Troubleshooting electrical problems on a Jeep Grand Cherokee requires the use of a voltmeter with an ohm scale. A wiring diagram for the vehicle is a major help for harness problems and for computer related problems. All components that require high amperage will have a relay between it and the switch. The higher the draw or amperage necessary to operate a component the larger the wire. It is not desirable to run high amperage into a switch.
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All relays work like a light switch. There are at least of four terminals. One terminal will have power supplied straight from the battery and the opposite terminal will run to the component. This circuit will be open as long as the relay is not actuated. The other two terminals are a switched power source of far less amperage and a ground. The switch that operates the relay has power from the fuse block or a buss bar that gives power to multiple circuits. When the switch is turned on, power runs through the relay to ground, activating the relay like switching a light on. This connects the power terminal from the battery to the component by closing the circuit. This way, the relay was activated remotely. All circuits are protected by a fuse or circuit breaker. Whenever there is a short, the amperage builds dramatically to more than the wire can handle and it heats up and can start a fire. The higher the amperage necessary to run a component the larger and lower resistance the wire should be.
Electrical Diagnosis Information
A lot of electrical diagnosis will be found to be a matter of common sense. All components must acquire their power from a fused source and they need a good ground for them to operate. If a component does not work, the first thing to do is make sure that it has power and a ground. The computer on a car is the grounding station to operate most of engine management system components. The injectors for example, get power from the fuse block any time the key is on, however they only operate when the computer injector driver circuit grounds them. The longer the computer grounds the injectors the more fuel that is injected into the motor. Injectors do not open farther, just longer. The same goes for most of the operating components such as the idle air control, variable valve timing, air injection pumps and so on.
When it comes to sensors for the input to the computer, the computer itself supplies the low voltage power and receives a signal through pulse signals or differential voltage. Always start at the fuse and then the component for power and work back to the fuse. Since covering every scenario would be a 200-page book, the most common problems will be listed below that can be done easily.
The yellow wire with a bold black stripe around it is the airbag actuator wire. It is very obvious and not a problem when doing electrical work. Do not probe this wire. If a yellow wire with a big bold black stripe is probed it will likely set off the air bag and do bodily harm. Enough said!
Check the battery for 12.5 volts. If it has less, charge it or jump it and if the car starts check across the battery to see if there are about 14.5 volts with the engine running. If not the alternator is bad. If it has the proper voltage, the battery is bad. If the battery was good, check the fuses and the starter relay. Pull the relay and check for power at one terminal with the key off. If there is no power, the problem is between the battery and relay. If there is power at the relay, have a helper hold the key in the start position. There should now be power to a second terminal. If there is no power, the problem is in the ignition switch or with the security system. If there is power, install the relay and check the main power at the starter large terminal. If there is no power, the wire from the starter to battery is corroded or a battery terminal is not tight or good. If there is power, have the helper hold the key in start and check the small wire on the solenoid of the starter. If there is power, the starter is bad. If there is no power the wire from the solenoid to relay is bad. A battery discharges faster than it can charge. It takes much more out of the battery to start a car than it does to run it. A battery with a bad cell will lose much of its amperage and cause the alternator to charge continuously at a high charge rate, which can over heat and damage an alternator. The best way to check a battery is to hold the voltmeter across the battery terminals and watch the voltage when someone starts the car. If the voltage drops below 10.5 volts, the battery has a bad cell and needs replacing. An alternator usually does not fail all at once. Usually one of the diode packs goes bad first and reduces the amperage and voltage it will put out. With the car running, the voltage across the battery should be 14.5. Anything much lower and it will take to long to charge the battery. Turn on the lights and the air conditioning. The voltage should remain the same. If not, replace the alternator.
Check Engine Light:
To correct a check engine light, an OBD II code scanner will be needed. The computer stores faults or failures in its memory and the scanner will access this memory and indicate what is not working. An OBD II scanner can be picked up at any auto parts store and is not expensive. Plug the scanner into the connector under the driver's side dash. It's not hard to find and all cars have these. The key must be turned all the way on with the engine off. Push the button on the scanner that says "Read." The computer will respond with a four-number code. Cross reference the number with the code description sheet that comes with the scanner and it will explain what the problem is. Once the part is replaced push the erase button and it will turn the light out.
The brake light switch is on the top of the brake pedal arm. The button on the switch should be pushed in with the brake pedal all the way up. If not, adjust it toward pedal arm. Look for hole in the arm where there was a rubber stop. If it is missing, just put a bolt in the hole and the lights will go off. If not the switch is bad.