Chipping Honda OBD0 and OBD1 ECUs for Programmable Tuning
Introduction
Socketing (chipping) a Honda ECU replaces the factory ROM with a 28-pin socket and a programmable EPROM, letting you run custom fuel and ignition maps, raise limiters, and datalog through free tools such as BRE and Turboedit (OBD0) or Crome, Uberdata, and Hondata (OBD1). This guide covers the classic chippable ECUs: OBD0 PR3 and PW0 (difficulty 3/10), OBD1 P05, P06, P08, P28, P30, P74, P75, and OBD1 PR3/PR4 (difficulty 4/10 if the latch is already on the board, 8/10 if you must add it), plus notes for the City Turbo T2 ECU (code 108, difficulty 6/10; a T1 should also work using the T2 ROM image). Important limits from the author's follow-up answers: automatic-transmission OBD1 ECUs cannot be chipped, and neither can ECUs made after 1995 (e.g. a P72 must be a manual, pre-96 unit). This is precision soldering on an irreplaceable board - damaged traces can total the ECU - so work carefully. Adapted from a community writeup by bmgjet on OzHonda (https://ozhonda.com/forum/showthread.php?162731-How-To-Chipping-Honda-ECU-s), including corrections from the follow-up discussion.
Reference
Tools Required
- Soldering iron (a 15 W iron works well for this board-level work)
- Desoldering braid
- Solder sucker
- Solder and flux
- Multimeter (for continuity and short checks)
- Small wire cutters or nail clippers (for cutting ROM legs)
- Hair dryer (City Turbo ECU only, to soften the chip glue)
- Isopropyl alcohol (for cleaning the PCB)
Parts Required
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EPROM chip: 27SF256 or 27SF512 (27SF128 also works on the City Turbo ECU)
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28-pin DIP socket
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OBD1 only: 74HC373 SMD latch (if not already fitted to the board)
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OBD1 only: 2x 0.004 uF SMD components (positions C50 and C49)
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OBD1 only: 2x 0.00001 uF SMD components (positions C91 and C92)
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Serial TTL converter (for datalogging)
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ROM image / base map for your ECU and engine
Safety Warnings
- ⚠ Check the board for torn traces after removing the stock ROM and check for shorts between socket pins with a multimeter before powering the ECU - a bridged or broken connection can destroy the ECU or leave the car dead.
- ⚠ Automatic OBD1 ECUs and post-1995 ECUs cannot be chipped - verify your exact ECU code and year before cutting anything.
- ⚠ Insert the EPROM with the notch in the correct orientation; a reversed chip will not survive power-up.
Step-by-Step Instructions
1 Confirm your ECU is chippable
Check the code on the ECU sticker. Chippable OBD0 ECUs: PR3, PW0. Chippable OBD1 ECUs: P05, P06, P08, P28, P30, P74, P75, and the OBD1 versions of PR3 and PR4. The City Turbo T2 ECU (108) can also be chipped. Automatic-transmission OBD1 ECUs cannot be chipped, and ECUs manufactured after 1995 cannot be chipped either - a P72, for example, must be from a manual car and pre-96. The full code on the side sticker cross-referenced against the PGMFI.org wiki bin page will also tell you the ECU's market of origin.
2 Remove the stock ROM chip
Warm up the soldering iron. The safest removal method is to snip the 28-pin ROM's legs individually with small wire cutters (nail clippers work too), then desolder the remaining legs from the PCB one at a time - cutting first is quicker and makes you far less likely to damage traces than trying to desolder the whole chip intact. Alternatively, use desoldering braid or a solder sucker on the back of the pins and lift the chip out whole. On the City Turbo ECU the chip is also glued down: after cutting the legs, warm the chip with a hair dryer to soften the glue so it can be pulled off.
3 Clean and open the through-holes
Use the solder sucker or desoldering braid to clear every leg hole so all 28 holes are fully open. Inspect the board to confirm no traces are torn. On the City Turbo ECU, clean off the leftover chip glue with isopropyl alcohol. A little flux on the holes will make the socket soldering easier.
