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Unread 03-11-2009, 07:15 AM   #1
svoss
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ECM, PCM, Electrical mystery

If i did not have to drive the jeep, reading and learning from the forum would almost be fun, there are alot of really smart and experienced guys (and gals i guess) that put some time into this to help out the rest of us.

1993 2.5L manual YJ, battery and alternator have been tested and are OK. SYmptoms are what i would expect from any vehicle with a bad alternator and subsequent discharged battery. For now, i can put a charger on it over night and drive it the next day. I have read on these jeeps the charging and voltage regulation is not all in the alternator. there is a link over to the computer, somepeople call it ECM some PCM. I don't know but i know its on the firewall behind the washer fluid reservoir. I've also read about a fusable link that may be involved.

I've read about the capacitor replacement and digging into your ECM (i would do this if i was sure that would help) the problems described in that post are different than i'm having, my jeep starts and runs, it just don't charge the battery.

I have had the alternator and battery out twice now and took to be tested, i took the vehicle up to the auto parts store and had them run their quickie tests. I really don't want to take it to a jeep dealer service. I'm hoping for some education and ideas. thanks in advance.

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Unread 03-11-2009, 07:54 AM   #2
TSEJEEPERS
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I would start with the cables. Are they corroded at all?
I am not sure if there is a Maxi fuse for the charging system or not.
You can lift the lid on the fuse box under the hood. The top has what is what marked on the inside of it.
Do you have anything that may be draining the battery? Aftermarket stereo lights ect?
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Unread 03-11-2009, 07:59 AM   #3
Old4X
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Check all your fuses with a meter. They can look good, but still be bad.
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Unread 03-11-2009, 08:08 AM   #4
Que89YJ
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Not charging

Check cables and grounds, check fuses. Both are great advice.
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Unread 03-11-2009, 08:22 AM   #5
letitloose06
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Your Jeep should have two, 50 amp maxi fuses for the charging system in the Power Distribution Center located in front of the battery.
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Unread 03-11-2009, 08:30 AM   #6
idaholtby
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Quote:
Originally Posted by letitloose06 View Post
Your Jeep should have two, 50 amp maxi fuses for the charging system in the Power Distribution Center located in front of the battery.
^^X2^^
The 50a maxi-fuses are F2 and F6 in the PDC.
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Unread 03-11-2009, 10:07 AM   #7
sentinal02
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easiest way to tell if your alt is getting to the battery, start the jeep, and set your volt meter to read 15 volts, or whatever your closest range is to that, and then put it across the battery terminals. if you're reading anything above 13 volts, change the battery because the alt is putting out and it's getting to the terminals. if not, touch the positive lead to the alt's output terminal and the neg to the alt's case. if you get above 13 there, then your alt is putting out, but not getting to the battery, check your fuses and the cables. if you're not getting power there, then it's either the alt or the field control from the computer.
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Unread 03-12-2009, 06:52 AM   #8
svoss
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Update on advice

pretty much ruled out cables, ground connections, fuses. I cleaned terminals, pulled ground connections to the block and firewall and cleaned those. The voltage at the battery is below 10 when running, after i shut it off and wait a little the battery recovers to 12 v. I put voltmeter on back of alternator and read below 10 v. I opened the fuse box in front of the battery and checked for continuity across each fuse, no problems there. I have pulled the alternator and had it checked twice, they are telling my at the store its fine and that i have a computer problem.
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Unread 03-12-2009, 07:28 AM   #9
Que89YJ
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alternator

It is control and output. If you have low voltage out the you need to check the input. The field control for the alt. is ground switched. It is the tn/blk wire going from pin 20 on the PCM to the alternator. Be careful when you try this next step. You need to check the alt. field by full fielding the alt.; This will force the alternator to max output but if it is also very bad for the electrical system if it is left like this for an extended period. Hook up your multimeter to the battery to monitor the voltage. Splice a short jumper to the tan/blk alternator input at the pcm on the same control line. Take the other side of the wire and ground it to the battery and monitor the voltage with your meter. You should see system voltage jump to a range of 16-18 Volts. If you see the voltage jump then you have a bad voltage regulator in the PCM. If the voltage does not increase then check continuity on the Tn/Blk wire from the PCM to the Alt. and the black wire off the alt. If the alternator is good as you have said then this should tell you what is wrong. (The alternator control could be Dark Green, my memory says something different then the wiring diagram).

