OBD1 P28 Minimum wires to Test ECU
 

What are the minimum wires to power up an OBD1 ECU? Specifically a P28 or maybe a stock 94 accord ecu.

I want to plug it directly to a battery and a check engine light so I can validate that its working. Its fine if I get 1 Cel or 50 as long as its powered and not a solid lit check engine light.

Here is what I think it may take. I'm not sure if the ones on Connector B are needed or not.
Connector A
13. MIL Green/orange
23. PG1 Power Ground Black
24. PG2 Power Ground Black
25. IGP Power Source Yellow/Black
26. LG1 Logic Ground Black/Red

Connector B
1. IGP Power Yellow/Black
2. LG2- Logic Ground Brown/Black

Connector D
4. Service Check Signal Brown or Brown/White


Brief explanation. I created a custom harness (engine side)and relocated the battery to the trunk. When I try to start the car I get no fuel pump and the check engine light is red. I swapped the ECU same result. I swapped the main relay same result. Checked all of the fuses and I don't see any issues there. All lights are working in the dash, the car cranks, When I jump the service check signal connector there is no change still solid red. I checked 23 and 24 and the grounds are good. I assume the fuel pump relay wont be enabled until the ECU powers up so I'm guessing that's not an issue. I suppose its possible that he Service check connector has a wiring issue but I did not modify that side of things so I don't think that's the issue.

I still need to validate the 2 wires on connector B but I ran out of light. I want to wire up the above test bed so I can test outside the car first. If I can duplicate the issue then ill know which wires I need to trace. Any input would be appreciated. I figured I would post here tonight that way I may get some inputs for tomorrow if I get stuck.

If I can get the ECU powered up I can then start troubleshooting any codes I will be most likely throwing. Also I don't recommend doing the harness the way I did it, pony up and get the rywire.

Re: OBD1 P28 Minimum wires to Test ECU
 

pgmfi.org ? View topic - Powering an ODB1 ECU on the Bench

That thread will have everything you need to know.

Re: OBD1 P28 Minimum wires to Test ECU
 

Wow, I started that thread back in 08 in the midst of Moates releasing their engine SIM. I sold many ECU connectors to Moates for their SIM back then...

Re: OBD1 P28 Minimum wires to Test ECU
 

Thanks Adam, lots of information. Very cool what they designed.
Unfortunately 17 pages and no one every actually answered the initial question. lol. I'm guessing that maybe it was obvious so they didn't bother. haha

I started work on this last night, I quickly realized that I didn't have any extra OBD1 connectors that I could use. I have one spare harness at the moment and I didn't want to pull it apart. I do have several OBD1 to OBD2 adapters so I just started pulling the pins from OBD2 connectors instead. I believe I have all of the above wires pinned and everything else removed, I want to keep this simple. I Just want to have power/ground and cel.

I plan to connect A23,A24,A26 and B2 to my jumper pack Ground.
A13 to a bulb, most likely in a cluster then the other side of the bulb to the jump pack ground
D4 I'm guessing needs to be direct to ground as well? Honestly I'm not sure if that one is ground or positive.

If I don't get anything ill also connect B1 to the positive as well.
Any thoughts? Maybe I should test on my old ECU just in case. lol

Re: OBD1 P28 Minimum wires to Test ECU
 

Thanks for posting Riv, you have any suggestions for me? Should I get the fire extinguisher ready? lol

D4 is a ground so that's easy.

Re: OBD1 P28 Minimum wires to Test ECU
 

Well no luck. I wired it up as listed above and no power, not sure what I am missing.

Re: OBD1 P28 Minimum wires to Test ECU
 

You need the VBU from what I would surmise.

Logic grounds are sensor grounds. TPS and MAP ground to the ECU.

IGP 1 and 2 are power inputs as well as for sensors.

https://cimg1.ibsrv.net/gimg/honda-t...53b80a6c71.gif

Re: OBD1 P28 Minimum wires to Test ECU
 

So you think I need D1 as well? I'm not sure what you are suggesting.
Are you saying A25, A26,B1 and B2 are not needed? I'm not sure it will hurt to have them wired but to simplify it would be good to remove them.

Re: OBD1 P28 Minimum wires to Test ECU
 

I've never done what you are trying, so I don't have a list. But I made my own harness, and know the pinouts pretty well.

The ecu uses the VBU pin for unswitched constant 12v.

Remember that the MIL pin does not output voltage. It is a ground.

Re: OBD1 P28 Minimum wires to Test ECU
 

Somewhere during my testing i blew the fuse on my Accessory connector and i didn't notice. I need to grab some fuses to keep testing. Maybe I have everything I need after all, we will see after I get some fuses.
I have the MIL grounded currently. I guess it needs to be a Positive instead?

I know the SCS is a ground but I figured the MIL was a positive.

Re: OBD1 P28 Minimum wires to Test ECU
 

Ok... swapped the MIL to a positive and I get a Solid CEL. I guess that's progress. I was actually really hoping I would get some normal Cel Codes.

I disconnected the SCS from ground and nothing changed.
Looks like the issue may actually be something outside the basic power and ground wires. Do we need fuel pump before we get the Solid Cel to shut off?

Just to be sure.... the SCS pin should be a ground right? Maybe that's my whole issue?

