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Truck dies after removing jumper cables


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So I'm trying to find out why my truck keeps dying. I jumped it off, let the vehicles run for several minutes, took the jumper cables off, and it runs for about a minute, then dies. I also noticed its idling weird. it will idle high, then get really low, then sound like someone is tapping the gas pedal. It will die soon after that. The alternator is brand new, I tried 3 different batteries (can't find my multimeter at the moment to make sure they are good, they are all about 5 years old). So I think I have it narrowed down to the battery, or a connection to the alternator. In the morning I am going to find my multimeter, and recheck the alternator connections. Am I missing something else that it could be? Would a bad battery make it idle like that? 

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If the alternator is working, it should run even with a dead battery. I suspect your new alternator was DOA.

Is that true even if the battery was drained to the point where it won't hold a charge?

one time I jumped my brothers old XJ off, drove it to the auto parts store, and left it running while they tried to charge the battery for me. Disconnected the battery while the engine was running and it kept on truckin without a battery for over an hour
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If the alternator is working, it should run even with a dead battery. I suspect your new alternator was DOA.

Is that true even if the battery was drained to the point where it won't hold a charge?
one time I jumped my brothers old XJ off, drove it to the auto parts store, and left it running while they tried to charge the battery for me. Disconnected the battery while the engine was running and it kept on truckin without a battery for over an hour
How on earth is this possible? Diesel is tho only thing I can come up with

 

Sent from my SM-G930P using Tapatalk

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If the alternator is working, it should run even with a dead battery. I suspect your new alternator was DOA.

Is that true even if the battery was drained to the point where it won't hold a charge?
one time I jumped my brothers old XJ off, drove it to the auto parts store, and left it running while they tried to charge the battery for me. Disconnected the battery while the engine was running and it kept on truckin without a battery for over an hour
How on earth is this possible? Diesel is tho only thing I can come up with

 

Sent from my SM-G930P using Tapatalk

it was a 97 XJ with an OBD2 HO. Not diesel at all haha.
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yup, most any vehicle will run purely on the alternator.  now, if it has a completely dead battery still installed, that's a whole 'nother issue.  for example we were having an heck of a time getting my buddy's bronco to keep running and it turned out the battery had died mid-drive.  once we separated it and got it jumped, we drove on the rest of the way home no problemo. :D

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Something is stopping the alternator from charging the vehicle. I'd suggest bench testing the alternator to make sure it's putting out voltage then go from there on checking whether you have a loose wire. You can also turn the key to the on position and see if you have voltage from the signal wire if you find your multimeter, normally 12V.

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excitement got the best of me. I went out and swapped the batteries just now. Jumped it off, let it charge for about 10 minutes, took the cables off, and it continued to run for a good 15 minutes. When it finally died the battery read 11.60. Which is an improvement, because when I took off the jumper cables, it read 10.3. So I'm still not sure what the culprit is. I would think if it were the alternator, it wouldn't have charge at all on its own, correct? 

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A properly function alternator should be putting out close to 14 volts DC at the battery with the engine running.

 

Did you measure the charging voltage with the engine running?

 

Your fully charged battery should measure 12.50 volts at least with no load.  That was the spec we used years ago to ship new batteries to the car factories when I worked a Gould Automotive Battery decades ago.

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I have heard that removing the battery cable while the engine is running could be hard on the rectifier diodes in the alternator. But i have never experienced this personally. I just wouldnt make a habit of doing this. Only do it once when diagnosing your engine stalling problem.

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Since you have a muti-meter try this easy test.....with the battery in and connected and the truck running take a voltage reading across the battery terminals....what is the voltage?

 

It should be somewhere  between 12.7 and 14.7 volts.....is that what you get?

 

 

 

 

 

And this may be off topic but I recently got one of these...

 

http://www.catautopower.com/25-amp-battery-charger.html

 

This is cat brand but there are other brands out there.....not only will it charge your battery....but it has an engine start feature (boost) that works (Winnipeg -40 trust me)

but it also has simple push button features that allow you to 1) check battery voltage.....2)do an in vehicle alternator check....3)and recondition the battery.

 

and you don't have to guess if the battery is fubar....if it is it tells you right on the screen.   . :MJ 1: .

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Hi GeorgiaJeeper,

 

Electrical problems can get confusing. Consider some basics:

 

1. For a vehicle to run, it generally needs at least 9 or 10 V available. This can come from a battery, or from an alternator. A vehicle can run on battery power alone, with the defective alternator. A vehicle can run on alternator alone, with the battery removed.

 

2. However, if the battery is very very low, it can "suck up" all of the alternators output, especially if the alternator is weak.

 

3. Do you have, or can you get a battery charger? Charge up the better of your batteries, if the charger output is relatively low output, this may take several days. After the battery is fully charged, disconnect the charger, let it sit for an hour, and then check voltage at the battery. A good battery will have about 12.8 V.

 

4. You can't really depend on the alternator to charge a fully discharged battery. It would take several hours, at least, of engine running above idle to fully charge a totally discharged battery.

 

5. As stated in another post, with a fully charged, good battery, a good alternator will typically raise the system voltage to around 14.4 V. It doesn't have to be exactly this. The voltage will be lower at idle, and may increase if you rev the engine. The voltage will be lower with a significant electrical load, such as headlights being on.

 

Why don't you try the above, especially with the fully charged battery, and see how things check out?

 

My guess is that ALL the batteries are very bad, and "eating up" all of the alternator output. 

 

Good luck!

 

Gene

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