If your vehicle's battery shows 12 volts when measured at rest but drops to 4 volts when you try to activate the starter, and there is no sound from the starter, it typically indicates a significant voltage drop under load, which can be caused by several issues. Here are some common reasons for this problem:
Weak or Failing Battery:
Even though the battery shows 12 volts at rest, it may not be able to provide enough current to start the vehicle. This can happen if the battery is old or has a weak cell. Try jump-starting the vehicle with another car's battery to see if it starts. If it does, the battery may need replacement.
Poor Battery Connections:
Check the battery terminals and cable connections for corrosion or loose connections. Clean the terminals and make sure they are tight and secure.
Faulty Starter Solenoid:
The starter solenoid is responsible for engaging the starter motor. If it's faulty, it may not send power to the starter motor. A multimeter can be used to check if power is reaching the solenoid when you attempt to start the vehicle.
Bad Starter Motor:
If the starter motor is faulty or worn out, it may not engage or turn over the engine. A "click" sound might be heard if the starter solenoid engages, but the motor doesn't turn.
High Resistance in the Starter Circuit:
Any high resistance in the starter circuit, such as corroded or damaged wiring, can lead to a significant voltage drop under load.
Seized Engine:
In rare cases, a seized engine can put so much strain on the starter that it causes a voltage drop. Try manually turning the engine over by hand to check if it's stuck.
To diagnose the issue, you may want to start by checking the battery and its connections. If those are in good condition, you can move on to testing the starter solenoid, starter motor, and the rest of the electrical system.