4 Solder in the 28-pin DIP socket
Insert the 28-pin DIP socket into the cleared holes and solder each pin, taking care not to bridge adjacent pins. When finished, check every pair of adjacent pins with a multimeter for shorts before going any further.
5 OBD1 only: install the 74HC373 latch if it is missing
Some OBD1 boards already have the 74HC373 latch fitted; if yours does not, solder it on first. There are two methods: (1) solder a 10-pin solid-center ribbon cable to the board pads, verify no bridges, then solder the ribbon to the latch; or (2) solder the latch directly onto the ECU - easy with the slim 74HC373 package, harder but possible with the wider 74HC373D if you bend the pins in slightly. Make sure the latch is mounted in the correct orientation.
6 OBD1 only: verify every latch connection
Test each connection with a multimeter before powering anything. ECU pad to latch pin mapping: 1-20, 3-15, 4-12, 5-16, 6-19, 7-9, 8-6, 9-5, 10-2, 11-3, 12-4, 13-7, 14-1 and 10, 15-8, 16-18, 17-17, 18-13, 19-14. Latch pin 11 connects to pin 22 on the MCU. Confirm continuity on every single pin before moving on.
7 OBD1 only: fit the SMD support components
Once the latch is confirmed, solder in C91 (0.00001 uF SMD, right next to the latch). Desolder and open the socket holes if not already done, then fit C92 (0.00001 uF SMD, on the back of the board between the socket pins). After the socket is soldered in, fit C50 and C49 (both 0.004 uF SMD, on the back of the board above the socket pins).
8 OBD1 only: automatic-to-manual jumpers and enabling the external ROM
If the ECU came from an automatic car and yours is a manual, take out both RP17 and RP18, and afterwards bridge RP18 using a wire - you will find them among the bank of resistors sitting just above the socket pins on the back of the board. To make the ECU run from the new external ROM instead of its built-in one, bridge J1 with a wire; J1 is on the back of the board to the left of C50.
9 Burn and insert the chip
Program a 27SF256 or 27SF512 EPROM with the correct ROM image or base map for your ECU and engine (Hondata's forum and site host many base maps; an Ostrich real-time emulator can be used in place of a burned chip). Insert the chip into the socket with the notch (half-moon) facing the correct direction. On the City Turbo, use the T2 ROM image.
10 Wire up datalogging
OBD0 port pins, reading left to right: pin 1 is Earth; pin 2 is RX, which takes data in from the computer; pin 3 is 5V; pin 4 is TX, which sends data out to the computer; pin 5 is unused. OBD1 (CN2-style port, reading top to bottom): pin 5 unused, pin 4 TX, pin 3 5V, pin 2 RX, pin 1 Earth. Datalogging needs just three of these - Earth, RX, and TX - through a serial TTL converter. On OBD1, J4 must be unbridged to enable datalogging. The City Turbo datalogging port is CN3, but no datalogging support existed at the time of writing.
11 Tune and log with the appropriate software
OBD0 ROM editors: BRE and Turboedit (the author could only get BRE working reliably for datalogging). OBD1 ROM editors: Crome, Uberdata, and Hondata; OBD1 datalogging programs: Freelog, Hondalog, and ECU Control. Get a proper base map loaded and have the car tuned for your setup. For deeper reference material, see PGMFI.org and Hondata's documentation.
Pro Tips
- 💡 A USB TTL converter works fine for datalogging, but the cheap ones are too slow for live tuning - they still deliver the data, just with lag.
- 💡 Cutting the ROM's legs before desoldering them individually is faster and much less likely to lift traces than desoldering the whole chip in one piece.
- 💡 If you have a Hondata board in another chipped ECU (e.g. a PR4), it can be moved into another chippable OBD1 ECU by socketing the new ECU - see Hondata's S300 socketing instructions. Hondata hardware is best installed by an official dealer.
- 💡 Converting a non-VTEC OBD1 ECU to VTEC control requires adding components to the board, or using a spare ECU output in Hondata/Crome Gold and re-pinning the loom - but the factory B18C intake manifold's air bypass valve still needs an output, so re-chipping the correct VTEC ECU is usually the cleaner path.
- 💡 When buying an S300, make sure it is genuine - counterfeit units are known to have problems uploading maps.
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