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I have a 2000 JGC 4.7 V8 and the instrument cluster dies 30 seconds after start and all the warning lights come on. The first light to show is the key with the circle followed by others. It starts and runs fine and has never stalled. I've checked door wiring and changed PCM but problem remains. If I unplug the alternator the cluster works and all is well. When measuring pin 2 voltage it is constantly fluctuating quickly between 3.8 and 5v before the gauges die and everything works. As I am watching it; the voltage jumps to 7.30v after about 45 seconds and stays there (no fluctuating) and it is at that time that my gauges die and I hear the door modules click and the windows and locks no longer work. If I unplug the instrument cluster at this point; the voltage begins fluctuating again (in the same range) but the windows and locks start back working normally. Given this; I ordered another cluster and installed it. Initially everything worked well for a day, but it is now back to what is was doing with the old cluster. I thought it was the alternator originally and installed a new one so the alternator is new. Thank you in advance for any help!
 

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The beast from Brazil
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If you disconnect the alternator now, does the instrument cluster come back and stays working ok? What is the battery voltage with the engine running?
The bus voltage seems to be high all the time, since normal bus voltage is arround 2V and fluctuating.
 

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If you disconnect the alternator now, does the instrument cluster come back and stays working ok? What is the battery voltage with the engine running?
The bus voltage seems to be high all the time, since normal bus voltage is arround 2V and fluctuating.
I just checked and the battery cold (hadn't started it all day) was at 11.64v. when it was running; it went up to 14.03v. I disconnected the alternator while it was running and the cluster did not come back. Previously with the old cluster I would turn the Jeep off; unplug the alternator and the cluster would come back - I never unplugged it while running. I tried it both ways now and it didn't bring the cluster back.
 

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The beast from Brazil
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The battery is or in a bad state, or it is not charged.
A fully charged battery should show 12.7V (half an hour after charging), a discharged battery will be around 12.2V.
Not telling that this will solve the whole problem, but its not ok.
Your jeep has a diagnostic disconnect plug mounted against the under side of the steering column under the dash. If you remove the shorting cap, you can disconnect most of the components from the data bus. PCM, TCM, passenger door module, SKIM and CD player (if equipped) are still connected. Now check what the bus voltage does. Now you will have to connect each component one by one to the bus via the diagnostic plug, until the fault appears. I don´t have the right manual with me now, but later on i can give you some schematics with the numbers in the diagnostic plug, so that you know which component(s) you will be connecting to the bus. If after connecting a component to the bus the fault appears, you have found the problem.
 

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Discussion Starter #5
The battery is or in a bad state, or it is not charged.
A fully charged battery should show 12.7V (half an hour after charging), a discharged battery will be around 12.2V.
Not telling that this will solve the whole problem, but its not ok.
Your jeep had a diagnostic disconnect plug mounted against the under side of the steering column under the dash. If you remove the shorting cap, you can disconnect most of the components from the data bus. PCM, TCM, passenger door module, SKIM and CD player (if equipped) are still connected. Now check what the bus voltage does. Now you will have to connect each component one by one to the bus via the diagnostic plug, until the fault appears. I don´t have the right manual with me now, but later on i can give you some schematics with the numbers in the diagnostic plug, so that you know which component(s) you will be connecting to the bus. If after connecting a component to the bus the fault appears, you have found the problem.
The battery is less than 6 months old. I will charge it tonight. Once I remove the shorting cap; how will I connect each module one at a time back to the bus via the plug? I have the schematic with the numbers. The cap connects them all together at once right? Also; will it start if I disconnected the SKIM? I am assuming that if the voltage is still high; it is one of the components that you mentioned.
 

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The beast from Brazil
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The battery is less than 6 months old. I will charge it tonight. Once I remove the shorting cap; how will I connect each module one at a time back to the bus via the plug? I have the schematic with the numbers. The cap connects them all together at once right? Also; will it start if I disconnected the SKIM? I am assuming that if the voltage is still high; it is one of the components that you mentioned.
Just use a short jumper wire and connect it between the two points you want to connect together. Indeed, the shorting cap connects all the pins together, making it one bus. The SKIM is permanently connected to the bus, at least, you can't disconnect it via the diagnostic port. You can ofcourse remove the SKIM connector to disconnect it from the bus, but then the engine will not run anymore. (only for three seconds it will run, but then shut down without the valid key signal from the SKIM). I would just remove the shorting cap and start the engine. Then only the PCM, SKIM, TCM, passenger door module and CD changer are connected to the bus. If the voltage stays below 2.5V, then those components are not faulty. Now connect one other component to the bus and see what happens to the bus voltage. repeat this with each component to find the faulty one. Pin 1 in the diagnostic port is connected to the PCM, so that pin must be used to connect to the others.
 