Re: OBD1 P28 Minimum wires to Test ECU
 

Well something is definitely not right,,,
I was doing some further testing and with MIL(A13) connected to a light and then a positive. If I ground pretty much any other wire it will be lit. Honestly I'm not sure what to make of that at this point.

Just to be Clear, with the following I get a Solid Cel, or maybe just a lit light. Without a 12 power to the ECU I'm sure its not actually powered up.
A13. MIL Green/orange Wired to Light and Positive
A23. PG1 Power Ground Black
or
A24. PG2 Power Ground Black
or
A25. IGP Power Source Yellow/Black
or
B1. IGP Power Yellow/Black

The following seem to make no difference.
B2. LG2- Logic Ground Brown/Black
A26. LG1 Logic Ground Black/Red
D4. Service Check Signal Brown or Brown/White

Re: OBD1 P28 Minimum wires to Test ECU
 

Service check pin should not be grounded unless you want some codes. Shouldn't matter essentially tho.

Logic grounds won't make a difference. They are grounds for the tps and map sensors.

IGP1 and 2 should both get 12v
PG1 and 2 should get ground
Mil goes to light then to 12v

Re: OBD1 P28 Minimum wires to Test ECU
 

I would love to get some codes, that would be a success. lol. Yep I have wired all of those and not had any luck.

Re: OBD1 P28 Minimum wires to Test ECU
 

Same wires on OBD2 ECU and same result. Looks like something more is needed.

Re: OBD1 P28 Minimum wires to Test ECU
 

Voltage backup as well? VBU.

Re: OBD1 P28 Minimum wires to Test ECU
 

VBU make no difference.

Re: OBD1 P28 Minimum wires to Test ECU
 

I think I mixed up logic and sensor grounds. Sensor grounds I think are for the map and TPS.

May need logic grounds. https://honda-tech.com/forums/drag-r...rence-3050717/

Re: OBD1 P28 Minimum wires to Test ECU
 

I'm not really sure how to move forward on this, I wonder if injectors are required. Maybe ill wire some up honestly by time I get this to work I may just have every sensor wired and no engine. haha.

Actually.... I do have another full harness and I'm sure I have spares of every sensor. I pulled the car in the Garage again finally, I was thinking about giving up but I think ill give this one more shot. Anyone have any ideas on what else may be needed to simply get a Valid Cel? Looks like emulator only has about 25 wires including everything. If I add in some injectors that would get me closer to the simulator.

I ordered some distribution blocks to make it easier to connect things.

Re: OBD1 P28 Minimum wires to Test ECU
 

Ok, I believe this is what the Simulator has wired up. I'm thinking that maybe only the fuel pump is needed from what I already have. The fuel pump is just a relay, I wonder if it goes to ground or power. Any ideas? I guess ill try that out and next step would be injectors. Injectors.. would I hook that to battery or ground?

Connector A(13)
01. INJ1 Brown
02. INJ4 Yellow
03. INJ2 Red
05. INJ3 L-Blue
07. FLR Fuel Pump Relay
09. IACV
11. EGR
13. MIL Green/orange
21. ICM
23. PG1 Power Ground Black
24. PG2 Power Ground Black
25. IGP Power Source Yellow/Black
26. LG1 Logic Ground Black/Red

Connector B(3)
11. CYP
13. TDC
15. CKP


Connector D(07)
04. Service Check Signal Brown or Brown/White
06. VTPS Vtec pressure switch
11. TPS
13. ECT
14. PHO2S
15. IAT
17. MAP

Re: OBD1 P28 Minimum wires to Test ECU
 

Actually... I don't think that Injectors are necessary to get he ECU fired up, if they are on they would be spitting fuel and flooding the engine. Man I really hope its just the FLR that's needed, maybe I just have that one wired up wrong.

Re: OBD1 P28 Minimum wires to Test ECU
 

Well I tried it with the FLR wired to both negative and positive with no change. I just get a solid CEL.

Re: OBD1 P28 Minimum wires to Test ECU
 

I remember that it will fire up without the distributor hooked up, but that's about the best I can do to help. I had all of the non-ignition stuff wired up before I ever tried to power it. I do feel like it was picky about which powers and grounds we're hooked up, so make sure you have all of those connected per your list above.

Re: OBD1 P28 Minimum wires to Test ECU
 

Yea I was considering going out and pulling connectors and testing on my wife's car until I could duplicate the issue I am having but I really don't want to accidentally create any issues with her car. The Prelude is OBD2 and so it my other Civic. I'm going to go and find a clean harness and plug that in and see where I stand. I'm seriously considering trying to adapt things over to a OBD2 Civic harness but honestly I feel like that would make things more difficult in the end. It crossed my mind to go full Race Car wiring as well but if I cant get the ECU to fire on the bench what luck will I have in the car. lol.

I pulled the custom harness from the Accord yesterday and untucked everything, not as big of a project as I expected., only took about an hour, and only lost 2-3 pounds in sweat. haha. I was going to install my old harness but I gave up on that pretty quickly, there was a good reason why I pulled it and the situation has not gotten better over the last 6 months. Ill just make a pick your part run and pick up a harness. Once I get the ECU to fire with that I should be able to do some testing, I think I still want to play with getting the ECU working on the bench.

Re: OBD1 P28 Minimum wires to Test ECU
 

Bit of a thread revival, can someone explain where the ECU gets its power from i always for its VBU but mine is not connected but ecu still has power, i can only assume the IGP cables also power the ECU not just ignition side?