Alternator

Whats the best way to test if an alternator is going bad
2001 XJ, 4WD, AW4, I6 4.0, A/C

Tryfan said:

Whats the best way to test if an alternator is going bad
2001 XJ, 4WD, AW4, I6 4.0, A/C

Click to expand...

Take it to the auto parts store.....

What are your symptoms?

You usually hear this loud noise. Sounds sorta like a goose in heat.

Get a volt meter. You need 13-14 volts across the battery when its running. If not, probably your alternator.

Tryfan said:

Whats the best way to test if an alternator is going bad
2001 XJ, 4WD, AW4, I6 4.0, A/C

Click to expand...

This may or may not be related to your alternator, but out of curiosity, how is your idle speed?

I tested the battery with ignition off and all electrics off except the hood light that comes on when you pop the hood and I got 12.7V. I started the engine up and let it run for a few minutes then tested the battery again and I was getting about 13.8V running. My idle is not perfect it tends to be a little erratic between 700 - 800 rpm. My symptoms are that if I don't use the jeep for a couple of days then I can't start the jeep, its a very slow crank and the battery seems completely dead, which makes me think it is the battery but I'm not sure. One night I left the interior light on for 10 minutes and then I couldn't start the Jeep - same symptoms. The battery is a Mopar and is about 3.5 years old.

I would say bad battery or bad connections to the battery. 13.8 is a good charging alternator. Can you test across the battery from the posts and then again across the cable ends. If there is a difference you may have a bad connection (corrosion between cable ends and batt post(s)). If voltage is the same I say it is time for a new battery.

Dave

Go to OEM Tech, and search up my primary electrical testing post (search for "Starter, Alternator, Starter" under my username, any order.)

That will tell you how to test your primary electrical system with common tools, and get reliable results.

Frankly, I trust that sort of test more than I trust that roll-around Bear ARBST tester that most parts houses are using (I used to work in a parts house, and I used to have to push one of those things around. I've had things come up bad that the Bear passed.)

The "goose in heat" sound is usually a failed/failing bearing - the alternator may still be fine electrically, although it's failing mechanically. It's also possible that the alternator could start failing electrically while still being sound mechanically, and you won't know it by sound. That's why a little testing is required.

Fortunately, the testing is not difficult. I didn't "sticky" the thread (as I recall) because I find it tacky to "stick" my own thread - but I'm hoping someone else did.

Thanks for all the advice I will continue with more testing

Just an idea but it sounds more like a parasitic load drawing your battery down, and the first thing that comes to mind is the underhood light. They are known for not shutting off and killing batteries, I would unplug it and see if your batt keeps dying.

I am going to start a band named PARASITIC LOAD!!:eyes:

dAVE

13.8V is a good charging voltage, but only if it's DC current. I say this because if a diode in the alternator were bad, you could see 13.8V and still return to a discharged battery after it sits. The diodes in the alternator will allow current to flow through them and discharge the battery if they fail "open."

That being said, I would still look at a failed battery or connection based on the weak start after 10 minutes of leaving the lights on. A good, charged battery will usually have at least 90-100 minutes of reserve capacity for such instances.

The battery was the problem, the alternator was OK.