Disabling VTEC VSS Check on P72 ECU
Technical guide for disabling the 25mph Vehicle Speed Sensor (VSS) requirement for VTEC engagement on the OBD1 P72 ECU.
To disable the 25mph Vehicle Speed Sensor (VSS) requirement for VTEC engagement on the P72 ECU, the specific memory address must be modified.
Warning
Modifying ECU calibration data can result in engine damage if performed incorrectly. Ensure you have a verified backup of your current ROM before applying these changes.
VTEC VSS Check Logic
The VTEC engagement logic relies on a speed threshold check. By modifying the value at address 547Dh, the VSS requirement can be bypassed.
Memory Map Reference
The following table outlines the relevant instruction sequence for the VTEC VSS check routine:
| Address | Opcode | Instruction |
|---|---|---|
| 11FD | C5 22 09 | RB 22.1 |
| 1200 | CB 24 | SJ 1226 |
| 1202 | C5 22 19 | SB 22.1 |
| 1205 | C5 F3 C0 32 | CMPB F3, #32 |
| 1209 | CA 1B | JLT 1226 |
| 120B | C5 D9 C0 44 | CMPB D9, #44 |
| 120F | CD 15 | JGE 1226 |
| 1211 | 90 9D 7D 54 | LCB A, 547D |
| 1215 | CE 03 | JNE 121A |
| 1217 | D8 31 0C | JBR off 31.0, 1226 |
| 121A | DB 1E 03 | JBR off 1E.3, 1220 |
| 121D | ED 19 03 | JBS off 19.5, 1223 |
| 1220 | E9 31 0B | JBS off 31.1, 122E |
| 1223 | E9 27 0F | JBS off 27.1, 1235 |
| 1226 | C5 22 08 | RB 22.0 |
| 1229 | C4 27 0A | RB off 27.2 |
| 122C | CB 17 | SJ 1245 |
Modification Procedure
To disable the check, set the value at memory address 547Dh to FF.
Tip
Setting the value to
FFeffectively masks the VSS input, allowing VTEC to engage regardless of vehicle speed, provided other engagement criteria (RPM, Oil Pressure, ECT) are met.