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Hi there, I am experiencing some trouble with my head unit, here are the details:

basic info:
- moved from Ontario to Alberta, nav was functional in ON, did not have map disk for AB, so did not bother trying to use till recently when I purchased the proper disk.
- nav is PCM 2.0 software 3.0, all other functions are normal
- after startup, get strange error: Unknown, unknown... image is attached
- info < shows no satellites detected
- date incorrect
- nav slow to startup
- when map starts, compass says north, points downward in display rather than upward as it usually does, does not rotate map as vehicle changes position
- current position in long / lat. is fixed and incorrect - somewhere in the middle of the ocean
- map will eventually come up, but vehicle position is fixed and never changes
- previously, when selecting AV, you would get a "not present" message, now you get a blank screen
attempted fix:
- removed head unit, checked all connections, particularly GPS
- removed spoiler, checked and replaced GPS antenna (fairly corroded), checked ground connection
- took to dealer to run diagnostics, they read error codes of 8005, 8006, and 8007, and provided me with a complete diagnostic printout, so I should hopefully be able to provide additional details if needed.
Any help would be appreciated.


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Good suggestions. I am pretty sure I have tried a rest, but I will give it a go again and post back the results just to be thorough. As for MOST / optic cable: where is the MOST module located - perhaps I can check the terminal ends of the optic cable first, then try just running an optic cable of my own through the cabin to rule a cable fault out.

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Most is not really a module, it is a kind of a light (sender) and a light sensitive sensor (receiver) both housed inside the PCM unit. In other words, the system begins and ends inside the PCM.

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As RFM says, the MOST (fibre optic) loop starts at the PCM, and connects to the optional equipment such as navigation drive, telephone module, amplifier, CD changer, etc. The fibre optic cables are coated orange, and transmit data and control signals from one device to the next in a ring, returning to the PCM unit. The two orange cables go in to black connectors.

If there is a break in the loop, it will not be able to communicate with any components in the loop - hence why I think you get the 'unknown' screen. I would start by looking for a disconnected plug or cable. Has any electrical work been done on the car recently?

Edit: This is what the plugs look like:


Edited by Richard Hamilton
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Hello guys,I have the same problem.... Error code 8007... Dealership hasn't a clue what it is. My PCM doesn't say unknown however. Everything works fine except the time, date and satellite reception ( no sats are present) what can this be??? Pls help!!!!! P.s I also changed GPS antenna in rear spoiler without fixing the problem!

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Thanks for the image. I am quite certain it is seated properly into the rear of the head unit, but am going to double check. I will post back with the results as I move along. There has been a lot of electrical work on this truck - but it has been me that has been doing it. I think a break in the loop is a very good possibility, I definitely feel it is behaving as if modules are not present. I understand what you are saying about the "loop" - but this cable must enter and exit modules in other locations in the vehicle. I am thinking as a first step to check the terminal connections at each module in the loop.

Where do you find the various components in the system typically?

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  • 4 months later...

I'm back at work on this error - still unresolved.

I did remove the head unit, checked all the connections, and paid special attention to the MOST connector - all seated nicely.

I would now like to check that same connector at the navigation unit and amplifier, but am not certain of their location. I have had a fair amount of this car apart and have seen all kinds of modules, but none so far with that MOST connector.

Can anyone shed some light on the specific locations - if I can get even a general or typical location, I will be able to ID the proper cable and check the connection.

I do not have the CD changer in this vehicle, but do have a subwoofer in the back where the spare would be - and there must be an amp in there someplace to drive it.



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I have PCM2.1 so it may be a little different but the Nav (DVD drive in my case) is under the passenger seat, not sure if you can see the connection from the back with the seat raised and all the way forward (could someone possibly have kicked it?). The CD changer as well as the amp are in the right side rear cubby compartment by the rear hatch. If you don't have a CD changer then the MOST connection should still be there but it should have a plastic bypass plug plugged into it that lets the fiber optic light signal continue. If you turn on the PCM and unplug any of these connectors you should see a red light coming out of one of the two brown cables, the light passes through whatever device it's plugged into and continues down the other brown cable to the next device in the loop.

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Thanks, I am sure it is a break in the signal someplace, and there are plenty of modules under the passenger seat along with another in the cubby, which doesn't look like a typical amp to me - but I am going to have a good search for that connector. When I pulled the head unit, I had a good look at the connector. The thing is, I cannot seem to locate that same connector or set of lines anywhere else. Well, not immediately apparent anyhow. Would you expect it to be bundled within a wiring harness along with standard wiring?

I'll post back with what I find.

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