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checking alternator

jdbwrx

NAXJA Forum User
Location
Avon, CO
So I'm chasing some electrical gremlins, and trying to find out if my alternator is OK. I have taken it to three different shops, two Napa, and a Carquest, but none of them have the right connector to check it. (I have a '99 by the way) Unfortunately, those are all my option within 45 minutes of where I live.

Is there anyway I can check the alternator myself to see if it's charging correctly?

Any help would be appreciated.
 
I strongly recommend buying a haynes guide. Your jeep should have a charge meter, and if not then it will have the "idiot light." If the charge meter is reading somewhere around 14 you should be fine. if you have the "idoiot light" a small indicator that says "charge" will come on if the alternator is not performing within acceptable range. I would try hooking your battery up to a charger. If it's a good battery, but is loosing it's charge over time while you drive, then the alternator is most likely not performing properly.
 
This is a continuing issue that I had starting last winter. The Jeep just kept dying. I ended up replacing the battery and the alternator last spring and things were fine all summer. This fall, it started doing it again, battery being dead and just not starting.

Part of the problem is, the Jeep is not my daily driver, so it it is not run on a regular basis. I have a yellow top (Carquest version) and it was fine all summer. I pulled all the leads off and trickle charged the battery a few weeks ago and it started up fine. Prior to that, if I started the Heep via a jump and let it run for a while (say 30 minutes), shut it off, and immediately tried to restart it, it wouldn't go. That's what makes me think it's the alternator. That, or the alternator just couldn't bring a completely drained battery back.

I don't have the idiot light, but I do have the volt meter, and it usually sits around 14+.

I need to go through and see if I'm drawing current anywhere, as it's on my list of things to do. I have a number of aftermarket electrical accessories, winch and lights, and don't know if I'm losing something there.

In the meantime, I was just trying to find out of it is the alternator or not, and was looking for ways to test it. I have a Haynes manual, but haven't checked it, so I will.
 
Just do the old trick of firing up the XJ and while it is running, unhook the battery. If it dies, then you need an alternator. If it keeps running, then you have a bad battery that is not keeping charge. If you have a fully charged up battery it can keep your car starting and running for short periods of time or short drives as well as show amps on the gauge. It can be confusing b/c you may think that the alt is fine and the battery is bad, when it could be just the opposite.
 
yeah, the haynes guide has instructions for measuring the brushes, and it tells you what range they should measure within. Sounds like you have a short somewhere in the wiring though. I had a fog light lead ground itself to the frame once, and that drained the battery. took me forever to find it. good hunting.
I don't suppose you drive an '89 pioneer somewhere in the St. Helens/scappoose area?
 
I read that taking the negative cable off while running on newer cars is a bad idea, that you can cause more problems with the electrical system.
 
If you have a voltage meter, the battery should be roughly 12.8 volts without the engine running (just battery voltage) and it should be roughly 14 volts with the engine running (alternator voltage). That's a quick and easy way to get started with a diagnosis.
 
jdbwrx said:
I read that taking the negative cable off while running on newer cars is a bad idea, that you can cause more problems with the electrical system.

that is true it can cause a voltage spike.
have you had the local autoparts store check the battery?
and if you have a dvom set it to the higest amp setting and un hook the pos. battery cable and put it in line (one lead on the battery post and the other on the cable) you should not see more than .035-.045 amps
make sure all loads are turned off ie:dome lamps,under hood lamps,door buzzer. if you have more you need to check for a draw.
just noticed you have a 99 if there is a charging problem the pcm should set a fault "generator field not switching proplery" at that point you will need a drb3 to preform the "generator full field test" and see if the pcm is capable of turning on the alt field.
justin
 
bigazxj said:
that is true it can cause a voltage spike.
have you had the local autoparts store check the battery?
and if you have a dvom set it to the higest amp setting and un hook the pos. battery cable and put it in line (one lead on the battery post and the other on the cable) you should not see more than .035-.045 amps
make sure all loads are turned off ie:dome lamps,under hood lamps,door buzzer. if you have more you need to check for a draw.
just noticed you have a 99 if there is a charging problem the pcm should set a fault "generator field not switching proplery" at that point you will need a drb3 to preform the "generator full field test" and see if the pcm is capable of turning on the alt field.
justin

Help the ignorant. I didn't understand most of that last part. What is the pcm (assuming the computer code)? What is a drb3? Oh and the battery is fine.
 
If your dash volt meter is indicating ~14 while the engine is running, and in the green with the key on and engine off, good chance that your alternator is good, and that either you have a bad cell in the battery, or something turned on that's discharging the battery when it sits. A battery should measure ~12.50 volts when totally charged, and 12.20 when 50% discharged, after setting overnight disconnected. You need a digital VOM to check it properly, since the difference between being discharged and charged is .3 volts.
 
jdbwrx said:
Help the ignorant. I didn't understand most of that last part. What is the pcm (assuming the computer code)? What is a drb3? Oh and the battery is fine.

sorry about that pcm= powertrain control module (computer)
and a drb3 is the factory scan tool we use at the dealer
hope that clears up any questions
 
my alt comes right up to 14v when I start, then slowly backs down to about 12v on the dash gauge. This always made me nervous but the jeep has been starting fine down to -5 or so, I guess it's ok.
 
It's normal for the alternator regulator to set the voltage high right after starting--the regulator senses a drop in battery voltage due to the current just used to start the engine--hence higher charging voltage to start with. As the battery charges, the regulator reduces the charging rate by reducing the alternator output voltage.
 
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