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Adding 1-Wire VTEC Control to OBD0 ECUs (PM6)

Hardware modification guide to adding 1-wire VTEC control to OBD0 Honda ECUs by populating unused automatic transmission solenoid circuits.

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Adapted from pgmfi.org wiki

The factory manual transmission OBD0 PM6 ECU (from the 1988–1991 Civic/CRX Si) does not have VTEC control circuitry. However, because Honda used the same PCB layout for both manual and automatic transmission ECUs, the manual boards contain unpopulated slots intended for automatic transmission lockup solenoid controls.

By populating these unused circuits with specific resistors, transistors, and diodes, you can create a high-current output channel. Custom ROM codebases (such as TurboEdit) can then use this channel to trigger a VTEC solenoid. This is commonly referred to as a 1-Wire VTEC modification because it drives the VTEC solenoid directly, while the VTEC pressure switch is bypassed in the software.

1. Required Components

Many of these parts are difficult to find in standard electronic component catalogs. However, they can be easily salvaged from a cheap, common OBD0 PM5 ECU (found in Civic DX/LX models):

Location Component Type Value / Specification Notes & Part Numbers
Q42 Power Transistor 2SD1594 (NEC D1594) SOT-89 or TO-220 package; salvage from automatic PM5/PM6.
Q16 NPN Transistor 2SC945 (C945) Standard NPN; can use NTE85.
Q28 Diode Array NIL3Z Salvage from PM5 ECUs.
IC17 Driver IC 8055P Low-side driver chip; salvage from automatic PM5/PM6.
C77 Ceramic Capacitor 0.01 uF Standard disc filtering capacitor.
R62 Resistor 10k $\Omega$, 1/8W Pull-down resistor.
R64 Resistor 51k $\Omega$, 1/8W Gate resistor.
R66 Resistor 10k $\Omega$, 1/8W Gate resistor.
R69 Resistor 110 $\Omega$, 1/4W Current limiting resistor.
R70 Resistor 1k $\Omega$, 1/8W Current limiting resistor.
D17 Rectifier Diode 1N4004 (or 1N1004) Flyback protection diode.
D26 Zener Diode ~68V Transient voltage suppression diode.
BR10 Jumper Jumper wire (0 $\Omega$) Solder bridge connection.
BR3 Jumper Jumper wire (0 $\Omega$) Optional; depend on board revision.

2. Step-by-Step Installation

  1. Prep the Board: Open the ECU casing and locate the unpopulated footprints on the right side of the PCB matching the component IDs listed above. Use solder wick to clear the factory solder from the through-holes.

  2. Solder Passives & Diodes: Install the resistors (R62, R64, R66, R69, R70), capacitor C77, and diodes D17 and D26. Pay attention to the polarity bands on the diodes.

  3. Install Transistors & Driver IC: Solder transistors Q42, Q16, Q28, and the IC17 driver IC. Ensure their orientations match the silk-screened outlines on the PCB.

  4. Install Jumper BR10: Solder a solid copper jumper wire across the pads at BR10.

    Note: Depending on your specific board revision, you may also need to install a jumper at BR3. Compare your board layout to a factory PS9 or automatic PM6 board to verify if the trace path requires it.

3. Output Pin Routing

Once the hardware is installed, the new output channel is routed directly to the ECU connector pinout. Under standard PM6 wiring layouts:

  • The output activates pin A8 (which is normally the automatic transmission lockup solenoid output on automatic harnesses).
  • To wire VTEC, run a single wire from your VTEC solenoid in the engine bay through the firewall and pin it directly into position A8 on the ECU harness plug.

Credits and source

Authors blundar, 90wagon, tatou100

Source Adapted from Hardware For One Wire Vtec on pgmfi.org wiki. Licensed under CC BY-NC-SA 1.0.