how do i test my alt?

i have a 96 xj with a 4.0 and i've been told my problems may be from a bad alt. i just want to know exactly how and war do i have to do to test it. also if i do need one, what is a good upgrade and wat should i look for thanks guys

Easy and fairly accurate way is to take it to your local parts store (advance, or autozone) They will do it for free, just make sure you grab someone who knows what they are doing. If you want to do it yourself grab a volt meter or watch your voltage gauge if the vehicle is equipped. Watch the voltage as you crank the motor... It should move from about 11.5-12.5 volts (engine off) up to ~13-14.5 volts after the engine is cranked... Anywhere out of that range and you should probably be concerned. I would put a load on while your testing to see how much draw from the electrical system... example, turn your headlights, wipers, and radio on and up to see if the voltage drops significantly while your motor is cranked. Make sure to check if your belt is tight enough/no slipping and/or your electrical connections are secure as well... that can cause a misdiagnoses of a bad alternator.

also if you start it then disconect i cant remember either the positive or the negative on the battery and if it still runs then your good.

What is it doing? Could just be bad cables or corroded terminals.

You can also start the Jeep, put a non magnetic screw driver on the center of the back bearing. If it sticks the alt is charging. How much?, dunno you need a voltmeter to know.

Just ditch your old one and get a Mean Green... I know certain Grand Cherokee Alternators are a high amp upgrade with only minor mods to make it fit... Do a search, there should be plenty of threads on alternators.

What is your problem? The Nippon Denso alternators are very reliable.

A good start would be to determine what the problems are. Any OBD codes? You can test voltage output quickly enough with a cheap voltmeter at the battery. Make sure the belt is tight enough. Run engine. Check voltage. If it's all right, turn on the headlights and a few other things, and if it stays within range, it's probably not the alternator.

BLKXJ33 said:

Easy and fairly accurate way is to take it to your local parts store (advance, or autozone) They will do it for free, just make sure you grab someone who knows what they are doing. If you want to do it yourself grab a volt meter or watch your voltage gauge if the vehicle is equipped. Watch the voltage as you crank the motor... It should move from about 11.5-12.5 volts (engine off) up to ~13-14.5 volts after the engine is cranked... Anywhere out of that range and you should probably be concerned. I would put a load on while your testing to see how much draw from the electrical system... example, turn your headlights, wipers, and radio on and up to see if the voltage drops significantly while your motor is cranked. Make sure to check if your belt is tight enough/no slipping and/or your electrical connections are secure as well... that can cause a misdiagnoses of a bad alternator.

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Easy? Yes. Accurate? Hardly. I used to work in a parts house, and I've had to service a couple of those benches. They test voltage output, but not current. Oops...

Disconnect a wire? Sure - if you want to buy control electronics afterwards. While that would work on vehicles with points and condensers, I sold a bunch of ignition modules to people who decided to try that (and blew the early ICMs,) and it's only gotten worse.

You want to test your alternator yourself? Here's what you can do for a "scratch test" that won't cause you any trouble...

1) Grab your DMM and a notebook and crayon. You don't have a DMM? Get one - you should have one anyhow, and you can get a decent one for about fifty bucks at Sears and better hardware stores.

2) With the engine OFF, check voltage at your battery. Write it down. It should be 12.0VDC or higher (12.6VDC is nominal, I've seen anywhere form 12.2-13.4VDC from a good battery with a full charge.)

3) Start the engine. If you want to check your battery and starter, have someone else crank the engine while you check voltage with the ignition coil disconnected - it should be at least 9.6VDC. Reconnect the coil and start the engine.

4) Check voltage at the battery. It should be significantly higher than when the engine was OFF - around 13.5-14.5VDC. If it's over 15.0VDC, you're overcharging - and you'll blow out your battery in short order. If it's "Engine OFF" voltage or lower, you're not charging. If that's the case, check the voltage at the output post on the back of the alternator - since you may have a wiring fault instead (there is a fuse or a fusible link between the alternator and the rest of the vehicle. That may have blown on you, for instance - and the alternator is trying to do its job without being connected to anything.)

If voltage at the battery is not good but the voltage at the alternator is; shut down, disconnect the output lead at the alternator, and check resistance between that ring lug and the battery + terminal - there should be "none" (there are conductor losses - but it should be less than 5 ohms total. If it's more, check connections for corrosion. If it shows open, find and repair the fault - it's probably the fuse.)

