stupid alt question?

[MudweiserXJ]

Prob a very stupid question but I have been having prob with my alternator, just changed them with ANOTHER used one. Gauge was still showingonly like 10 volts. Wasnt sure if my gauge was just bad or what, and this is a trail only rig so cant drive it to auto parts to have alternator checked, but here is my question...... shouldnt jeep keep running if I take battery cable off? I checked on my 96 and it keeps running but wasnt sure if the 89 was the same. BTW it is a 89 4.0. Anything other than alternator could cause this? I just replaced it with another one and both was reading 10 volts max on dash gauge.

Thanks

[90Pioneer]

Did you forget to plug in the the connector for the gauge on the alternator? I belive it is a 3 wire connector. Mine did the same thing, read about 10 volts with a new alternator. Plugged that connector back in, and presto, gauge reads at 14.

[MudweiserXJ]

Originally Posted by 90Pioneer

Did you forget to plug in the the connector for the gauge on the alternator? I belive it is a 3 wire connector. Mine did the same thing, read about 10 volts with a new alternator. Plugged that connector back in, and presto, gauge reads at 14.

yeah its plugged in, my main question is should the 89 run when I disconnect the battery after it is started. My 96 DD I can start it and unhook battery and it will still run, thats the old way I knew to check for a bad alternator but wasnt sure if the 89 was diffrent. Cause as soon as I pull the battery cable it dies. So if someone could check on their 87-90 XJ.

[MudweiserXJ]

......................

[Appleby]

Hey, I think you probably have it diagnosed. To make sure, there are several other ways you can check your alt. Test the output to the batt with a voltmeter at the battery. Voltage between pos and neg on your battery should be 12 volts with the engine off, and approx. 14 to 15 with it running. Check the condition of your battery, even a new one can have a problem. Make sure that it will hold a charge to 12 volts. On your "89 XJ you may have an older style alt with a test hole in the back. The hole is next to the wiring harness connector. With the engine running and a voltmeter attached, put a small screwdriver in the hole and ground the tab to the case of the alt. Check the reading. It should be 15 volts or higher ( it's unregulated, so don't do the test any longer than you need to ) If it's low, then for sure it's the alt. One other way is to disconnect the neg batt clamp, and connect a test light between the clamp and the post to test for a short. If the light comes on, you've got a short. Unplug the alt. If the light goes out, it's the alt. If the light stays on, you've got another problem. I hope this helps.

[8Mud]

No test hole on the CS, alternator. http://www.alternatorparts.com/cs130_sbpage1.htm does a pretty good job, of explaining the CS alternator.
Have noticed there are a few different wiring possiblilities for the CS alternator, near the plug in the back, most every Jeep Iīve seen is marked F/I at one pole on the connector.
Testing with a volt meter at the battery is a good suggestion. In Jeep volt meters are often a bit flacky.
If your alternator is putting out 10 volts (actually by test), a bad diode is a strong possiblity.

[MudweiserXJ]

Thanks y'all, but my main problem is, I dont have a volt meter or any way to test it. I did change batteries, between a new one I put in the 89 and the one that is in my 96So should the jeep still run if i disconnect my battery after started? The 96 does and thats the way we used to check chevrolets. Thanks

[8Mud]

A long time ago, they advised against running the motor with the battery disconnected. Canīt really remember which vehicle it was, believe it was Chev. with the CS alternator. I gnerally pay attention to the warnings in the FSM.
Iīve avoided doing this since reading the warning, on most every vehicle. Something to do with, the high frequency voltage regulator, in the newer alternators, possible voltage surges and the electronics.

[TheRasper]

i would certainly avoid doing that for a testing procedure. On my 93, the voltage Reg. is in the PCM, and has to be sent off for a rebuild. The notion of a voltage regulator being an easy and cheap fix are gone with these rigs, and should not be tempted for failure. I understand you cannot drive it down, and haven't the testing equip. yourself, but I'd pull it and carry it down to the store for a bench-test. In the ling run, you'll be glad. Too much at risk the other way.
Best of Luck to you...
TheRasper.

[TheRasper]

*long run*... geeez, it's late...

[Roxtar]

Yes



If you pull the cable and it dies, the alternator is bad.
You can bring the alternator into any auto parts and they can test it on the bench for you. They don't need the Jeep there.

[MudweiserXJ]

Originally Posted by kid4lyf

Yes



If you pull the cable and it dies, the alternator is bad.
You can bring the alternator into any auto parts and they can test it on the bench for you. They don't need the Jeep there.

OK Alternator tested good, Battery good, Borrowed a volt meter, and it tested 12V at battery with XJ off, AND 12V at battery when running.....All wires look good, and YES it still dies when I dosconnect battery cable while running.

Any where else I should look at fixing this?

[8Mud]

Alternator should be putting out close to 14 volts, after start up, checked between the battery terminals. After it has been running for awhile, it sometimes settles in a little lower, but rarley 12 volts. Voltage usually fluctuates some, with RPM, not a lot, but some.
Look on the back of your alternator, the black plug, one of the poles is marked with a F over an I.
Sounds kind of like your alternator either has a faulty regulator (internal) or possibly a diode. Used to be a diode was either good or bad, but have seen some on the CS type alternator that were kind of bad, made for low output. Need the voltage going to the F/I pin on the alternator for it to work right, should be getting battery voltage at the connector, with the ignition on.
Have also seen a bad cell in the battery mess up voltage reading, a good test is while starting the Jeep, hold a voltmeter across the pos and neg poles of the battery, when everything is OK, it shouldnīt read any/much lower than 9 volts.

[TheRasper]

As I said before, I don't know if your system is the same as mine, but if so it is your voltage regulator, which on mine is in the PCM. (I am even more convinced now that you say the Voltage is at 12 no matter what.. You are'nt showing proper charge because the regulator is not telling the Alt. you need it.)If that is the case, it is likely that you did not need to replace the alt. Consult you FSM to see if your system is regulated by a PCM contained voltage regulator.

note: if this is the failure, you may not have to buy a PCM. I was $$$ ahead to ship mine off to a rebuilder and have the voltage regulator fixed in it. I would suggest you shop around and compre cost and downtime for a repair to yours vs. purchasing a new one.

Best of luck with your repair.

TheRasper

[brtb]

I know Advance Auto stores here have a machine they can test bare alternators with, out of the vehicle... might want to check and see if any stores around where you are can do that.