YJ losing battery juice while running-battery/alternator good - JeepForum.com

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post #1 of 21 Old 01-11-2010, 10:50 AM Thread Starter
Dannyboy6292
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YJ losing battery juice while running-battery/alternator good

Hey guys,

I have had my jeep in the shop now three separate times. First time they said it was the starter was wired bad, after that they said there was a short in the wiring somewhere. I went and tested the battery and it was fine. The VAT (I believe that is the name) came out fine and apparently the alternator is working fine. That being said after charging battery and them “fixing” the jeep issue I drove away and it started like a champ. The start got gradually rougher-slower after driving a bit until it would not crank. I pop started it back to shop and now they are working on it. What could be killing the battery when it is driving. I do have an auxiliary cooling fan hooked directly onto the battery but not sure that, that could be causing the battery draining issue.



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post #2 of 21 Old 01-11-2010, 11:03 AM
Why J?
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Does the fan run constantly?

When did you add the fan, in relationship to this battery draining issue?
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post #3 of 21 Old 01-11-2010, 11:06 AM Thread Starter
Dannyboy6292
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Does the fan run constantly?

When did you add the fan, in relationship to this battery draining issue?
The fan runs when I start the car. It has its own separate on/off switch.
I just bought the jeep about 2 weeks ago and have been dealing with this issue since. I know its a chevy 350 engine conversion.
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post #4 of 21 Old 01-11-2010, 11:07 AM Thread Starter
Dannyboy6292
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I can choose to leave the fan off when running but the engine would overheat eventually. When I get down the last time or so there is enough juice to crank engine, when driving I can actually see the lights and stuff dimming as I am driving. The fan slows down etc. It has me baffled.
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post #5 of 21 Old 01-11-2010, 11:08 AM
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Is the alternator actually charging the battery?
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post #6 of 21 Old 01-11-2010, 11:09 AM
Que89YJ
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# times doesnt speak well for the mechanics youre taking it too. The alternator could be undersized for the loads you have going on the jeep. Depending on how you have the fan wired it could draw 40 amp continuous by itself. I see your running a 350 so I am assuming you have the larger gm alternator that is listed in the alternator mods-if not its a cheap good place to start. The alternator becomes less efficient as it becomes hotter. You can see a decrease in the output as much as 40% depending on how hot it gets. You should just be able to see what the current draw is from your system by hooking up a current clamp around the battery output of it and do the same at the alternator lead to the battery. Check the alternator cold and hot and under full load and with nothing but the engine running. If it is slow to die while the vehicle is running and you are not killing the battery when you have the ignition off. It would seem like its the alternator being undersized for the load.
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post #7 of 21 Old 01-11-2010, 11:13 AM
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Quote:
Originally Posted by Dannyboy6292 View Post
I can choose to leave the fan off when running but the engine would overheat eventually. When I get down the last time or so there is enough juice to crank engine, when driving I can actually see the lights and stuff dimming as I am driving. The fan slows down etc. It has me baffled.
Sounds to me like the alternator is not charging the battery.

check alternator, and wiring.
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post #8 of 21 Old 01-11-2010, 11:24 AM Thread Starter
Dannyboy6292
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Sounds to me like the alternator is not charging the battery.

check alternator, and wiring.
Yeah, I told these guys I thought it was the alternator from the get go and they said it was good. I will follow up when I hear today. Thank you.
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post #9 of 21 Old 01-11-2010, 11:33 AM
Mark05059
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the alternator could be "good" but not have enough out put to keep up with what you have in use...


also the voltage regulator could be not calling for juice from the alt

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post #10 of 21 Old 01-11-2010, 11:34 AM
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Hey Danny, take a look at the GM alternator in the alternator swap writeup. It should bolt up to your 350 and give you 140amp out which should keep you with enough juice to make FP&L jealous. If the battery was load tested and good then do the swap. Just curious, how long have you been driving on her after you did the fan mod? Did you use the Ford fan?
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post #11 of 21 Old 01-11-2010, 12:03 PM
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Quote:
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Yeah, I told these guys I thought it was the alternator from the get go and they said it was good. I will follow up when I hear today. Thank you.
Alternator being good and actually charging the battery are 2 different ball games.

I would find a new shop.
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post #12 of 21 Old 01-11-2010, 12:11 PM Thread Starter
Dannyboy6292
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Quote:
Originally Posted by Que89YJ View Post
Hey Danny, take a look at the GM alternator in the alternator swap writeup. It should bolt up to your 350 and give you 140amp out which should keep you with enough juice to make FP&L jealous. If the battery was load tested and good then do the swap. Just curious, how long have you been driving on her after you did the fan mod? Did you use the Ford fan?
Truth be told I bought this from a guy about 2 weeks ago for 5 grand. The tires and wheels alone on it are worth close to that much. There is a brand new 350 and clutch in it as well. Therefore I really dont know what it was like in the past. I checked out the alternator swap write up and will find out what alternator is in there now.
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post #13 of 21 Old 01-11-2010, 12:13 PM Thread Starter
Dannyboy6292
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Alternator being good and actually charging the battery are 2 different ball games.

I would find a new shop.
Yeah, the only one open on New Years day when I bought the thing was a full service Tire Kingdom. So I paid them already for the work and so they need to fix it.
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post #14 of 21 Old 01-11-2010, 05:08 PM
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You need to get a voltmeter and check the voltage at the battery and at the alternator while the engine is running. These voltages need to be with in 1 volt of each other( i personally like them as close as possible) While the engine is running and no load on the electrical system the minimum voltage should be no less than 13.5 volts and no more than 15.0. If the voltages at both locations are more than a 1 volt you need to locate the bad connection then. this could be anywhere from a firewall connection to a bad amp meter if it has one. Good luck>
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post #15 of 21 Old 01-11-2010, 05:44 PM
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Agree get a voltmeter. When you check the connections check terminal to terminal then terminal to connector.
This picture might describe better:
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