DPO stuck to on

Hello , I’ve ran into a rather strange situation that I’m having a hard time wrapping my head around and I’m looking for a wiring expert to maybe explain and help me make sense of it .. hopefully that will lead me into the right direction to a diagnosis for the issue ..here is a little back story of what’s happened

I had a Haltech elite 1500 wired into a customers car and everything was working as you would expect .. wired a couple relays to cooling fans and everything worked as it should .. time has passed

at this point the vehicle owner has removed the engine , cut out the inner aprons for some “drift mod” and painted the bay , and then ran the body harness and fuse box threw the fire wall and into the cockpit for a “shaved bay “ look (1997 240sx ) .. reinstalled everything and ready to test the car …

when the key is turn to “ on “the fans run before it has hit my threshold .. relays still wired as they were before .. switch active state from on to off .. they still run .. as soon as the 12v switch source hits the relay switch the fans come on as if the dpo is a constant ground .. I re wire the relays to eliminate his mess of wiring under the dash .. same symptoms … the fuel pump is still being tripped by the factory body harness and works as it should .. comes on for my pump prime time limit and shuts down .. so I decided to move the fan relay dpo wire over to the fuel pump dpo output that was just confirmed working … fans run constant still when the 12v switch hits the relay and the fuel pump plugged into the fan DPO works as it should .. ( changed the setting in computer for different DPO’s obviously ) .. so now I decide to wire the fuel pump direct from battery using a relay (85)-dpo (86)-12v(switched) (87/30) battery-pump .. completely isolated from any chassis wiring (12v switch from ignition) .. key on and the pump runs constant now just like the fans ..somehow I can’t wire any external relays anymore or they will run constantly .. I’m not too sure what he may have done in the mess of things under the dash in the cockpit but somehow things wired threw the body harness relays will work (only fuel pump tested ) and external wired relays won’t … I’m trying to understand how this is possible .. is the ecu back feeding a ground somehow ? Yes the relays have fly back diodes and I’ve also tried solid state relays with the same results .. even being the SSR’s don’t use a 12v switch from ignition … I just didn’t think the ECU’s DPO’s could be grounded unless you tell them to ground .. and why does it work if running threw the chassis OEM fuel pump relay and not a external ? Maybe I’m overlooking something obvious .. any advice or suggestions is encouraged and appreciated but knowledge and education is what I seek on this matter because it just doesn’t make sense on how this could be happening .. thankyou in advance

I would start with the ECU. Is it's power / ground reading battery voltage? Check the resistance between the ECU ground pin and the chassis ground. I would suspect a missing / floating ground, or a short between something in the wiring harness that results in the DPO pins (when off/floating) being less than the switched power to feed the relay 86 pin. That voltage difference has a current flow, and thus the relay triggers, when it's logically "off"

Good Luck...

Thanks for the advice David .. you are a super star in these forums … I will check some of these things … I still don’t understand how it could work when wired threw the chassis harness and relays but not when using a external … in my head, the circuit is a loop .. if there is a break in the loop then we don’t get power . The only thing connecting the loop in my external relay is the DPO of which has a fly back diode to prevent inductive kickback and latching the dpo … is there a scenario where the DPO could be energized connecting the circuit on my standalone relay but when using the factory relay it’s not ? This is the part that is hard for me to understand ..

The way you indicate that the fuel pump is wired implies that the DPO is providing a ground output.

If that is so, I highly suspect that you have a short to ground that was probably caused by the rewire. I would lift the wire at the header for the DPO output and see if everything still latches when keyed on.