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  A bad starter solenoid in your 1987 Jeep Wrangler will prevent you from using the vehicle until the starter solenoid is replaced. There are several similarities between a failing battery and a failing solenoid, so being able to determine the slight differences will help you with diagnosing the real issue.
  
  

Failure to Turn Over

• The starter solenoid is responsible for providing the starter with enough electricity to turn the gear located inside the starter. In turn, that gear turns the flywheel and the engine comes to life. When a solenoid completely fails, the engine will do nothing when the key is turned. If the idiot lights on the dash dim and the engine fails to turn over, the battery has been ruled out as the culprit.
  
  

Clicking Sound

• Often, the solenoid will not completely fail and the result will be several clicking sounds. The engine will fail to turn over and will not start. If the failure is with the battery, the starter will only click a few times. If it is the starter solenoid, the starter will continually click for as long as you attempt to start the engine.
  
  

Slow Starting

• If the engine turns over slowly there is a possibility that the starter is failing. If this happens in conjunction with the headlights dimming while attempting to start the engine, it is likely that the starter is going bad.