jeep58

My jeep has the check gauge light coming on along with check engine light. I disconnect battery and reset the ecm and everything works next day the check gauge lights up again with the engine light on. When this is going on the a/c and the cooling fan motor will not work along with the power windows. Again disconnect battery and reset ecm all works. I have replace the ecm a year or so ago unfortunately I do not have the paper work with it anymore, Does this sound like the ECM is bad again? The one I replaced it with was a remanufactured unit, thanks for any input!!!

dave1123

Why did you replace the PCM (Powertrain Control Module) without doing any diagnostics! That's an expensive jump. Are you getting any trouble codes? Were there any gauges working or not working? There's over a quarter of a million miles on the one in my 2000 and it never gave a single fault and still hasn't.

Start by taking apart all the electrical connectors you can find and checking them for corroded or loose pins. Make sure none of the pins or sockets are pushed out the back of the connectors and they are clean. Check and clean all the ground connections as well. Pull the instrument cluster and check the plugs on the back. It's only held in by a spring clip bezel in front of it and 4 phillips screws.

Get any trouble codes and then come back. You can get them with a scanner or by the "key" method. Turn the ignition on until the warning lights come in, then off, on, off, on and wait. Any trouble codes will appear in the odometer one after another. Then "done" will appear. If there's no codes stored the problem was intermittent and has cleared itself.

jeep58

Thanks for the reply, I changed out the ecm the jeep would start and shut down, after new ecm everything was fine this was a year ago. I checked the charging 13.8 volts the battery less than a year old tested with a load tester it is ok. What connections should be looked at ? Why is this intermittent right now I just used the jeep and everything is good ie: cooling fan come on and off temp rides at about 212 or a little higher, oil pressure is about 45 psi hot, volt gauge reads just under 14 volts. As I said it will show check gauges with engine light illuminating then it will clear its self out!

jeep58

Oh also no codes pending or otherwise with scanner, I checked it with my scanner when the check gauge light was on !

Noah911

I have a decent guess..

It sounds like you could have an issue in the PCI Bus system to me? The PCI Bus is a one-wire communication system. It allows the various modules (computers) on the Jeep to communicate with each other and share information. PCI Bus problems are usually intermittent, and include symptoms such as what you have described; having certain systems not engaging - like the power windows being inoperative, the a/c, fan, and the check gauges/engine light. (Disconnecting the negative battery terminal reboots the system and may temporarily resolve the issues)

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 (located under the drivers side dash adjacent to the fuse box). The PCI Bus wire is in pin #2 of the data link connector.

You could backprobe and test the wire with a digital voltmeter. See if you find a problem? If you do this be careful not to damage the portion of the terminals that make contact with a scan tool.

I have some copy & paste about it for you:

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

End of copy & paste

If someone else reads and thinks this sounds plausible as well.. they may comment on it?

dave1123

You go, Noah! That's an excellent explanation and even I understand it, and that's saying something! I never had to mess with the PCI Bus, but I understood how it worked. With minimum voltages in that range, grounds are super important. I was aware of the violet tracer being the bus, but not specific voltages or resistances. These vehicles being almost 20 years old, anything could be wrong with that wire. I know the PCM constantly monitors the bus, checking in with her kids, so it speak and sends and receives data from them. All on that one wire! One time when I plugged in my scanner is said "no communication with system." I pulled it out, blew on the plug, and reinserted it. Bingo! it worked. So much for keeping it under the seat. every now and then I spray it with electrical cleaner.

jeep58

Ok Sounds like you know what's going on I will try and check what I can it is getting a bit complicated for me, PCI BUS is the culprit ! Maybe bad ground somewhere, violet tracer wire starting to get hairy thanks for the explanation!

jeep58

Come to think of it the radio that the kids put in has an amp under the seat, when the radio is on you can hear a humming coming from the speakers as you drive goes with the rpms, maybe a clue here?

jeep58

So where would pin #2 located on data link connector on jeep , there are two rows on connector? Thanks

Noah911

Does this help?

jeep58

Thank You!

jeep58

Ok so the check gauge light comes on and off, I checked pin number 2 and got 3.35 volts, with engine running but the check gauge light was out when I did this. So I start to unplug modules and keep checking pin 2 for readings?

Noah911

That's right. One-by-one.. If it is consistently greater than 2.5V, that is what you should be doing. Checking the voltage reading after every time you disconnect a module. Once you have the corrupt module unplugged, you should see a voltage change - bringing it back down within the normal range of 0.5V - 2.5V to that wire, like it should be normally.

jeep58

Thanks Again!

Noah911

You're welcome sir.

I am curious to know the end results of testing, and your final outcome? It sounds like you are on the right path to figuring out what is causing the issues. Would you mind posting an update for me when you have it?

I will be checking and following to see your progress. Let the forum know if you have any questions, or develop further concerns as you go forward.