When your Mercury Grand Marquis' alternator or battery begins to go bad, it can cause many issues, including rough running, stalling, unstable idling, hard starting and a no start condition. Either of these components could be the culprit so you'll need to troubleshoot the issue to rule one of them out. The two components work with each other, with the alternator charging the battery.
Check the voltage of the alternator by starting the car and turning on the radio, A/C and turn signals. Touch the leads of a multimeter to the positive and negative terminals of the battery. The voltage should be between 13 and 14 volts; if it is not, then there is a problem with the alternator.
Locate the alternator at the front of the engine connected to the serpentine belt. It will be on the top passenger side of the belt system.
Check the wires on the rear of the alternator. There will be two wires that are secured to the rear of the alternator by nuts. Check the tightness of these nuts. If they are loose, tighten them then recheck the voltage.
Check the wiring harness going into the rear of the alternator. If it is loose, push it in until the locking button clicks into place then recheck the voltage.
Examine the wires on the rear of the alternator for any breakage. If there is breakage, repair the wires by splicing or replacing them, then recheck the voltage.
Recheck the system after all tests and repairs have been performed; if the problem still persists, replacement or rebuilding of the alternator is required.
Check the voltage of the battery without the car running using a multimeter. The state of charge scale should read: 12.65V--100 percent charge, 12.45V--75 percent charge, 12.24V--50 percent charge, 12.06V--25 percent charge and 11.89V or less--no charge. If it is 75 percent or less, recharge the battery and recheck.
Check the tightness of the battery cables. If they feel loose, tighten them then recheck.
Examine the terminals of the battery. If there is white or green buildup on the terminals, clean them off then recheck.
Check the battery cables for any breakage. Repair any breakage then recheck the battery.
Trace the ground wire back to where it mounts to the body. Verify that the ground is securely bolted to the body. If it is loose, tighten it then recheck the battery.
Recheck the battery after all repairs and tests are performed; if the battery still fails, you'll need to replace it.
