When running an automatic OBD0 ECU (such as a PM6 from a Civic/CRX EX or LX) in a manual transmission vehicle, the ECU's transmission control routine checks for the presence of the automatic lock-up solenoid. If this solenoid is not detected, the ECU triggers a Check Engine Light (CEL) for Code 19 (Automatic Transmission Lock-up Control Solenoid).

While the standard method to convert an ECU involves desoldering internal jumpers (such as RP1/RP2) or modifying the circuit board, this guide details a simple external harness modification developed by community member defensio. By faking the solenoid load using a resistor on the wiring harness, you can run a manual car on an automatic ECU without opening the ECU casing.

The 220-Ohm Resistor Hack#

To bypass the Code 19 check, you must wire a 220Ω resistor between the ECU's lock-up solenoid output pin and ground. This mimics the electrical resistance of the factory solenoid coil, satisfying the ECU's current-draw monitoring circuit.

Wiring Specification#

• Target Pin 1 (Output): Pin A8 (Automatic Lock-up Solenoid output on OBD0 automatic ECUs).
• Target Pin 2 (Ground): Pin A2 (ECU Power Ground).
• Resistor: 220Ω (1/4 Watt or 1/2 Watt resistor is sufficient).

Harness Modification Steps#

1. Locate the wires corresponding to Pins A2 (Ground) and A8 (Lock-up Solenoid) on your OBD0 wiring harness plugs.
2. Strip a small section of insulation from both wires near the connector.
3. Solder a 220Ω resistor inline between the two wires, bridging them.
4. Insulate the exposed solder joints using electrical tape or heat-shrink tubing to prevent short-circuits.

Walking Through the Build (Visual Guide)#

![Exposing Pin A2 on the Harness Plug](DSCF1531.JPG)
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![Pin A8 Spliced and Soldered](DSCF1533.JPG)
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![Solder Connection with a 220-Ohm Resistor](DSCF1534.JPG)
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![Adding a Mode Selection Switch](DSCF1535.JPG)
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![Solder Joints Heat-Shrunk and Bundled](DSCF1536.JPG)
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![Rear view of the completed harness plugs](DSCF1537.JPG)
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![Side profile showing clean harness wrap](DSCF1538.JPG)
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![Completed harness ready to plug into the ECU](DSCF1539.JPG)

Advanced Application: OBD0 VTEC Solenoid Repurposing#

This harness hack is highly popular for OBD0 VTEC conversions (such as running a JDM B16A engine on an automatic USDM PM6 ECU).

On JDM OBD0 VTEC ECUs (like the PW0 or PR3), Pin A8 is the native control pin for the VTEC Solenoid. Because USDM automatic PM6 ECUs also use Pin A8 to drive the automatic lock-up solenoid, you can upload JDM VTEC software onto a chipped USDM automatic ECU, and Pin A8 will seamlessly act as the VTEC solenoid output.

Using a Mode Toggle Switch#

By wiring a Single Pole Double Throw (SPDT) switch between the ECU Pin A8, the 220Ω resistor (tied to Ground), and the engine's VTEC solenoid wire, you can create a switchable harness:

• Manual / VTEC Mode: Connects Pin A8 directly to the VTEC Solenoid.
• Auto Check Bypass Mode: Connects Pin A8 to the 220Ω resistor to bypass the automatic transmission diagnostic check.