Would like a second opinion on this series 60

[TomHanson]

I have this '98 pete 379 with a 12.7 ddec 3 in the shop here. Jakes sound like they're stuck on low, no matter the position of the switch. Has an active fault of
[ACTIVE] Aux. output #3(S3) - ENG BRK MED - open circuit (high side)
Flash: 31
ECM: Mas
ID: S 51
FMI: 3
Start: 451
End: --
Duration: 03:10:37
Count: 1
Start Time: Feb21,23:51

I removed the valve cover and used a test light on each solenoid while the engine was running and activating jakes. I found no matter what setting the switch was on, only the front solenoid would get power. I removed the switch in the dash and tested it, which it is fine. I went through the troubleshooting tree, and it seems all the wiring checks out. I can activate the low side manually through the ecm and see there is power to the front solenoid. I activated the med/high side, and there was no voltage to the center/rear solenoid. I swapped in a test ecm, and could manually activate low/med/high and see there is voltage coming to the solenoids now. What I find odd - I'm still getting the exact same fault code active, but the jakes work correctly on low/med/high now while running. My test ecm is programmed for a Volvo with a 12.7, so maybe that could be the case? Any input would be appreciated, thank you!

[Mal4shtf]

That harness is trash? You moved it around swapping ecms and now they work but the ecm can still detect a short to high? That harness is like 230$. Maybe replace the jake solenoids to and e done f%cking with it for 500$?