P28/S300 Issue on Accord

[JosiahFaison40]

Car: 1999 Honda Accord EX (5-speed)
ECU: P28 — all capacitors replaced, J12 removed, and J1 jumped
S300v3 serial: 67277
Mods: Ghost key push button from Jordan Distributions, eBay cold air intake, full speaker system (removed for testing), 3-to-2-wire IACV conversion with Rosko conversion plate

1. Symptoms / History

• Installed S300v3 onto the P28 and initially uploaded my tune — S300 was turning on (not sure if sensors were reading).
• When finishing the installation of the 3-to-2-wire conversion, the S300 showed no live data. At that time D3/D4 (green) were lit but D2 rapidly flashed red.
• I reseated the S300v3 a few times to make sure it was correct; now when the S300 is hooked up it shows no lights at all.
• Plugging my original stock ECU back in makes the car work normally (no issues).
• Sent my ECU and S300v3 combo to be tested on an engine sim, which came back all good. Also, I had the P28 board switched out just in case.

2. What I’ve checked/measured (measurements taken at the OBD1 ECU connector side of conversion harness):

2.a. Baseline (ECU unplugged):

• A25 (IGP2): 11.92 V (key ON)
• B1 (IGP1): 11.92 V (key ON)
• A7 (fuel pump relay): 11.90 V key ON, drops to <0.4 V key OFF
• Grounds: A23, A24, A26, B2 → 0.0 Ω to chassis (good)
• Conversion harness continuity for D19–D22 → OK

2.b. Continuity (ECU unplugged):

• D19 → chassis: OL (open) — no short
• D20 → chassis: OL — no short
• D21 → A23: 0.0 Ω
• D22 → A23: 0.0 Ω

2.c. ECU plugged in (S300 removed), Key ON:

• A25: ~2.17 V (should be ~12V)
• B1: ~2.17 V (should be ~12V)
• D19 (VCC1): ~5 mV (should be ~5V)
• D20 (VCC2): ~5 mV (should be ~5V)
• D21 / D22: ~2 mV (should be ~0V)
• D4 (SCS): ~-0.9 mV (I forgot what this should be but this is only testing if the ECU is initialized, I think)

2.d. ECU plugged in, TPS / MAP / IACV disconnected (Checked after each sensor removed and still no change):

• A25: ~1.90 V ((should be ~12V))
• B1: ~1.88 V (should be ~12V)
• D19/D20 remain near 0 V (should be ~5V)
• D4 measured ~8.9 mV and slowly increasing to >22 mV if probe left in pin (odd behavior)

2.e. Connector isolation:

• Plugging Connector A (only) → A25/B1 collapse to ~1.8–2.0 V and constant CEL.
• Plugging Connector B (only) → A25/B1 collapse to ~1.8–2.0 V and no CEL.
• Plugging Connector D (only) → A25/B1 remain ~11.9 V (no collapse).

With ECU unplugged, A25/B1 read ~11.9 V (so vehicle/harness switched feed is fine until ECU is inserted).

2.f. Fuse & relay checks:

• ECU fuse (driver panel): 12.19 V both sides (battery and load side)
• Fuel pump fuse (driver panel): 12.18 V both sides

IGN1 relay continuity OK; replaced both fuses and the 7.5A for clock backup in passenger side panel — did not fix the issue.

3. Other notes:

• Wiggle-testing the harness/connector at A25 didn’t change the low voltage significantly.
• My multimeter is a cheap one from Amazon so it could be slightly off but from what I can see that isn't the issue.
• I've tried some of these tests with the S300V3 installed and removed but still got the same issues.

4. What this data shows / my current conclusion

• The harness and vehicle switched feeds are good (A25/B1 read ~12 V when ECU is unplugged).
• When the P28 is plugged in, A25/B1 collapse to ~2 V and the ECU 5 V rails (D19/D20) sit near 0 V — the P28 is not powering up its internal 5 V rails in-circuit.
• The collapse happens when either connector A or connector B is plugged into the ECU (connector D alone does not cause the collapse). That localizes the problem to something tied into pins present on A and B (power distribution/components near connector A/B on the board).
• Grounds are good and fuses/relays appear fine, so the load is being introduced by the ECU or something that only connects when the ECU is mated to the harness.
• This strongly points to an internal ECU fault (power regulator, shorted pass device, shorted diode/MOSFET, bad cap, or a connector/pad problem on the P28 board).

5. What I’ve tried already (recap):

• Checked & replaced fuses (ECU, fuel pump) and reseated/swapped relays
• Unplugged TPS, MAP, IACV (no change)
• Removed S300 when testing the ECU (no change)
• Confirmed grounds and harness continuity for VCC / sensor ground rails

6. Planned next steps (DIY repair path)

• Open ECU and inspect PCB around connector A/B and the power regulation area (looking for lifted pads, cold joints, heat damage, bulging caps, solder bridges).
• Do diode/continuity testing on board traces from A25/B1 to power components to find a low-ohm path to ground.
• Use a series lamp (12 V incandescent), test off a battery with an inline fuse (5 A) to safely apply 12 V to the board and identify the hot/shorted component. I have a Sylvania 7443 bulb available for this.
• Replace/repair the faulted SMT/THT part (likely regulator, diode, MOSFET, or cap) and re-test.

7. Questions to the community:

• Has anyone seen a P28 behave like this (A25/B1 collapsing to ~2 V only when connector A or B is mated)?
• Any suggestions for the most likely components to check first around Connector A/B on a P28?
• If you have a common “spot” or part that fails on these boards after S300 installs, please share (photo/part number/repair tip appreciated).

[JosiahFaison40]

This is the pinout diagram I'm using for all measurements (02D01720-1500 is the board in my ECU):

P28_02D01720-1500_Pinout.jpg

[Spunkster]

You should try the ECU on a known working vehicle to see if it works correctly there. If it does, then there appears to be a wiring issue in your vehicle.

[JosiahFaison40]

You should try the ECU on a known working vehicle to see if it works correctly there. If it does, then there appears to be a wiring issue in your vehicle.

I don't know anyone else with a 6th-gen Accord so I'm not sure how I would test it. I tested a lot of the wiring in my vehicle with this test tho. Any recommendations would be appreciated.

[Spunkster]

IT does not need to be an accord. It can be any vehicle compatible with the s300/OBD1 combo.

[JosiahFaison40]

Since the ECU and S300 were both tested on an engine sim, I really think it's something with my car wiring. From my testing, I haven't found anything in the wiring that would cause the voltages to drop like that. The problems are only present when the P28 is attached, which makes me think it's something really niche that I'm overlooking. Unfortunately, this will probably require multiple hours of testing my car's wiring, which I will start on tonight.