Help finding datasheet MM056 9B08

[ICTravis]

I have Google-ed quite a bit and tried the datasheet look up sites in the sticky above. Called a few electronic engineers I know...  Nothing.  Just want to know what functions this performs for now.  Thank you!

[miguelvp]

Maybe adding where the board belongs to might help.
Could be a custom chip or just a custom label for some common IC

[ICTravis]

Sure.  It is a component in the digital speedometer in an early eightys Mitsubishi automobile.  This chip resides underneath a futuba 3 digit display part number: 4-GT-50Z or 457175-0050.  I believe the chip contains the logic for lighting up the display.  Currently the display stops reading above 136kmh.  Wanting to reverse engineer the circuit in order to attempt to figure out how to modify it to continue displaying at faster speeds.  In another domestic market a nearly identical speedo is reported to display up to 180+ and there are digits to go to 299.  Unfortunately no information is available as far as schematics or otherwise for either my speedo or the 180+ speedo and I do not have a 180+ speedo at my disposal.  I have already verified that the pickup that converts the speedo cable rotation into an square wave signal for the speedo continues its modulation well past the displays limit.  Something else worth mentioning is that around this IC are pads and printing indicating additional circuitry not present on this board so I am hoping this suggests that maybe some part of it is for increasing the display limit.

And back to my initial request.  If anyone is able to find anything about this chip I would love to know specifically the steps you took to find the information so that next time hopefully I can do this on my own.  Frustrating having dug this far into it and not knowing what to do next...

Edit:  One last thing is that there are 42 pins on the chip if that is of use at all

Thank you,
ICTravis

[ICTravis]

Same number of pins similar part number prefix similar function.  Am I getting close?

http://www.micrel.com/_PDF/mm5450.pdf

[amyk]

The logo is that of Nippon Denso, and both from that and its age, you are basically not going to be able to find any datasheet for it, much less one in English and not Japanese; my guess is that it's a mask-ROM MCU programmed as a frequency counter and calibrated to display speed. If you trace the schematic and figure out how the display is driven and where the input signal comes from, it shouldn't be too hard to replace it with an MCU of your own choosing (e.g. PIC, AVR, 8051, etc.)