The odometer on the Buick is driven from an AC frequency that is fed to the computer. The freq. is generated by a sensor on the tranny. The computer then sends a negative going square wave to an 8 pin DIP located next to the odometer. The chip then sends a stepper motor drive signal to a small motor which in turn drives the numerical display. The problem is the 8 pin chip is prone to failure because it is underrated. The chip is a Cherry Semiconductor CS8442. It is next to impossible to buy in lots less than 100. To overcome this problem I bought a NTE 1857 16 pin dip H stepper motor driver for $12.00. The chip will sink a load at 350 ma versus the Cherry unit at 85 ma. By removing the digital display and referencing a schematic a negative going pulse can be added along with ignition voltage to make the unit work on the bench. Tie pin #10 of the NTE chip positive for ccw generation. Tie pin #9 low for full step motor sweep. Heatsink the center pins (4), insulate the new circuit board and use prints (can be found on-line) for both the old chip and new chip to run four wires from the new chip to the old board to drive the motor. This is nothing more than a simple H configuration stepper motor driver circuit. Cherry Semi apparently wants to keep all automotive electronics out of the hands of the common guy. This work around solves that problem.
Bookmarks