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Unread 04-26-2010, 11:23 PM   #1
nsx2000
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Battery not charging - detailed troubleshooting

Yes I was an idiot for not unplugging the battery before I started doing some work. I crossed the positive cable from the top of the alternator to the alternator casing and heard several loud pops. I fried my alternator so I replaced it (it was putting out 31V after I hooked it back up!). Only the replacement was bad and I drove it to the parts store.

Once I got there my Jeep would not start. I removed the alternator and had them test it and it failed. So I put in a new alternator after testing that it was good. One problem down. Now for the dead battery.

They charged the battery for me and after a few hours we found that the battery was leaking all over the charge station. Bought new battery. This is where it gets interesting...

Using Voltmeter-
1. Positive alternator post to frame, engine running - 13.5-14.5V
2. Positive to negative battery post, engine off - 12.5 or 12.8V (can't rem)
3. Ohm test from neg battery terminal to alternator case, engine off - no resistance
4. Ohm test from positive post of alt to the positive post on battery, engine off - no flow (don't know if this is good or not)

MAIN PROBLEM: The voltage warning light turns on after 10 when starting, and the battery is not getting charged. After driving back to the house I tested the voltage between the positive and negative posts with the engine still running and got 11.8V.

What the heck is still wrong???
Please help

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Unread 04-27-2010, 09:01 AM   #2
meep
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There is likely a big-a$$ fuse in the underhood fuse block.... 100-200A. Probably held in w/ two screws. it blew when you shorted the terminals the first time. That's why the 2nd alt showed 31V.... it was feeding into zero load.

Not absolutely positive the jeep is wired this way, but this is where I'd start.

M
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Unread 04-27-2010, 10:49 AM   #3
nsx2000
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Quote:
Originally Posted by meep View Post
There is likely a big-a$$ fuse in the underhood fuse block.... 100-200A. Probably held in w/ two screws. it blew when you shorted the terminals the first time. That's why the 2nd alt showed 31V.... it was feeding into zero load.

Not absolutely positive the jeep is wired this way, but this is where I'd start.

M
I checked all the fuses in the two fuse blocks under the hood along the driver side and cannot find anything blown. I have also pulled the fuses from inside the car under the steering column. Maybe there is a fuse hidden somewhere else?

CORRECTION: The original alternator was the one that was putting out 31V after I fried it. The first replacement alternator was putting out correct voltage but no amperage as well as having bad bearings. I have replaced it with a second, properly tested and functioning, alternator. Sorry for the confusion in my first posting.
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Unread 04-27-2010, 11:08 AM   #4
tjkj2002
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Start thinking PCM as that controls the alternator.
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Unread 04-27-2010, 12:34 PM   #5
merlinTec
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Hi
Yes PCM controls the charging but you will have a fusible link blown up by the battery in the main power lead.
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Unread 04-27-2010, 01:03 PM   #6
nsx2000
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Quote:
Originally Posted by merlinTec View Post
Hi
Yes PCM controls the charging but you will have a fusible link blown up by the battery in the main power
lead.
This is a 2006 Jeep Grand Cherokee Laredo V6

You do not by change have a picture or diagram of where this fuseable link is do you? The cable up by the battery looks ok, but I lose it as it passes by the driver valve cover. And it is extremely cramped under the bottom and can hardly see the starter, which is where I guess the main red cable is going.

Am I on the right track?
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Unread 04-27-2010, 02:09 PM   #7
meep
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fusible link... good call.

nsx-- not knowing exactly where it is myself.. it will look like a wire, inline with the alt + line. probably crispy charred in appearance, 2-4" in length. Follow the big red wire from the alt and see where it goes. To starter? To main fuse block? I doubt it's in a wrap, that wouldn't make sense.

I really really doubt the ecu was harmed.
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Unread 04-27-2010, 02:53 PM   #8
merlinTec
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Hi
The way to test is volt meter from alternator + to battery + if you have more then 0.5 volts you have a fault in that wiring (expect you have more like 14volts) with engine running.
You have to excuse me as we don't have the v6 petrol in the UK in the GC but going by the parts picture the link does look like it be down by the starter.
Hope this helps
wiring.jpg  
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Unread 04-27-2010, 02:59 PM   #9
nsx2000
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Well I hope it is not the computer, but the Jeep dealer says it probably is after all the stuff I have done so far. I asked them about a fuse-able link and the parts guy couldn't find it in his diagrams, and the service guy said there wasn't anything there like that on a 2006. Says I'll just have to have them test the computer to see if I did fry it or not.

Guess I can try to trace those wires down tomorrow tonight...I don't suppose I can inspect the computer myself?
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Unread 04-28-2010, 10:01 AM   #10
meep
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start jeep.

1. ensure the v at battery is close to v at alt +. try merlin's test above... should be very, very close. If different, bad + wire. if same, proceed.

2. ecu sends juice to energize the field coil on the alt. the more power needed, the more juice sent. Need to measure.... on older DCX cars it was a 2-wire system-- easy--- measure across the two small wires. it'd be anywhere from 1-12 volts or so. would drop with engine rpm increase. hopefully these are 2-wire field coils.
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Unread 04-29-2010, 02:43 AM   #11
kreso
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PCM doesn't send anything. It provide ground to the alternator. So to test you can connect connect - wire from alternator to ground. If all other is OK your battery should be charged
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Unread 04-29-2010, 10:23 AM   #12
nsx2000
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Well I took the wiring harness out last night and merlinTech was 100% correct. There is a fuse-able link just before the starter. Completely smoked and destroyed. Now to see how much a new harness is vs replacing the bad wire manually. Sounds like I have to get this from the dealer since it is so new.
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Unread 04-29-2010, 11:53 AM   #13
meep
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kreso,

sounds like it's a 1-wire system then.

Glad you find it nsx!

FYI just going a little further, if it does provide ground to the FC as stated, it may not be a "straight" ground. the field coil will get (+) from the mains, and ecu/pcm (which does it in these vehicles?) will regulate charge rate by varying the resistance to ground, either by transistor type (variable resistance) or the cooler-running pulse-width modulation (more likely). either way, you could measure from (+) to that field coil gnd lead to see what the controller is doing. The corresponding Voltage will decrease with engine rpm, increase with electrical load.

Good call on it being a negative system kreso. good info!
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Unread 04-30-2010, 08:01 PM   #14
nsx2000
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Recap

So, I thought I would share how this all turned out.

The fuse link down by the starter was broken/fried. There were three ways to fix this:

1. Buy a new Battery/Alternator Wiring Harness from Jeep - $170
2. Buy a used one - cost unknown
3. Repair the cable

I went with fixing the cable after a friend helped me who has redone harnesses before. He use to make custom Jeeps so I'll hope we made the right choice.

This fuse point can be tested by testing for continuity from the alternator positive cable to the battery positive cable. The fuse link is on the alternator side of the loop (with the starter in the middle of the alternator and battery). So I replaced the cable going from the alternator to the starter.

How we made the cable:
Pulled the bad cable out of the harness running from the alternator to the starter. I chose to use a fuse holder with a large 40A fuse to serve as my fuse link.

Afterthoughts:
In hind sight I think I should have gone with a fuse link again. I have been told that fuses can eventually wear out and you have to replace it again. I'll just have to see how well the 40A fuse holds up. I chose the 40A fuse after looking at the amp rating of a 10AWG copper wire.
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Unread 05-01-2010, 12:16 AM   #15
fatfreddy
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You can buy fuse link wire.Or use a 40 amp circuit breaker.
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alternator , battery , charging , dead , short

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