Written breakdown of the Bomarc reverse-engineered board set 37820-P28-A51 (Engine Control Computer for Honda/Acura, 8 sheets, 2005). Stock OBD1 P28 covered SOHC VTEC D-series cars across markets: 92-95 Civic EX/Si (USDM D16Z6), Del Sol VTEC (D16Z6), and EDM/other Civic and Del Sol codes (P28-G0x / P28-Q50 etc.) including D16Z9 applications. Related OBD1 boards share a lot of this layout; optional circuits on the sheets may not be populated on every board. Match your revision (02D011F0-1500 / 02D01720-1500, auto vs manual).

For RAM/ROM maps and board photos see the P28 ECU reference0. Harness side: OBD1 wiring0, ECU connections0.

Warning

Third-party reverse-engineered drawing, not a Honda factory schematic. Values come from a scan - unclear bits are called out. Confirm critical nets on a real board before cutting traces.

Drawing info#

Legend (sheet 1)#

Files#

• P28_ECU_Civic.pdf1 - preferred 8-page landscape scan (one sheet per page). Hosted by imtheman03130 in Honda-Civic-P28-ECU0; that is the legible scan used for the sheet images and transcription below.
• FinishedP28PinoutCORRECTED.png0 - corrected harness pinout graphic from the same repo
• The common 11-page portrait file (P28 Schematics.pdf / bomarc-services-p28.pdf) is the same Bomarc 8 sheets, just a lower-res split scan

Connector pinout (harness side)#

Corrected community pinout for P28 / P06-class OBD1 plugs A, B, and D. Connector C is not on this graphic. Integra wire colors appear where the source diagram listed them; that does not mean Integra used a P28 from the factory.

P28/P06 harness pinout with multi-model empty/optional slots noted

Connector A (26-pin)#

Connector B (16-pin)#

Connector D (22-pin)#

Pin numbers on the Bomarc sheets below are the ECU edge pins as drawn. Cross-check colors against your harness.

Sheet images#

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Sheet 1 - Power supply#

Title block: POWER SUPPLY, sheet 1 of 8, dated 2/27/05.

Connector feeds#

Power nets leave this sheet toward the logic pages (intersheet hex nets).

Main parts#

Passives worth noting#

• R21 100 Ω on Ign B+ into the protection network
• SR4418 plus D5 on the +12 V branch (series protection)
• NF1 filter on Batt B+ with C19 (33/35) and R22 2200 Ω; C20 0.01 and R49 10 kΩ on the VREF side
• D6 marked 1AZ21V / 21VV class on the Batt B+ sense path into MCU pin 31
• +5 V into the U17 region; +4.3 V near 2SB1068 (PNP) with R23 22.02, R24 10 kΩ, R25 45.3 kΩ / 1%
• Crystal on MCU-1: XTL1, CA1/CA7 10 pF, crystal marked 10AKSS3C
• U20 gates with R27 10 kΩ and C45 0.001 into the reset domain; C82 0.001 ×2 near the second NAND

Sheet 1 is Ign B+, Batt B+, grounds, and the regulated rails for everything else. TA8903 pin map is only partly annotated on the drawing.

Sheet 2 - Misc inputs#

Title block: MISC. INPUTS, sheet 2 of 8, dated 3/1/05.

Switch inputs#

Hybrid block into MCU-1#

Large multi-device hybrid feeds INTO PART OF U1 at pins 40 and 56 (as labeled). Bus nets out of the hybrid are labeled ID0, ID1, ID4, ID5, ID6, ID7.

Several M5 stages condition gear, VSS, A/C, SCS, VTEC pressure, alt FR, and brake before they hit MCU-1. Gear drive 3 has an external 2SA1195 stage into ID6.

Glue#

• Comparator/op-amp path near ID0 with R94 300 kΩ, C6 0.63/35, R89 10 kΩ (amplifier labeled near pins 6/7/8 into ID0)
• Note on the sheet: For glue chip rail see U10 on sheet 4, U22 on sheet 3

This sheet is digital/switch inputs, not the analog sensors.

Sheet 3 - Analog sensor inputs#

Title block: MORE INPUTS, sheet 3 of 8, dated 3/2/05.

MCU marking on this sheet: MSM66207-251, MCU-1, board 02D01720-1500.

Note

Suffix -251 is what Bomarc printed. Other P28 baseroms use different MCU IDs (230, 304, etc.). Do not assume every P28 is a 251.

Sensor connections#

ICs#

Discretes#

• P/S front-end: 2SA1175, DM4 S255, R101 20 kΩ, R29 23 kΩ, R102 4700
• Several S233 / S253 diode packs for protection (DM5, DM8, DM11, etc.)
• Op-amps around HEGO/TPS with R85 10 kΩ, R78 100 kΩ, etc.

Sensors are muxed through the 4051s into the 66207 ADC. Channel-to-sensor mapping is only partly annotated on the drawing.

