Here's why:
* Separate Circuits: The gauges for temperature, oil pressure, and transmission position (Park, Reverse, Neutral, Drive, etc.) are likely on separate circuits within the instrument cluster. Since the temp and oil pressure are accurate, their circuits are working correctly. The issue is isolated to the transmission position indicator circuit.
* Faulty Sender (Unlikely): While a faulty transmission position sensor (sometimes called a neutral safety switch) *could* cause issues, it would likely affect the car's starting ability or operation (preventing starting in Park, etc.). Since your car is operating normally otherwise, this is less likely the culprit.
* Internal Gauge Cluster Problem: The most probable cause is a fault within the gauge cluster itself. This could be:
* Faulty wiring: A short circuit or broken wire within the gauge cluster's internal wiring harness related specifically to the transmission position indicator.
* Failed component on the circuit board: A component (resistor, capacitor, integrated circuit) on the gauge cluster's printed circuit board responsible for interpreting the signal from the transmission position sensor might have failed.
* Bad gauge itself: Though less common, the gauge needle itself or its mechanism could be malfunctioning, sticking, or misreading the signal.
What to do:
1. Check all related fuses: While less likely given other gauges work, ensure all fuses related to the instrument cluster are intact.
2. Professional Diagnosis: The most efficient way to diagnose this is to take it to a qualified mechanic or auto electrician. They have the tools and expertise to test the circuits within the instrument cluster and determine the precise cause of the malfunction. They might be able to repair the circuit board, replace a faulty component, or suggest a replacement gauge cluster if necessary.
3. Consider a replacement cluster (less ideal): Used gauge clusters for 1987 Oldsmobiles can sometimes be found relatively inexpensively, but this is a more significant repair and involves removing and installing the cluster.
Don't ignore the issue. While it might seem minor, a faulty gauge cluster can sometimes indicate larger underlying electrical problems. Professional diagnosis will give you the best solution.
