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BUS repair instructions

BUS instructions:
http://forums.cnet.com/7726-7811_102-3252337.html

Car Tech forum: The Famous Jeep Grand Cherokee Electrical Problem
by: trevtaylor November 16, 2007 3:05 PM PST
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Grand Cherokee 2000 Electrical Problems
by jeepkid1 - 2/20/10 11:21 PM
In Reply to: Grand Cherokee Electrical Problems by frabuk

Hi,Frabuk. Sorry. My info might have been too long. I'll try this again, these are solutions from an "expert" with Jeep.

How to Fix the Famous Jeep Grand Cherokee Electrical Problem

PCI Bus Diagnostics for 1999-2004 (WJ) Jeep Grand Cherokee

PCI Bus Diagnostics for 1999 Jeep Grand Cherokee

PCI Bus Diagnostics for 2000 Jeep Grand Cherokee

PCI Bus Diagnostics for 2001 Jeep Grand Cherokee

PCI Bus Diagnostics for 2002 Jeep Grand Cherokee

PCI Bus Diagnostics for 2003 Jeep Grand Cherokee

PCI Bus Diagnostics for 2004 Jeep Grand Cherokee

The PCI Bus system is a one-wire communication system. It allows the various modules (computers) on the vehicle to communicate with each other and share information. This information sharing reduces wiring complexity.

The PCI Bus wire is either yellow with a violet tracer, or white with a violet tracer, depending on year and model. The easiest place to access this wire is at the data link connector, which is the connector that the scan tool plugs into for diagnosis. On LHD North American Jeeps, the data link connector is located under the driver side of the dash, adjacent to the fuse block. It is easily visible without removing any panels.

The PCI Bus wire is in pin #2 of the data link connector. Normally I remove the connector from it's bracket and pull it down for better access. This allows me to backprobe the connector with my voltmeter. You do not want to damage the portion of the terminals which make contact with the scan tool connector.

Here are the symptoms you are probably experiencing with a PCI Bus issue:

The A/C blows normal volume but the air is not cold, because the compressor will not engage, even with the 'snowflake' or 'A/C' button pressed.

The instrument cluster backlighting is ON at all times (when the key is on), even with the headlamps OFF.

Power windows are inoperative.

The overhead console displays 'lines' instead of information.

The gauges are not reading correctly.

These symptoms may come and go. Often PCI bus problems are intermittent.

The PCI Bus system operates on a 0 to 7.5 volt square wave. If you were to connect an oscilloscope to pin 2 of the data link connector, on a system with no problems, the oscilloscope would display a nice square wave. Most people do not have a $2000 scope, so I have devised an easier method. You will need a digital voltmeter to continue.

Set the meter to 'DC Volts'

Connect the red lead of the meter to pin 2 of the data link connector

Connect the black lead of the meter to ground. Any bare metal surface under the dash will work.

Turn the key to 'ON'. Or, start the engine. Sometimes the PCI Bus problem is easier to duplicate with the engine running.

Observe the volt reading on the meter.

If the volt reading is zero, or very close to zero, it is possible that the PCI bus is OPEN, or SHORTED TO GROUND. A different test will be required, which is detailed later in this article.

If the volt reading is close to 12 volts, or battery voltage, the PCI Bus is shorted to voltage, and diagnosis should be fairly easy.

If the volt reading is varying between 0.5 and 2.5 volts, the fault is not present at this time. Verify this by inspecting the operation of the windows and other symptoms.

Normally what I see on problem vehicles is a reading of 4 to 6 volts. This indicates that a module (one of the vehicle computers) is pulling the bus voltage too high, causing interference and a loss of communication between modules, which leads to the symptoms you are experiencing.

If the PCI Bus voltage reading is consistently above 2.5 volts, we need to determine which module is causing the problem. There is no quick way to do this. Each module on the bus must be unplugged until the voltage returns to the 0.5 to 2.5 and fluctuating range.

