Chips and dips

You can see there are two large chips which dominate the board. The one on the right isn't particularly interesting, being a gate array, which it's best to think of as a really dumb but really fast calculator and switch.

The chip on the left is a single-chip microcomputer from Hitachi, running at 10MHz, with 2K of RAM and 32K of ROM. In 1995, it was pretty good. This is the chip that I'm going to focus on. It has 84 pins, very few of which are connected to anything.

1. XTAL - to crystal oscillator; also pin 84. The oscillator should be double the desired clock frequency.
2. Vss - Ground (0V); also pins 24, 41, 42, 64, 83. The chip will not operate if any pin is left unconnected.
7. P14 - input/output pin
16. Vcc - Power (5V); see also pin 55. The chip will not function if either pin is left unconnected.
17. MD0 - input pin for setting the operating mode; also pins 18, 19. Hard wired to mode 7 - single-chip mode.
18. MD1
19. MD2
20. STBY - low input causes chip to enter low-power standby mode.
21. RES - low input (or output) causes chip to reset
24. Vss; see pin 2
25. P30 - input/output pin
29. P34 - input/output pin
30. P35 - input/output pin
31. P36 - input/output pin
32. P37 - input/output pin
35. P42 - input/output pin with enough power to drive an LED
41. Vss; see pin 2
42. Vss; see pin 2
43. P50 - maybe connected, maybe not
46. P53 - maybe connected, maybe not
49. P56 - input/output port with built-in pull-up
50. P57 - maybe connected, maybe not
55. Vcc; see pin 16
58. P72/FTI2 - maybe connected, maybe not
59. P73/FTI3/TMRI - maybe connected, maybe not
64. Vss; see pin 2
65. AVss - Analog ground - hard wired to pin 64
70. P84/AN4 - maybe connected, maybe not
73. P87 - input/output pin
74. AVcc - Analog reference voltage
78. P93/RXD2/PW2 - maybe connected, maybe not
80. P95/TXD1 - maybe connected, maybe not
81. P96/RXD1 - maybe connected, maybe not
83. Vss; see pin 2
84. EXTAL - to crystal oscillator

Connector photos

 This is a tentative list, based on guesswork and the Haynes manual. I'll update it as I investigate.

1. Power transistor
2. Tacho signal
3. Condenser/resistor
4. ECCS relay - control
6. Gnd (MAYBE NOT!)
7. Diagnostic connector pin 1
9. Cooling fan signal
11. ???
13. Gnd
14. Diagnostic connector pin 9

15. Diagnostic connector pin 2
16. Air flow meter
18. Engine temperature sensor
19. Exhaust gas sensor
20. Throttle sensor - signal
21. Engine temperature sensor (NOPE! IT'S ANOTHER Gnd)
22. Crank angle sensor
23. Diagnostic connector pin 6
24. Engine warning light
29. Throttle sensor - Gnd
30. Crank angle sensor

32. ???
33. ???
34. Ignition start
35. Neutral switch
36. Condenser/resistor
37. Throttle sensor +ve
38. ECCS relay +ve
39. Gnd (YES!)
41. ???
43. Power steering oil pressure switch ???
45. ???
46. Battery +ve
47. ECCS relay +ve
48. Gnd

101. Fuel injector number ?
102. ???
103. Fuel injector number ?
104. Fuel pump relay - control
105. ???
108. Gnd
109. ECCS relay +ve
110. Fuel injector number ?
112. Fuel injector number ?
113. AAC valve
116. Gnd

Several connections that should have been present, according to the Haynes manual, were not. These were numbers 17, 27, 31, 40, and 107. Given the briefest inspection of the board, it appears connections 101-116 are 12V, whereas the rest are 5V.

Pin-outs from a similar model are available here