The most common problems with the Wrangler occur with computer controlled devices and easily repairable components. The majority of problems in today's cars are computer related. The computer keeps a record of such failures in its memory and will display these failures in the form of a coded number. This number can be accessed and converted to a language with a code scanner and a code identification book.
Check the battery with a voltmeter. Battery voltage should be 12.5 volts. If it shows less than proper voltage, jump-start the car, if possible. If the car will not jump-start, charge the battery and try again. If the battery will not take a charge, replace it. Once the car is running, check the battery for a charge from the alternator. Voltage at the battery with the engine running should be 14 to 14.8 volts. If it is under 13.8 volts, the alternator will need to be replaced. Batteries normally never die fast. Usually a cell goes bad first and the battery can still have 12.5 volts but not enough amperage to start a car. The same goes for the alternator--with one diode out, the amperage and voltage will drop.
Check the battery terminals for looseness and corrosion. While checking the battery voltage, have a helper turn the key and try to start the car. With the key in "start," read the battery voltage, which should be no less than 10.5 volts. If it is lower, there's a bad cell in the battery that will need to be replaced. If the voltage remains the same and the starter does not engage, check the voltage at the large positive cable at the starter. If there is no voltage, the wire from the battery to the starter is bad. If there is voltage, go to the next step.
Check the main fuses in the fuse and relay box on the driver's side fender well. If the fuses are good, check the relay. Pull the starter relay out and with the key off and check for power at one of the terminals in the relay plug. If there is power, turn the key on and have a helper hold it in the "start" position while the plug is checked for a second terminal to have power. If no power, there is a problem with the ignition switch or the security system. If there is power at the second terminal, then check the small wire on the starter solenoid for power when the key is in the "start" position. If there is power, then the starter is bad.
Pull a spark plug wire, insert a test spark plug into the wire and lay it against the block for a good ground. Let go of the spark plug and have a helper try to start the engine. Watch the spark plug for a spark. If there is no spark, there is an ignition problem. If there is a spark, reinstall the spark plug wire. If no spark was present, turn the key on and check the coil for power at one terminal. Check the opposite, negative terminal for the power to oscillate as in a duty cycle when the starter is engaged. If it does show an oscillation, the coil is bad. If there is no oscillation, there is a problem with the triggering of the coil. Check the crank position sensor for a signal. The crank sensor is located in the top of the transmission bell housing on the driver's side.
Check the fuel pump fuse and relay the same way you did with the starter. If these are good, locate the fuel Schrader valve on the fuel rail on top of the injectors. Use a small screwdriver and push in on the Schrader while a helper turns the key on. If fuel pressure is present, check the injectors next. If there was not fuel pressure, the fuel pump is faulty.
Pull the electrical connector off of the fuel injector and check for power at one terminal with the key on. Probe both terminals at the same time and watch for an alternating ground or a duty cycle as the starter is engaged. If there is no power to the fuel injectors, check the fuse. If there was power and no duty cycle, the computer injector driver circuits are at fault. This is under the assumption that there was no check engine light on. If the check engine light was on, go to next step.
Plug the code reader into the OBD II port under the driver's side instrument panel and turn the key to "run" (keep the engine off). Hit the "read" button and the reader will interrogate the computer, and it will reply with a code number.
Write the 4-digit number down and use the code interpretation sheet that comes with the code reader to reference the code. For instance, if it says that the crank angle sensor is bad, this controls the timing of the spark by giving a reference to the computer of top dead center on the number 1 cylinder. If the computer does not see this signal, it will not fire the ignition. There are numerous things that will prevent a start, and anything that the computer operates will be displayed. If no codes are present, that could cause a no start condition; the vehicle should be taken to a shop for a compression test and a more advanced diagnosis that can be done at home without the necessary (expensive) equipment.