Module examples are:

PCM (engine computer)...engine compartment

TCM (transmission computer)...engine compartment

BCM (body computer)...under dash near fuse block

ABS (antilock brake computer)...engine compartment

Instrument cluster

Airbag computer...under center console/armrest

Driver door module (the window switch assembly)...remove door panel to access

Passenger door module (the window switch assembly)...remove door panel to access

Radio

Audio amplifier...under back seat

A/C control head...in dash

Overhead console computer

Immobilizer module...remove steering column covers to access

I begin with the interior modules, unplugging the ones that are easiest to get to. Continue to unplug modules until the bus voltage lowers to a normal level; 0.5 to 2.5 volts and fluctuating up/down randomly. Keep in mind that unplugging some modules will cause other symptoms...try to ignore those for now and focus on the voltage reading.

Once you find a suspect module, reconnect everything else and observe the bus voltage. Reconnect the suspect module and try to duplicate the symptoms. Verify that the suspect module is actually the problem several times before you spend money on a replacement part. Be sure to cycle the ignition key every once in a while during testing. The trick here is to be able to consistently verify that the bus problem is present, then verify that it is not present when the suspect module is unplugged. Try to not let the system 'fool' you.

If the bus voltage is around 12 volts, or is the same as battery voltage, and ALL modules are unplugged, then the bus wiring is shorted to voltage. This problem may not be easy to find. You will have to trace the bus wire extensively. The good news is that this scenario is very unlikely.

If the bus voltage is always near 0 volts, a different method will work.

Disconnect the vehicle battery under the hood.

Connect your meter the same way, but switch to the 'ohms' setting, to check the resistance of the bus. A bus that is shorted to ground will have a very low resistance, possibly below 10 ohms. In a situation where the bus is shorted to ground, a module could be at fault, but more likely the bus wire has rubbed through somewhere and is touching bare metal. Keep in mind that the whole frame and body of the vehicle is ground, so that makes this scenario more likely.

If the ohm reading is 'OL' meaning infinite resistance, then the bus wire is OPEN and must be traced out for continuity. However...if every module is unplugged, and the bus wiring is OK, the meter will display 'OL' for bus resistance.

Read the following information, which is from Chrysler:

Measuring PCI voltage is the first place to go if no communication is possible with any modules. This tells what type of failure has occurred and this dictates the next step. If communication is possible with only one module, an open in the bus is likely.PCI bus vehicle are very easy to diagnose bus problems on. Since each module has a termination resistor of a standard value a measurement of bus resistance (with the battery disconnected) gives a very good idea of the condition of the bus. For example: Termination resistance 350 to 750 ohms would be a normal bus. 0 ohms would indicated a shorted bus OL would indicate an open bus 3300 ohms would be one dominant module only 10800 ohms would be one non-dominant module only
 

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The data on the bus is in the form of pulses. Digital. So the voltage will fluctuate within a range.As suggested o.5v to 7.5v when read with a digital voltmeter. An analog meter will pull down the signal.
 

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Car5Car, great write-up. Thank you.
My 2003 Wrangler (4.0, AT Sahara) starts and runs fine, but none of my gauges work. I even bought another cluster at the junk yard to test, same result.
I tested the harness behind the cluster for B+ and ignition power, and checked the ground connections.
I have very low voltage 0.08-0.25V on the bus wire. I tried disconnecting the battery, every module and tested the VT/YL wire for continuity and to ground. The wire is good.
I tested the bus voltage at every module while the engine is running, disconnecting the module and back in and tested the bus wire. Same voltage everywhere.
Any recommendation would be appreciated.
Thanks
 

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this is a great article, a little OLD and attaching a new topic may go unread by others .
Maybe posting in TJ WRANGLER TECH.

Any way did you check continuity (engine OFF) from PCM to ICM ? OR PCM to DLC?
Did you SCAN for codes?
 

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Thanks for the quick reply. I looked all over and searched the service manual for ICM you referred to. I assume it's ignition control module. According to the manual, my 2003 Sahara has the following bus-enabled modules. PCM, TCM, sentry, cluster, radio, aribag and DLC. I checked for continuity for all of these modules. I scanned for codes and getting P0888. I installed a new transmission control relay, checked voltage and ground at the relay connections. Checked continuity to the PCM for the transmission control relay circuit. I bought a new PCM from eBay and the seller programmed it to my VIN. The engine does start and runs great. Transmission shifts smoothly through the gears, but for the life of me, I can't get the cluster gauges to work. I don't want to buy more modules so I think I'll have to bite the bullet and take it to a shop to have them hook it up to DRBIII scanner.
 