If voltage is not present at the alternator output, then plan on replacing your alternator.

If voltage at the battery and at the alternator output post are both good, start turning on electrical accessories while monitoring voltage at the battery. It should dip slightly as you turn things on (say, a few hundredths of a volt for each accessory,) but it should remain above the engine OFF value you recorded at first. You are now checking the current output of your alternator - which those parts house benches can't do. The smaller shops that just do starter/alternator rewinds usually can (mine does,) but the larger chains don't bother - and I shan't go into why. Turn on your radio, lights, fans - whatever you've got that's electrical. Voltage should remain a few points above the engine OFF value, even with everything turned full on.

5) Shut down. Check your battery voltage with the engine OFF immediately, and again about fifteen minutes later. It should be comparable to what you wrote down in (1), above. You're doing the second check to make sure your battery is taking the charge fully, and not getting a "surface charge" - which is useless.

Of course, this all begs the question - why for do you suspect your alternator? What trouble are you having?

Nice 5-90!!!! I couldn't have explained it better my self.

I cringed when they said "disconnect the battery with it running". These are 75 ford trucks people. I have a friend who blew out the dash(speedomoter/tach) control board on her BMW just getting a jump.......$1200 Ooops. I won't jump people with my mercedes because I am scared of that. One voltage spike and its over.

silverslk said:

Nice 5-90!!!! I couldn't have explained it better my self.

I cringed when they said "disconnect the battery with it running". These are 75 ford trucks people. I have a friend who blew out the dash(speedomoter/tach) control board on her BMW just getting a jump.......$1200 Ooops. I won't jump people with my mercedes because I am scared of that. One voltage spike and its over.

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That's why I won't own a European car to begin with. I had a buddy with a 1992 BMW 325is (I may have that year wrong, but it sounds about right...) and his tuneup cost more than my whole truck. He had to get a steering rack - paying for that job would have covered any four vehicles I've owned, with some left over.

And, I've worked on a few (brakes on that 325is, rear main on a Benz 450SEL, and have you ever tuned the six carburettors on an old Jag V12? Easier to do when drunk, believe me...)

No, thanks. Not this little black duck! As vehicles have "advanced" and gotten more electronic, they're more sensitive to voltage spikes and such - which has changed troubleshooting procedures.

(Oh - I had to sell a Duraspark module to someone who blew his out on a 1975 Ford pickup once, and he wondered why he suddenly had no electrical power... I never did like the old Ford Duraspark...)

5-90, You must sit here all day and post... All your posts are a page long and too damn detailed for the average reader, ha ha... Very knowledgeable though. Its an alternator, just check the voltage-- that easy. And as far as the parts store testers... I don't disagree with the fact that some suck... I know bc I used to work at one myself. But their are some decent testers these days that save you the trouble of having to make it such a complicated test. And for the average person... its more complicated with a DMM, unless you know a little about electricity. In most cases Ive seen in the offroad world... either you kill your alternator or its good. The brushes are either worn down, the circutry/voltage regulator is shot-- probably because you soaked it in mud/water, or the bearings are shot from belt being too tight or mud/water. A simple test will tell you whether or not you need to ditch your old one. And as far as Nippen Denso goes MJR, they aren't bad No complaints from my standpoint, I'm running one in my jeep now thats 13 years old.

silverslk said:

I cringed when they said "disconnect the battery with it running".

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I couldnt agree more... wrong way to do it

I would like to say that the voltage regulator is in the engine controller on 91-up models. Most of the time it's not the alternator or engine controller for that matter on these years but either a draw running the battery down when the vehicle is off or a bad battery. Since the engine controller senses the voltage, battery temperature, and other things it monitors then charges accordingly. If the battery does not come up to the voltage the computer expects it can pull down the load on the alternator trying to bring it up (which can overcharge a bad battery although the voltage can be within specs). Of course as these vehicles get older you can see wiring problems also especially if the battery has leaked acid anywhere.

