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    Thread: What MCU is on the OBD1 & 2 Clusters?

    1. Member Keebler64's Avatar
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      06-14-2010 10:49 AM #1
      I've been looking at quite a few pictures of gauge cluster PCBs, but I've been unable to discerne what is actually powering the board.

      An MCU is a Microcontroller Unit. It's basically like your CPU in your computer. The most common types are 8bit. But can be of any type (8, 16, 32, 64bit) with varying types of architectures. I really don't see a 1995 vehicle requiring much processing power, and would assume it's an 8bit MCU, probably from an OEM supplier like Bosch or similar with a core like that of an ARM or Atmel variety.

      The big square thing in the middle is the MCU.


      Edit: The image is from Toby Erkson's page at aircooled nut.

      If anyone is able to look at the chip(s) and give me a listing of what is written on the chip. That would be great. Also, if you're aware of any JTAG or debug ports on the board. That would also be helpful.

      Additionally. I do not have a Bentley, but if anyone has a wiring diagram of what wires go from the Cluster to the ECU, Circuit Block.

      Thanks.

      PS: I know I could take my OBD1 cluster apart, but right now my car is perfectly stock and functional, and I don't want to experiment on it directly.
      Last edited by Keebler64; 06-14-2010 at 10:57 AM.

    2. Member Sallad's Avatar
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      06-14-2010 11:30 AM #2
      Why are you asking this? Are you planning on swapping chips around? Again, why, wouldn't it be easier to just go buy a working one from a junkyard?
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    3. Member Keebler64's Avatar
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      06-14-2010 11:40 AM #3
      No. I'm just curious as to what runs the cluster. What gets reported to the LCDs, and how they operate, and so on. It's just a random habit I have. See what controls devices, then figure out ways to manipulate them.

    4. Member Sallad's Avatar
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      06-14-2010 11:45 AM #4
      You asked about "debugging" ports. I know in the elevator trade, when we are wiring up the controllers (56 x 3 wire bundles coming into them) We have "testing ports" on our PCB's. They are just little pins that are all over the board, for attaching a meter to, for testing of voltages, etc.

      Is this what you are asking about? If you want to trace how they work, I'd start at where the power comes into the board, and follow the "traces" on the board and follow the power around the PCB.
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    5. Member Keebler64's Avatar
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      06-14-2010 11:54 AM #5
      Quote Originally Posted by Sallad View Post
      You asked about "debugging" ports. I know in the elevator trade, when we are wiring up the controllers (56 x 3 wire bundles coming into them) We have "testing ports" on our PCB's. They are just little pins that are all over the board, for attaching a meter to, for testing of voltages, etc.

      Is this what you are asking about? If you want to trace how they work, I'd start at where the power comes into the board, and follow the "traces" on the board and follow the power around the PCB.
      Typically, debugging or JTAG ports are pin ports/wire termination points in 2/3/5/6/10 groupings. Usually they're without actual PINs, just empty voids where you're able to insert a test plug into. The pins/ports are usually directly connected to the SPI (Serial Peripheral Interface) or I2C pins of the MCU. This allows you to plug a Programmer cable/dongle in, and run various types of debug software. Or serial terminals.

      For instance, figure out the communications protocol of the chip to establish a way to talk to the device. Then output the data, and figure out a way to overwrite or change it, or redirect it.


      EDIT: Case in point. OBD and VAGCOM, CANBUS was developed for the purpose of diagnosing problems, and changing data in the ECU to modify or repair problems in the software. I'm not looking for a way to actually change the stored data, such as ODO readings. More like, add functionality to the clock display, or add in a custom MFA. Or any variation of them. OR, say you've got a dead Cluster. Figure out a way to repair it.

      Speaking of Odometer. Is the data stored on the cluster or ECU? (Say when you swap clusters from an OBD2 to OBD1 car, or pretty much any vehicle with a digital odometer display). I would think it's stored in the ECU, to prevent such a problem of inaccurate milage reporting. Especially since, unlike older vehicles where there is a cable directly connected to the transmission via gear system to an analogue counter.
      Last edited by Keebler64; 06-14-2010 at 12:00 PM.

    6. Member Sallad's Avatar
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      06-14-2010 12:03 PM #6
      The ODO reading are stored in the cluster, as far as I know. Because when I did my engine swap (16v) I had two separate ECU's in the running car (CIS-Motronic, then Digi2) and all the time, my ODO stayed the same reading.
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    7. Member Keebler64's Avatar
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      06-14-2010 12:06 PM #7
      Quote Originally Posted by Sallad View Post
      The ODO reading are stored in the cluster, as far as I know. Because when I did my engine swap (16v) I had two separate ECU's in the running car (CIS-Motronic, then Digi2) and all the time, my ODO stayed the same reading.

      After the swap, did it continue to accurately count miles? Did your transmission change with the 16V? (Other than my 2002 TDI, this is the newest VW I've ever owned. Everything else was 1985 or older. So I'm just now getting accustomed to the electronics.)

    8. Member Sallad's Avatar
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      06-14-2010 12:19 PM #8
      The cluster gets the ODO reading from the VSS (Vehicle Speed Sensor) it's located on the top of the tranny (3 wire plug) As I kept the same tranny, the cluster reading never changed.
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    9. Member Keebler64's Avatar
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      06-14-2010 12:28 PM #9
      Quote Originally Posted by Sallad View Post
      The cluster gets the ODO reading from the VSS (Vehicle Speed Sensor) it's located on the top of the tranny (3 wire plug) As I kept the same tranny, the cluster reading never changed.
      Thanks, I really need a Bentley. But don't have the cash for one. I found my Rabbit Bentley in a scirocco I purchased from some guy in field for $50. (I got the car for $50).

      Anyway, the PCB doesn't have to have any Debug ports. If I can figure out what MCU is on there. Then I can find the datasheet and pinout for it.

    10. Member Keebler64's Avatar
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      06-14-2010 04:48 PM #10
      Rather than wait for someone else to do it. I decided what the hell?, And just pulled my own anyways. I mean, it's only 4 screws. Anyway.

      It's an:

      ITT
      CDVG-E440
      G 035125.702
      5014. 10/S

      Currently doing a lookup on it's operation.

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