I have seen many posts on this board with people incorrectly diagnosing starting issues, or giving advice that requires a lot more wrench turning and guess work than necessary. I figured I could make a little diagnostic write up to better inform people about how to test the system, and how it works in our Jeeps.
One thing to keep in mind is that the starters ONLY job is to turn the engine over. If your starter is cranking over the engine but it's not firing, your problem lies elsewhere.
The first step in properly diagnosing the starting issue you are having is to verify your voltage. If you are not getting enough power to the starter, you cannot properly start your jeep. Grab a voltmeter (NOT a test light, those are useless here), and a twelve pack. I prefer Guinness or New Castle, as they have a high Jive-osity ratio when working on my jeep, but anything will suffice. You may need a friend to give you a hand turning the key while you take some voltage readings.
1. First, we start with the battery. This is where you get your power from. If you have a load tester, hook it up to the battery posts (there is no need to remove any cables). Make sure your battery has a decent charge. 12V - 12.6V is acceptable here.
2. With your voltmeter on the batter posts, turn the key to the start position, and read what the voltage drops down to at the battery posts. If it drops below 10.5V or so, you either have a bad connection at the posts, low battery, or a bad battery. Make sure the posts are free of corrosion, charge it up, throw a new one in, do whatever. If you have the battery tested at a shop, keep in mind that you cannot correctly test a batterywithout it being fully charged. Once you have your freshly charged/new battery, throw that sucker in and if you still don't start, proceed to the next step.
3. Have you cracked one open yet? If not, do so.
4. Once you have verified that your batteryis able to produce enough current, you need to check your feeds to the starter. We will start with the ground, since that is more common of a failure from what I have seen. With your volt meter, put the positive lead on the positive battery post, and the negative lead on a clean metal surface on the starter body, or the solenoid mounted on it. Any clean metal surface on the starter will suffice. Verify that the voltage reading you get at rest is the same as the battery reading at rest. Have your friend (you gave him a beer too, I hope) turn the key, and note the the lowest point the voltage drops to. Remember the reading we got when trying to crank at the battery? You do not want to see more than a 0.5 volt decrease at the starter when cranking. If you drop more than that, you have resistance in your ground circuit. Trace it back until you find the point where the voltage is being lost. That includes the cable, the connection at the batteryend and the end that mounts to the motor, the connection where the starter mounts, everything in between. If you are having trouble tracing it back, just start at the starter and move the volt meter back one connection at a time until you get a good voltage reading. That will tell you exactly where you are losing your ground. Once everything checks good, we need to check the live feed to the starter.
5. Put the negative lead on the negative post of the battery, and the positive lead on the large post of the solenoid on the starter, where the live batter cable runs to. Perform the same tests as we did on the ground cable to check the live feed. Everything good? Let's check the ignition feed now.
6. Keeping the negative lead where it is, put the positive lead on the small wire connected to the starter. This is the ignition terminal, when you turn the key to the start position it sends power to this terminal to engage the starter. Take the same readings as we did on the cables. Note that this wire should ONLY be live with the key in the start position. If you have low/no voltage here, then you are not getting power through the ignition, and should check for a bad/broken wire, ignition switch, etc. In a Manual jeep, check to make sure the Neutral Safety Switch and related wiring is in working order.
7. If you have sufficient voltage present at the starter and still not cranking properly, then your issue is most likely the starter (or a seized engine). More than likely you can replace the starter and be on your merry way.
COMMON STARTING ISSUES:
RAPID CLICKING: 99% of the time it is low voltage to the starter. Check battery and feeds as detailed above. Basically you have enough current to engage the first coil in the starter solenoid, which pulls the plunger in and engages the drive into your flywheel, but not enough to power the holding coil which keeps it in while cranking. This causes the solenoidto release, and the re-activate since the current in the system is brought back up. this will happen rapidly, hence the click-click-click-clicking. VERY rarely it is an imbalanced solenoid on the starter. The voltage readings above will diagnose it properly.
NO CLICKING AT THE STARTER: Check for a loss of ground to the starter, or a loss of voltage at the battery. Check the ignition terminal on the solenoid for power when trying to start. If none of those circuits have any issues, the starter is bad.
JUST A CLICK: Again, verify the voltages. The solenoid could have enough power to engage and feed current to the starter, but a loss of current in the ground/cable/battery could be causing the starter to not crank properly. If the voltages are fine, change the starter.
STARTER SPINS, BUT DOESN'T CRANK THE ENGINE: The drive in your starter is blown or the gear is chewed. Change the starter (or drive if you are so inclined). Also check the teeth on the flywheel, make sure they aren't torn up. It is also possible (but unlikely under normal circumstances) that your drive shaft is snapped, or the planetary track in the starter is worn, in which case a new starter is in order.
STATER STAYS ENGAGED AFTER ENGINE STARTS: If the starter stops spinning when the key is off and the engine stops turning, check the ignition feed to the starter, make sure the ignition switch isn't faulty and supplying voltage to the starter when not in the start position. The drive could also be binding mechanically. If the engine stops turning and your starter is still spinning even with the key off, you have a low voltage issue. Basically what is happening is you have enough voltage to crank, but it is low enough that the contact diskin the solenoid is welding itself to the contacts due to heat from loss of amperage. Find your voltage drop in the battery/cables. I also suggest changing the starter/starter solenoid, as this causes the contacts to become distorted and chewed, and will cause problems in the starter.
CRANKS SLOW: Again, check voltages, change starter if necessary. Notice a pattern?
A NOTE ABOUT HAVING AUTOZONE TEST YOUR STARTER.
Autozone, C.A.P. and the likes cannot PROPERLY test your starter. I say this because every store I have seen test them just puts power to it to see if it spins and to check that the drive pops out. There are many problems that cannot be determined through that alone. Worn or gummed up brushes, a slipping drive, and worn contacts in the solenoid are just a few of the issues that can make a starter turn over on the bench, but not in your jeep. If have yours tested and they tell you it is fine, take it with a grain of salt.
Feel free to keep this thread alive by adding your input, suggestions and any more information I may have missed.