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Unread 07-26-2010, 04:36 PM   #16
TJdualsport
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so jerry are you saying if your battery dies the alternator is close behind in death? the ac trick works for me. if your clutch is turning on and off really fast its because your low on freon, and thats because you got a leak somewhere.

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Unread 07-26-2010, 06:45 PM   #17
Jerry Bransford
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Originally Posted by TJdualsport View Post
so jerry are you saying if your battery dies the alternator is close behind in death?
No, it's just that the alternator requires a good strong 12v from the battery to work. Once the alternator starts getting a good solid 12v from the battery after being recharged or replaced, the alternator starts producing full power again.

None of the old generators required battery voltage to work and some of today's modern alternators (like the GM one-wire alternator) don't require battery voltage either. But most other automotive alternators, like what are installed into Jeeps, do require 12v from the battery which is called the alternator's "exciter voltage". If the battery is disconnected from a TJ, the engine will eventually die. With a missing or completely dead battery, the Jeep can't even be push-started. If jump started, the engine will die after the jump start battery is removed if the battery is missing or dead.
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Unread 07-27-2010, 06:03 AM   #18
tjdriver
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if your clutch is turning on and off really fast its because your low on freon, and thats because you got a leak somewhere.[/QUOTE]


its not low on freon. it works fine when it idles normal or when being driven. must be some sort of protect mode or something cause it only does that at the low idle.

the idle is up to approx 600rpm now. i drove it to work and it runs fine. it only dropped down to the low idle twice with 2 hours of running yesterday and did not low idle at all this morning.

seems it may be headed in the right direction.

thanks for all the help fellas.

p.s. i didn't change/do anything to it yesterday other than put a new oil pan gasket in.
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Unread 11-18-2011, 04:32 AM   #19
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Originally Posted by Jerry Bransford View Post
No, it's just that the alternator requires a good strong 12v from the battery to work. Once the alternator starts getting a good solid 12v from the battery after being recharged or replaced, the alternator starts producing full power again.

None of the old generators required battery voltage to work and some of today's modern alternators (like the GM one-wire alternator) don't require battery voltage either. But most other automotive alternators, like what are installed into Jeeps, do require 12v from the battery which is called the alternator's "exciter voltage". If the battery is disconnected from a TJ, the engine will eventually die. With a missing or completely dead battery, the Jeep can't even be push-started. If jump started, the engine will die after the jump start battery is removed if the battery is missing or dead.
Reading your post may of verified my suspicion. My Jeep has suddenly displayed low idle after warm up. This manifested itself right after I found that the battery was drained too low to start the engine after leaving the lights on for about 30 minutes (Factory Fogs too). A quick jump start and the battery seemed to accept charge as it can start on its own; however I believe it is no longer fully charging and would likely fail a load test. The engine is so clean inside & out with only 59,000 making me doubt the Idle air controller or throttle position sensor is at fault. Battery is original & 8 years old so I am going to start there. I have found that your posts are amongst the most informative & your suggestions consistently practical. I rarely feel the need to post an issue because I often find that you have already addressed it on some thread. I am sure that your posts have helped many people and I for one would like to say thanks for taking the time to make those posts! Posts like yours enable me to resolve problems from my couch, minimizing the amount of time I spend under the hood...
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Unread 11-18-2011, 07:15 PM   #20
joe108
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When my battery was going bad, all the guages went wild!, they looked like windshield wipers, it ran bad, and the key buzzer was screaming. After I replaced the battery it was back to normal. As mentioned the jeep/computer likes a good steady 12 volts or it throws a fit.
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Unread 11-18-2011, 07:47 PM   #21
matthew900
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I ran in to this same problem last week after I left the lights on for a full weekend. I think its a combo of the battery being to low to run the alternator properly and the computer's idel settings getting wiped. I charged the battery fully with a battery charger and my problem was solved. Hope that helps,

Matt
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Unread 11-18-2011, 08:32 PM   #22
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If my battery gets low and it barely starts then the computer will forget how to idle.

The only way I got it to idle is to disconnect the battery and touch the positive cable to ground, this will reset the pcm and it will idle normal again. It will also forget how to idle if the battery is dead and you coast start the jeep.
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Unread 11-18-2011, 10:57 PM   #23
Mictrik
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Originally Posted by matthew900 View Post
I ran in to this same problem last week after I left the lights on for a full weekend. I think its a combo of the battery being to low to run the alternator properly and the computer's idel settings getting wiped. I charged the battery fully with a battery charger and my problem was solved. Hope that helps,

Matt
Exactly what I did, problem solved. Jerry Bransford is completely correct about the stock alternator needing an "exciter current" from the battery to generate a magnetic field. Probably residual magnetism in the inductor allows for "self excitation" of the field winding when rotational speed is sufficient.
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Unread 11-18-2011, 11:02 PM   #24
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Originally Posted by alex211 View Post
If my battery gets low and it barely starts then the computer will forget how to idle.

The only way I got it to idle is to disconnect the battery and touch the positive cable to ground, this will reset the pcm and it will idle normal again. It will also forget how to idle if the battery is dead and you coast start the jeep.
Hmmm. I think that is the wrong way to go about relieving the problem. My bet is that all you are doing is reseting some of the logic within the computer resulting in raised idle. The raised idle probably provides enough rotational speed to the alternator to "self excite" & in turn charge the battery. have you tried applying a load by running the ac compressor or some lights? Perhaps your issue is of a different nature?
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Unread 12-11-2012, 07:11 AM   #25
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Originally Posted by Jerry Bransford View Post
When the battery is low enough, the engine will barely idle and it can even stall. Have the battery load tested. The alternator depends on the battery to put out power so if the battery is dead or near dead, the alternator will be dead or near dead too.
Quote:
Originally Posted by Jerry Bransford View Post
No, it's just that the alternator requires a good strong 12v from the battery to work. Once the alternator starts getting a good solid 12v from the battery after being recharged or replaced, the alternator starts producing full power again.

None of the old generators required battery voltage to work and some of today's modern alternators (like the GM one-wire alternator) don't require battery voltage either. But most other automotive alternators, like what are installed into Jeeps, do require 12v from the battery which is called the alternator's "exciter voltage". If the battery is disconnected from a TJ, the engine will eventually die. With a missing or completely dead battery, the Jeep can't even be push-started. If jump started, the engine will die after the jump start battery is removed if the battery is missing or dead.
Your posts are always spot on jerry. Time to replace my battery.

Stratton.
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Unread 12-11-2012, 07:54 AM   #26
JEK3
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My battery crapped out a couple weeks ago. Now that I think about it, the idle is much smoother since I replaced the battery.
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