One minor clarification from 5-90's post "If voltage is not present at the alternator output with everything connected then you have an open wire/fuse/fusible link coming from the battery, but when the output wire is disconnected with the engine running and you have no voltage at the ouput stud on the alternator but have power on the field wires and the engine controller it switching the ground for the field then plan on replacing your alternator."

silverslk said:

I cringed when they said "disconnect the battery with it running"

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BLKXJ33 said:

I couldnt agree more... wrong way to do it

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Yup.

MJR said:

One minor clarification from 5-90's post "If voltage is not present at the alternator output with everything connected then you have an open wire/fuse/fusible link coming from the battery, but when the output wire is disconnected with the engine running and you have no voltage at the ouput stud on the alternator but have power on the field wires and the engine controller it switching the ground for the field then plan on replacing your alternator."

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Eh - I can't remember everything...

BLKXJ - I work from home, and I are the boss. I'm usually at my desk online anyhow (research,) so it's not much trouble. And, since I finally learned to touch-type, it doesn't take anywhere near as long to write all that as you might think.

Besides, I'd rather see someone test something properly than use a method that is likely to cause other problems. Why buy trouble, when you get enough for free?

While the tests I gave seem complex on the face of them, they're actually quite simple once you've done in a couple of times, and it's going to give you better results than the bench down at the local. Just for fun, I connected an ammeter in series with an alternator I was testing once upon a time, and the bench was drawing about half an ampere. That's about enough to run a decent flashlight with a good bulb, but it's not even enough to feed an ignition system on a 1950 Studebaker... I ended up getting a batch of wirewound power resistors and making a "load box" that was good for a consistent 50A - not perfect, but loads better. It's amazing how much the failure rate went up after I built that load box...

5-90 said:

Eh - I can't remember everything...

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I know the feeling, just when you get done you thought of something else or just take certain things for granted since you do it all the time.

MJR said:

I know the feeling, just when you get done you thought of something else or just take certain things for granted since you do it all the time.

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Yeah. Do something for a couple dozen years, and you quit thinking about the little stuff (it's all automatic.)

5-90,

Thanks for your detailed numbers. I tested the voltage at my battery under a number of the scenarios you described, and here is what I got:

Everything off: 12.64V

"On", engine off: 12.4V

Cranking: 11V

"On", engine running: 13.94V

"On", engine running, AC on, lights on, radio on: 13.92V

Shutdown: ~13.9, dropping ~0.01V every second after shutdown


For my case, it seems that the alternator is in decent shape? Of course, I did this w/a cheap $20 multimeter, but the numbers should at least be in the ballpark.

One wierd thing I noticed was that when they key was turned on, but the engine was off, the PCM made a wierd, high-pitched hissing noise, and faintly beeped about once every second. Is this normal? It may have been doing it while the engine was running, but I couldn't hear it over the engine noise.

MJCfromCT said:

5-90,

Thanks for your detailed numbers. I tested the voltage at my battery under a number of the scenarios you described, and here is what I got:

Everything off: 12.64V
"On", engine off: 12.4V
Cranking: 11V
"On", engine running: 13.94V
"On", engine running, AC on, lights on, radio on: 13.92V
Shutdown: ~13.9, dropping ~0.01V every second after shutdown

For my case, it seems that the alternator is in decent shape? Of course, I did this w/a cheap $20 multimeter, but the numbers should at least be in the ballpark.

One wierd thing I noticed was that when they key was turned on, but the engine was off, the PCM made a wierd, high-pitched hissing noise, and faintly beeped about once every second. Is this normal? It may have been doing it while the engine was running, but I couldn't hear it over the engine noise.

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Your numbers look good (the second one was unnecessary, but the rest are fine.) That "trickle down" effect after shutdown is normal - that's the "surface charge" of the battery bleeding off through the various "keep alive" circuits in your vehicle electronics. As long as it stabilised at or near the initial "Key OFF" value, you're fine.

As for the PCM effects, I'm not sure - what's in there? I've not worked on one of those in a while, so I don't recall what they're supposed to be doing. However, if everything else seems to be working normally, I'd be disinclined to worry overmuch abou it. You don't need the headache, you know?:laugh3:

You don't need the headache, you know?

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You are right, bad to go creating problems.

Ok, so I have a 1987 Cherokee Chief, 4.0L. I just put a new alt in and its still not charging. I have run into this problem before on other jeeps i have owned. But i dont know whats wrong. can some one help?