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Matt Hardy
Matt Hardy, ASE Certified Technician
Category: Jeep
Satisfied Customers: 920
Experience:  ASE Master Auto Tech, ASE L1, Jeep/Chrysler Dealer Tech, 18 yrs exp
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1999 Jeep grand cherokee: oil pressure..voltage..check engine light

Customer Question

1999 Jeep grand cherokee. at start up, Gauges work temporarily then the tach konks out followed by the oil pressure, temp, voltage and fuel. of course i get the check gauges light and the fuel level warning light comes on. i even had the check engine light come on also. scanned the PCM and it had no trouble codes. any ideas???
Submitted: 7 years ago.
Category: Jeep
Expert:  Matt Hardy replied 7 years ago.

Hello, this sounds like a PCI bus problem. I have a diagnostic method for this. See next post.


Expert:  Matt Hardy replied 7 years ago.

PCI Bus Diagnostics for 1999-2004 (WJ) 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:


  1. 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.

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

  3. Power windows are inoperative.

  4. The overhead console displays 'lines' instead of information.

  5. 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.

  1. Set the meter to 'DC Volts'

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

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

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

  5. 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:

  1. PCM (engine computer)...engine compartment

  2. TCM (transmission computer)...engine compartment

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

  4. ABS (antilock brake computer)...engine compartment

  5. Instrument cluster

  6. Airbag computer...under center console/armrest

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

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

  9. Radio

  10. Audio amplifier...under back seat

  11. A/C control dash

  12. Overhead console computer

  13. 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

Customer: replied 7 years ago.
thats a pretty cool write up. but it really didn't get me too much farther.... i'll throw an accept for that and put it in a doc file for later.

check this out...

when the gauges go south, and i jiggle the door lock switch, the gauges come back to life. the door locks are operating intermittently but when i jiggle the switch, the gauges come around.... then they die and i can repeat the process. the gauges are now dying maybe one at a time or in pair or something as where before they were all going out at once...

am i onto something here...