Bad bcm on an 04 kj?

Jeeper4life · **Posted:** Fri Nov 08, 2013 3:21 pm

I still have some kinda electrical issue going on with my 04. It kinda started with the power door lock fuse. It would blow that fuse every now and then. Well the bcm shares that fuse also I believe. Well I have just been replacing the fuse and it hasn't done it in a while. About a month ago however, I was sitting in a drive thru and didn't catch it exactly but I think everything, gauges and what not shut off like I turned the jeep off and bam came right back on like I had just started it up. The jeep didn't die or shut off or anything. Just the weird dash issue. It's only done that once so far. Now the last issue is whenever it gets cold out something is draining the battery slowly. If it sits for around 8 hours or so it's really slow to start. The battery is fairly new but a cheap auto parts store battery. I didn't want to get it a decent battery if all it's gonna do is kill it right away. So anyone got any ideas on where I should start?

Jeeper4life · **Posted:** Fri Nov 08, 2013 6:34 pm

Battery and alternator tested good according to the parts store. The battery for some reason was half charged.. I didn't figure the alternator was bad, It would of triggered the dash light if it was I think. Where should I start next?

MoD · **Joined:** Sat Mar 17, 2012 12:53 am **Posts:** 1343

i would start with a volt meter in the fuse box.

Jeeper4life · **Posted:** Fri Nov 08, 2013 7:53 pm

And do what? I know very little about vehicle electronics.. but I'm slowly learning..

MoD · **Joined:** Sat Mar 17, 2012 12:53 am **Posts:** 1343

well i would determine that the drain was in that fuse. to do that you will leave the key off and look for current draw. then you would trance down the wiring and make sure what is killing it. after that...well...it depends what it is

Jeeper4life · **Posted:** Fri Nov 08, 2013 9:16 pm

Awhile back, my wife and I where working on it. What we did was, she held the multi meter on positive and negative sides of the battery to get a base reading. Then I went inside and pulled out each fuse one at a time to see if we could get a drop in voltage, then I would put it back in if it didn't drop and move on to the next one, and we got nothing..don't know if you have ever done something like that. I just kinda get the feeling something is slowly drawing on the battery if it sits for 8 hours or more. Its only REALLY noticeable when its cold out. Seems like everything I'm reading in my FSM is stating replace the BCM or Junction block. I'm thinking about taking it into the stealership just possibly have them diagnose the issue then I bring it home to research and fix the issue.

Billwill · **Posted:** Sun Nov 10, 2013 4:30 am

Jeeper4life wrote:

Awhile back, my wife and I where working on it. What we did was, she held the multi meter on positive and negative sides of the battery to get a base reading. Then I went inside and pulled out each fuse one at a time to see if we could get a drop in voltage, then I would put it back in if it didn't drop and move on to the next one, and we got nothing..don't know if you have ever done something like that. I just kinda get the feeling something is slowly drawing on the battery if it sits for 8 hours or more. Its only REALLY noticeable when its cold out. Seems like everything I'm reading in my FSM is stating replace the BCM or Junction block. I'm thinking about taking it into the stealership just possibly have them diagnose the issue then I bring it home to research and fix the issue.

You do not diagnose a battery drain issue by putting the voltmeter across the terminals of the battery!!!

First thing to do is make sure that the battery connections are clean and tight, the negative lead going to chassis is clean and tight and the earthing strap going from engine to the chassis is clean and tight!

Then put your voltmeter over the battery terminals and with the engine OFF you expect to see at least about 12.5 volts.
As you start the engine you must not see the voltage across the battery go below about 9 volts...that would indicate a bad cell on the battery.

As the engine idles you expect to see the voltage across the battery sit at between 13.5 to 14.5 volts..if not then there is a charging issue.

Now to diagnose a good battery dropping its charge while standing.

Disconnect the negative lead of the battery. Set your MUltimeter to read "DC Amps"...you normally have to move the red lead to a different possition on the Multimeter marked "DC Amps...max 10 Amps"

Do not attempt to start the engine...you will blow the fuse in the Multimeter.

Connect the Black lead of the meter onto the negative terminal of the battery and the Red lead of the meter onto the negative battery connector that you have previously dis-connected.

With lights off, radio off etc. your meter should show no more current draw than about 40 milli amps (0.040 Amps)..put the meter on the lower Amp reading of about 1.0 A to get more accurate readings. If the current drain is in the order of about 1 Amp or greater then you have a problem. Now pull out fuses one-by-one to watch for a current drop.

Finally disconnect the thick lead that goes to the alternator and watch for a current drop....if a Diode in the Alternator has shorted out it will cause a backflow of current.

The PCMs and BCMs on these Jeeps are pretty robust...do not go replacing them until you have researched the problem further! :wink:

Jeeper4life · **Posted:** Sun Nov 10, 2013 8:24 am

cool I had forgotten someone told me to do it like the way your talking, when i get home from work tonight I will mess around with it! Thank you for help!

Bad bcm on an 04 kj?

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