ISS or a related circuit.
3. Inspect wiring: Carefully inspect the wiring harness for any damaged or corroded wires leading to and from the ISS. Pay particular attention to areas that might be subjected to movement or abrasion.
It's generally recommended to have a qualified mechanic diagnose and repair this issue. Improper diagnosis or repair could lead to further damage to the engine or transmission. However, checking the ISS and its wiring is a good starting point for a DIY approach. Remember to consult a repair manual specific to your 1997 Saturn SC2 for detailed diagrams and specifications.