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Discussion Starter #7
The battery is less than 6 months old. I will charge it tonight. Once I remove the shorting cap; how will I connect each module one at a time back to the bus via the plug? I have the schematic with the numbers. The cap connects them all together at once right? Also; will it start if I disconnected the SKIM? I am assuming that if the voltage is still high; it is one of the components that you mentioned.
Just use a short jumper wire and connect it between the two points you want to connect together. Indeed, the shorting cap connects all the pins together, making it one bus. The SKIM is permanently connected to the bus, at least, you can't disconnect it via the diagnostic port. You can ofcourse remove the SKIM connector to disconnect it from the bus, but then the engine will not run anymore. (only for three seconds it will run, but then shut down without the valid key signal from the SKIM). I would just remove the shorting cap and start
the engine. Then only the PCM, SKIM, TCM, passenger door module and CD changer are connected to the bus. If the voltage stays below 2.5V, then those components are not faulty. Now connect one other component to the bus and see what happens to the bus voltage. repeat this with each component to find the faulty one.
Ok so just so I am clear. Am I still measuring measuring voltage from pin 2 of the data link connector once I've removed the shorting cap? Also; should I use the jumper wire to connect one component to a prong on the shorting cap? I understand the concept, but I just want to be clear about exactly how to connect each component one at a time.
 

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The beast from Brazil
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With the shorting cap removed, the data connector is also disconnected from the bus. So you should measure on pin 1, so that you see the bus voltage from the PCM and other connected components.
The shorting cap only serves to short all pins from the diagnostic connector together. So to connect any component to the PCM, you put one side of the jumper wire into pin 1 from the diagnostic connector and the other end into the pin number from the component you want to connect to the bus (=PCM). So you said you have the schematics with the diagnostic connector pin out, right?
 

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Discussion Starter #9
With the shorting cap removed, the data connector is also disconnected from the bus. So you should measure on pin 1, so that you see the bus voltage from the PCM and other connected components.
The shorting cap only serves to short all pins from the diagnostic connector together. So to connect any component to the PCM, you put one side of the jumper wire into pin 1 from the diagnostic connector and the other end into the pin number from the component you want to connect to the bus (=PCM). So you said you have the schematics with the diagnostic connector pin out, right?
Ok that makes sense now. Yes; I have the pinout. Strange thing is I went out to move it back to the garage and the battery was dead. That's strange because the battery was fine (12.44v cold) earlier this week before I put in the new cluster. The temp did drop here, but I didn't think it was enough to bother the battery at all. It's practically a new battery.
 

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The beast from Brazil
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Ok that makes sense now. Yes; I have the pinout. Strange thing is I went out to move it back to the garage and the battery was dead. That's strange because the battery was fine (12.44v cold) earlier this week before I put in the new cluster. The temp did drop here, but I didn't think it was enough to bother the battery at all. It's practically a new battery.
It being a new battery is not a guarantee that it is ok. I would charge it and check the voltage half an hour after charging, it should read 12.7V
If this is ok, it is possible that some component drains the battery, even with ignition off.
 

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Ok that makes sense now. Yes; I have the pinout. Strange thing is I went out to move it back to the garage and the battery was dead. That's strange because the battery was fine (12.44v cold) earlier this week before I put in the new cluster. The temp did drop here, but I didn't think it was enough to bother the battery at all. It's practically a new battery.
It being a new battery is not a guarantee that it is ok. I would charge it and check the voltage half an hour after charging, it should read 12.7V
If this is ok, it is possible that some component drains the battery, even with ignition off.
As of last night after fully charging the battery; the PCI voltage gradually started running high (3.2 - 5v), but I still had all communication and everything was working properly. Will I still be able to troubleshoot the diagnostic junction port?
 

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The beast from Brazil
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As of last night after fully charging the battery; the PCI voltage gradually started running high (3.2 - 5v), but I still had all communication and everything was working properly. Will I still be able to troubleshoot the diagnostic junction port?
Able to troubleshoot yes, but it will not be that clear when you have found the faulty component. The bus voltage, measured with a digital volt meter, should normally not going over 2.5V. If it does, some component is causing this. It is also possible that a short between 12V and the bus wire exists inside a connector. Many times this happens inside connector C200 in the passenger footwell area. Water ingress due to a leak (via windshield seal or antenna wire) can cause corrosion inside the connector, which could cause a (partly) short and increase the bus voltage.
 