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My bad I get caught in acronym madness, different years and models*.... With ICM in this case I was thinking INSTRUMENT control module,

How is continuity from PCM (C3 30) to cluster (C2 12) ?
Did you do the instrument cluster actuator test?

Do you still have P0888?
YES - RELAY stuck on - is there a REMOTE starter... AKA POS added to jeep?



* add EMIC to the alphabet soup ElectroMechanical Instrument Cluster.
 

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PCM (C3 30) to cluster (C2 12) wire is good. No loss and not grounded. The P0888 is still there. I installed a new transmission control relay, checked voltage and ground at the relay connections. The weird thing is that with the P0888 code the transmission has issues, either no shifting or hard shifting. Mine shifts normal and runs and drives smooth.
 

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2001 Wrangler Same Problem

Hoping you guys can help as part of this thread:

Having same issue as others - jeep runs great but gauges do not work (never had since I got from PO). There is an aftermarket radio installed but looks like all of the connections are correct.

Replaced PCM (different reason - while not a california model it had a california PCM which means someone screwed something up in the past)
Tried different cluster
ODBII scan shows no codes and MIL off (although its not)
Instrument Cluster test gave: b0, b8, and Panel Open messages.
Checked the Multifunction Switch (the Panel Open code) and appears to be fine

Weird thing noticed is that SKIM connector is not plugged into anything (which also has that PCI BUS wire as part of the circuit) and there is no SKIM in this vehicle as far as I can tell (took a look at key assembly and no ring or sensor present).

I did the VDC test at data link connector (pin #2) and was getting low numbers - less than 0.1

Went on to check the #2 PCI BUS pin at the data link connector as suggested for OHMS with negative battery off - getting reading of about 1.24 to 2 ohms.

2 questions:
1) Could this be because the SKIM connector is not connected as part of the circuit and if so can I jump the Violet/Yellow wire from another module to it to complete the circuit?
2) Any suggestions how to find that ground issue?
 

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You said you replaced the PCM. You may need to expand on that a bit. Are the part numbers exactly match? Was the PCM programmed to your VIN?
In my experience, I ordered a PCM with the exact same part number from eBay. The seller offered a service to program my PCM to my VIN number which I did want them to do.
Once I installed the PCM I had the same issues as you. After weeks of troubleshooting and testing I ended up buying another PCM programmed that to the VIN and my gauges started to work.
When I was testing I disconnected every module that were BUS enabled and I was still getting very low voltage on the BUS circuit at the PCM so that's when I decided to get another PCM.
Good luck, I hope this helps.
 

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PCM was replaced by a repair shop out here - took it in because could not figure out the O2 problem and thats what they found. They used a shop out of Tennessee that uses rebuilt or used PCMs and program to the VIN. The first one they installed they thought might be broken because they couldn't get the gauges to work so they sent it back, got another one, same problem, and then ended up spending hours on the phone with them trying to trouble shoot. At that point (which was like a month) I said I'd just find the problem myself.
 

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If you disconnect all the BUS modules (airbag, radio, TCM, cluster, etc.) what voltage do you get on the buswire at the PCM?
 

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I'm interested to know the answer to that question too?

I would assume it should be .5-2.5 volts oscillating with a multimeter because I believe the PCI Bus is wired in parallel...
I'm trying to rule out a bad PCM.
It's connector C-3 Pin C30 on my PCM.
No Gauges and No communication from the Skim to PCM P1686
02 Wrangler 4.0 Manual
 

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Yeah it does. I got 1,100 ohms between pin2 and pin4 at the data link connector with an ABS module, skim, cluster, and airbag, but I'm dealing with a PCI Buss problem too. That's why I am curious about that voltage with everything disconnected.
 
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