You put a new starter in your car 1991 Mercury Sable and when you hooked up the positive cable to battery started turn over without key Ignition whats problem?

The most likely cause of your 1991 Mercury Sable's starter turning over without the key in the ignition after installing a new starter is a faulty ignition switch. The new starter is likely working perfectly, but it's receiving power when it shouldn't. The ignition switch is responsible for controlling power to the starter solenoid, and if it's malfunctioning, it can send power even when the key isn't turned.

Other possibilities, though less likely, include:

* Wiring problem: A short circuit in the wiring harness between the battery, ignition switch, and starter could be sending power to the starter. This is especially possible if wiring was disturbed during the starter replacement. A wire might be touching ground where it shouldn't, or touching a live wire.

* Neutral Safety Switch (NSS) problem: This switch prevents the starter from engaging unless the car is in Park or Neutral (automatic transmission) or Neutral (manual transmission). If this switch is faulty, it might be sending the signal to engage the starter even without the key. This is less likely, given the starter immediately turns when connecting the positive cable, but it's still a possibility.

Troubleshooting Steps:

1. Check the ignition switch: This is the most probable culprit. You'll likely need a wiring diagram to properly test the switch. A faulty ignition switch needs replacement.

2. Inspect wiring: Carefully examine all the wiring connected to the starter, battery, and ignition switch. Look for any frayed, damaged, or improperly connected wires. Pay close attention to where the positive cable connects to the starter; make sure it's not touching anything it shouldn't.

3. Test the neutral safety switch (NSS): This is more involved and may require a multimeter to test the circuit's continuity in different gear positions. Consult your car's repair manual for instructions specific to your model.

Important Safety Note: Disconnecting the battery negative terminal before working on any of the electrical components is crucial to avoid electric shock. Also, be cautious when working around the battery, as it contains corrosive acid.