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As of last night after fully charging the battery; the PCI voltage gradually started running high (3.2 - 5v), but I still had all communication and everything was working properly. Will I still be able to troubleshoot the diagnostic junction port?
Able to troubleshoot yes, but it will not be that clear when you have found the faulty component. The bus voltage, measured with a digital volt meter, should normally not going over 2.5V. If it does, some component is causing this. It is also possible that a short between 12V and the bus wire exists inside a connector. Many times this happens inside connector C200 in the passenger footwell area. Water ingress due to a leak (via windshield seal or antenna wire) can cause corrosion inside the connector, which could cause a (partly) short and increase the bus voltage.
I haven't been able to check C200 yet, but I did take the cap off of the data juntion Port and my voltage was still high on PC bus. I disconnected the PDM and TCM, but there was no change. When I unpluggrd the alternator; the voltage stayed below 2.5v. Is there anyway to to test the SKIM and/or PCM? What am I missing? I also noticed that the backlight on my instrument cluster stayed on last night without the key in the ignition. The only way I could get it to turn off was to turn the key to "on" position and then back off.
 

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The beast from Brazil
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So you removed the shorting cap from the diagnostic juntion port and the bus voltage on pin 1 of the diagnostic port was still high? At least you have removed a large part of possibilities now.
Did you remove the small connector from the alternator, which feeds the alternator field? Trying to find a common point were the voltage can jump over.
The SKIM can be ruled out by disconnecting its connector. You could do the same with the PCM. If then the bus voltage is still high, there must be some connector having a short internally.
Edit: The only connector i could find in the alternator field circuit and in the PCM PCI bus circuit is C103. This connector sits just above the PCM. It is a 14 pin connector with a gray colour. C102 sits in the same area and has 14 pins, but has a black colour. I would check this connector on both sides and eventually use contact cleaner to remove any corrosion and/or moisture.
 

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Discussion Starter #15
So you removed the shorting cap from the diagnostic juntion port and the bus voltage on pin 1 of the diagnostic port was still high? At least you have removed a large part of possibilities now.
Did you remove the small connector from the alternator, which feeds the alternator field? Trying to find a common point were the voltage can jump over.
The SKIM can be ruled out by disconnecting its connector. You could do the same with the PCM. If then the bus voltage is still high, there must be some connector having a short internally.
Edit: The only connector i could find in the alternator field circuit and in the PCM PCI bus circuit is C103. This connector sits just above the PCM. It is a 14 pin connector with a gray colour. C102 sits in the same area and has 14 pins, but has a black colour. I would check this connector on both sides and eventually use contact cleaner to remove any corrosion and/or moisture.
Yes; I removed the small connector from the alternator and used pin 1 to test the voltage. The voltage was consistently fluctuating between 3 and 4.5 volts. It didn't bring the gauges back; but the voltage went down. Do you think it's necessary to test the SKIM? I don't know how to check it if the Jeep will stall with it being disconnected. Also; wouldn't it shut down if I disconnected the PCM? I've tried to avoid disconnecting the battery because everything resets and makes it extremely difficult to troubleshoot. Is it ok to leave it connected while I check and clean these connectors?
 

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The beast from Brazil
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For now i would first check the C103 connector, before doing anything else. It is easily accessable and as far as i could find the only spot were both the bus wire and alternator field wire pass through. Disconnecting the SKIM after the engine runs, should not stall the engine, however you never know if disconnecting the connector during operation would cause a voltage spike and fry some electronic component.
Disconnecting any of the PCM connectors would cause the engine to stall and is not a good idea to do, it will most probably fry the PCM.
 

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For now i would first check the C103 connector, before doing anything else. It is easily accessable and as far as i could find the only spot were both the bus wire and alternator field wire pass through. Disconnecting the SKIM after the engine runs, should not stall the engine, however you never know if disconnecting the connector during operation would cause a voltage spike and fry some electronic component.
Disconnecting any of the PCM connectors would cause the engine to stall and is not a good idea to do, it will most probably fry the PCM.
Ok. I will check the C103 connector and clean it. Do I have to disconnect the battery to do it? Trying to avoid resetting everything.
 

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The beast from Brazil
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Ok. I will check the C103 connector and clean it. Do I have to disconnect the battery to do it? Trying to avoid resetting everything.
No, if the ignition is switched off, you can disconnect the C103 connector without disconnecting the battery.
 

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Ok. I will check the C103 connector and clean it. Do I have to disconnect the battery to do it? Trying to avoid resetting everything.
No, if the ignition is switched off, you can disconnect the C103 connector without disconnecting the battery.
Ok. I will update later tonight.
I cleaned the connector and have the same issue. I did notice something tonight though that has me baffled. The voltage on the diagnostic junction port pin1 is fluctuating but still high (above 2.5v). However; when I put the cap back on and measure the voltage on pin2 of the data link connector; the voltage is static at 7.3v. I checked and rechecked by moving between the two while the engine is running as well as turning it off and checking the voltage on each individually and making sure to turn the engine off in between moving from pin1 to pin2. Is there something on the PCI bus going to the data link connector that is not going through the data junction port? I can't figure out why there is a discrepancy....
 
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