Your Jeep Alternator

Alternator 56-78 AMP for Jeep CJ & SJ

The headlights, air condition (not that it is needed much when off roading), ignition coils, engine cooling fans, radio, and all those other things you have become accustom to and may even take for granted are all possible due to your electrical system that is powered by…. your Jeep’s gas tank. What? You don’t agree? It is true, I jest not. The link that makes it all connect is the alternator – it takes mechanical power from the crankshaft, transmits it via the fan belt or serpentine belt, and turns the alternator. So what does that mean to you? It means the main function of your Jeep’s alternator is to convert power from the gasoline engine that drives you along the roads and trail you adore, to electrical energy to keep the battery in tip top shape.. Bad alternator means no fun for you or your Jeep!

Alternator 61-74 AMP for 1987-90 4 CYL YJ & XJ (Tagged 1101171 or 1101172)

So how do you know when your alternator is going bad? Well one revolution of the alternator puts out three separate voltages. Each of the three phases has its own windings in the alternator and each of the windings has its own pair of diodes. Each one of these can fail, one set at a time. If that happens, the alternator can still charge the battery but only limited. In other words, it isn’t going to keep it fully charged however you could still drive and not even realize your alternator is beginning to fail you. One system of the alternator fails you are left with 2/3 charge, two fail – 1/3 charge and well if all three fail you are sitting idle once your battery runs dead.

Alternator for 1993-94 V8 ZJ
Alternator for 1993-94 V8 ZJ

You can test your alternator by using a voltmeter. Start your Jeep and make sure all your accessories are off – headlights, radio, fans, etc. Rev your motor to a fast idle. Set the Voltmeter to the DC scale (not AC or Ohms). Measure your Jeep’s voltage across the battery terminals – red lead of the voltmeter on the positive terminal, black on the negative. The voltage will probably read around 14 volts. If it reads less than 12 volts you probably have a failed alternator, if not, continue testing. While still testing, starts throwing on your Jeep’s accessories – heater, fans, radio, headlights and whatever else you have that pulls a draw. Rev up your motor and watch the voltmeter. If it continues to read 14 volts you are good to go, if not and it reads less than 13 volts, chances are that your alternator is failing you. Before stopping your testing, make sure you check your battery terminal connections and check to see that your belts are tight. These things can change the voltage readings.

Alternator Metric Hardware for 1980-90 Jeep CJ & YJ

A dead give away that your alternator is failing is noise. Your alternator rotates on bearings and when these fail, you will hear a loud grinding noise.

Alternator 85-100 AMP for 1987-90 6 CYL 4.0L Jeep XJ (Tagged 1101167 or 1101170)

Written by Andrew

"You're telling me it's got four wheels, two seats and goes faster than the speed limit? Good, I'm driving."


  1. I always considered myself to be mechanically challenged, but I understood exactly what you were saying. I’m not ready to start working on it myself, but understanding the parts is a good place to start.


  2. Terrific article! I could have saved myself some bucks if I had read it sooner. I replaced the battery thinking I was pretty smart for realizing what the problem was. I had already disposed of the old battery before I figured out that wasn’t the problem at all. Turned out to be the alternator. It’s not the most expensive repair I’ve had done but it wasn’t cheap either. I guess I should be thankful for that. The battery probably needed to be replaced anyway. I think it was close to 4 years old. Next time, I’ll let the garage tell me what’s wrong instead of trying to repair it myself to save a couple of bucks.


  3. I have to admit, than when I read about car parts and their functions it might as well be written in Greek. I’m as bad as the women. If I put the key in and it doesn’t start I haven’t the first clue about what might be wrong. I’ve thought about taking a basic car repair class at our local college. Some of the guys think it’s hilarious that I pay to have my oil changed.


  4. It sounds like a pretty important piece of the vehicle. I’ve heard of it before but never really understood how much it did.


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