Last edited by Que89YJ; 03-12-2009 at 08:46 AM..
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Unread 03-12-2009, 11:25 AM   #10
svoss
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Alternator Schematic?

as i held the alternator in my hand looking at all the electrical connections with them positioned from about 9 o'clock to 2 o'clock starting at the 9 o'clock connection.

9 position medium size wire
11 and 12 o'clock two tiny wires that feed into a black plastic looking connector.
1 o'clock is another medium size wire
2 o'clock is the big wire off of the battery positive.

I understand your warning about not wanting to do this "full fielding" test for long.

I want to make sure i understand what you are describing. splicing a short jumper from pin 20 to the battery is going to be a 5 ft stretch of wire from one side of the firewall to the other to go from the computer to the battery. If that is what you mean, what is a good way to do that with the PCM wire bundle screwed in place? Is that what you mean or do you mean putting a jumper from the appropriate terminal on the alternator (that heads over to Pin 20) from there to the negative on the battery?

Sorry i did not look at the colors of each wire but will. Is there a good way to figure out which pin is pin 20? I will try to take some pictures and get a good look at the wire colors tonight. I do not know how to add pictures to a message but i could email them to you direct?
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Unread 03-12-2009, 01:12 PM   #11
laybackman
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Yes the PCM or ECM is the 'brain'
Let's take step back a bit here.
You have no CEL light on and no stored codes.
You have a good ECM.
You have a good alternator.
You have a good battery.

If that is true then you have to have a bad wiring issue.

How did you check the battery cables? If they are original I'll bet the are completely corroded underneath the insulation. They will be green and not allow the alt to charge the battery. Add a jumper cable to the - side of the battery and got to good ground. Take the other battery cable and attache it to the + side of the battery and follow it back to the alt. Carefully place that cable on the post on the back of the alternator. You have just piggy backed the cables on your battery. What happened?
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Unread 03-12-2009, 04:20 PM   #12
svoss
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Trying to Clarify

The CEL comes on and throws some codes 12, 33, 41, 55

From what i recall, one code means the battery was disconnected, one means something about AC (which i do not have), one was someting about voltage or alternator field circuit, one means end of codes.
I never said i did not have codes, what i do not have is the CEL problem and symptoms that others described where the engine won't start or dies and that leads into the ECM capacitor replacement thread. In my original post i said all previous work was pointing toward a computer problem and i was looking for advice.

No I have not cut into the cables to see if they are completely gone. If i put a voltmeter on the big post on the back of the alternator then to ground its not showing 14 volts, more like 9v. Based on all i've read from sources here and the autoparts store, that points to a computer problem. If i can put jumper cables from my truck to the jeep battery and it fires it up and shows 14 volts at the battery, there i have put a properly operating charging system in parallel with the jeeps, that to me would rule out bad cables on the jeep because i am still using the jeep cables to transfer power to the starter and to ground.

the latest idea sounds like it will be adding a jumper cable in parallel from the alternator to the battery. If i test from that post on the alternator to ground and read 9.6volts, i'm not following how that will show me anything different.

Sorry for any confusion, maybe this message will help clear things up.
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Unread 03-12-2009, 06:55 PM   #13
laybackman
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There are two a 50AMP fuses in the PDC that protects the battery and starter. If the starter works then they must be OK.
The alternator field control circuit is inside the alternator. Code 41 is telling you it isn't good. Now the plug on the back of the alt has three wires including a ground wire which must be in place.

If you are only getting 9 Volts out of the alternator that tells me that the ECM signal is good but the alternator can only manage to produce 9 volts. The signal from the ECM is not on a sliding scale it is either on or off in my opinion.
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Unread 03-12-2009, 07:05 PM   #14
svoss
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so are you thinking the alternator is bad? autoparts store has tested it twice and says its fine but i have a computer problem. I removed the computer just now and took it apart, the gel is preventing me from doing anything other than looking. i did figure out the numbering and see pin 20 it is a dark green wire. what do i need to do to tell the auto parts store to give me a new alternator. or is there another test like someone posted i should try. thanks
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Unread 03-12-2009, 07:19 PM   #15
svoss
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I temporarily plugged the computer back in, hooked up the battery and started it up. the CEL comes on immediately, no delay, the voltage across the battery is 9.89Volts, the voltage from the big wire on the alternator to ground is 9.89 v. the medium sized terminal ohms zero to ground on the alternator. the others are hard to get to with the engine running.
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