Sheet 4 - TDC / CKP / CYP#

Title block: MAGNETIC POSITION SENSORS, sheet 4 of 8, dated 2/28/05. Board note: 02D01720-1500 ABY-D.

Sensors#

Conditioner#

Comparators shape the three channels, then feed PART OF U1 at pins P3, 41, 36, 44, 50 (as labeled). Intersheet net H and hex (1) leave the CKP path.

Glue#

Standard Honda three-sensor distributor interface. HIC-1 internals are not expanded on the drawing.

Sheet 5 - Data link and knock#

Title block: MISCELLANEOUS, sheet 5 of 8, dated 2/28/05. Board: 02D01720-1500 ABY-D.

External#

Blocks#

Legend table lists U11 NJM2904 and U19 NJM2903.

Diagnostic serial goes through discrete line drivers into MCU UART pins. Knock is optional.

Sheet 6 - Computer control logic#

Title block: COMPUTER CONTROL LOGIC, sheet 6 of 8, dated 3/1/05. Board: 02D01720-1500 ABY.

This is the main digital sheet.

Devices#

Bus (as drawn)#

• Multiplexed address/data from MCU-1 through RM3 and U4 HC373
• ID0-7 into U9 HC541, then RD
• Control lines between MCU-1, ASIC U7, PPI U8, and MCU-2
• NMI / RST area: R35 220 kΩ, R56 1600 Ω 1%, R85 4.7 kΩ, etc.
• Intersheet nets (8), (9), (2), (3), (5) continue to injector/output sheets

This is the usual OBD1 story: MCU + ASIC + 82C55 + optional EPROM. MCU-2 is clearly labeled MSM6434-13 on the restored sheet (older transcription guessed MSM66513). "Demon 2" looks like a socketing/dev label, not a Honda production name.

Sheet 7 - Injectors, main relay, A/C#

Title block: INJECTORS + 2 OUTPUTS, sheet 7 of 8, dated 2/28/05. Board: 02D01720 ABY-D.

Injectors#

Drivers#

Main relay and A/C#

Pre-drivers use DTC144 packs (ASIC pins 22-25 side), DTC114 stages, RM7 (8E222/331), DM10 S235, and intersheet M(6) into the ASIC. ASIC also shows pins 32/33 toward the ST413A pre-drive.

ST460 hybrids are the usual low-side injector switches (common failure parts on OBD1 boards). Drive comes from ASIC U7, not raw MCU pins.

Sheet 8 - Outputs#

Title block: OUTPUTS, sheet 8 of 8, dated 2/28/05. Board note on this sheet: 02D01720-1200 ASY-8 (other sheets say 02D01720-1500). PPI marked MSM82C55A2, part of U8.

Outputs#

Drivers#

Control comes from ASIC U7 (left: pins around 28-31) and PPI U8 port pins labeled on this sheet: PA6, PA3, PC2, PB6, PA6, PC1, PB2, PB3, PB1, PC5, PC6, PB5. Intersheet (5) and (6) continue shared enables.

VTEC, purge, MIL, IAC, fans, and A/T lock-up are PPI/ASIC low-side outputs on this drawing, not direct MCU pins. Optional labels again cover multi-market boards.

How the sheets fit together#

Batt B+ / Ign B+ / GND
        |
     Sheet 1
   TA8903 + regs
    +5V / +4.3V
        |
   +----+----+
   |    |    |
  S2   S3   S4
 switch analog TDC/CKP/CYP
   |    |    |
   +----+----+
        |
     Sheet 6                 Sheet 5
  MCU-1 / MCU-2 /          DLC + knock
  ASIC / 82C55 / EPROM
        |
   +----+----+
   |         |
 Sheet 7   Sheet 8
 injectors  VTEC, MIL, IAC,
 relay, AC  fans, purge, ICM

Limitations#

1. Source is a raster scan, not a vector netlist. Small text can still be ambiguous even on the clarity-restored pages.
2. Optional circuits are drawn even when a given board leaves them off.
3. Some hybrid passives and a few PPI port digits remain hard to pin down; MCU-2 is labeled MSM6434-13 on sheet 6.
4. Wire colors use Bomarc's abbreviations; regional harnesses can differ. Bomarc pin labels (e.g. sheet 1 D2 as pwrgnd) do not always match the community harness pinout table above.
5. The multi-model pinout graphic documents empty slots on purpose. Not every pin is live on every ECU.
6. Bomarc is not Honda factory documentation.

Downloads#

See also: P28 ECU reference0, Add IAB to P280, Disable VTEC VSS check0, Introduction to ECU chipping0.

Credits#

• Legible scan, per-sheet images, and corrected pinout: imtheman03130 - Honda-Civic-P28-ECU0 (P28_ECU_Civic.pdf, P28ECU/Page1-8.jpg, FinishedP28PinoutCORRECTED.png).
• Underlying reverse-engineered drawings: Bomarc Services, 37820-P28-A51 (2005).
• Written sheet notes on this page are a Hondabase transcription of that scan, with unclear bits called out.