Modifying PCM??? High power audio build.

[Floridaplur]

I’m not sure if this is possible, but I’ll tell my story and what I am trying to correct.

My 2018 CX5 NA is a car audio demo vehicle with about 30k watts rms. I have a high output alternator that is disconnected from the PCM and wired directly to ignition source to charge at 14.4v at all times and keep my 405ah lithium bank charged. This is working perfectly. The issue I’m running into, is that once the voltage drops from 14.4 to 13.8v or lower for more than a few seconds, the battery light comes on, and if I keep going the entire engine dies (while the dash also starts flashing everywhere and going nuts). When this happens, it throws codes P0192:00-28 (Fuel Rail Pressure Sensor A Circuit Low), P0629:00-28 (Fuel Pump A Control Circuit High), as well as the obvious P2503 (Low voltage). It’s strange, being the car prior to the PCM bypassed alternator charged between 12.8-13.1v almost all the time, so the PCM is sensing the amount of voltage drop, and not the actual voltage…from there it appears to be lowering fuel pressure to the rail and the car enters some sort of limp mode and shuts the engine down. I’ve done a bit of research, and think this may be low voltage being sent to the suction control valve, but this is entirely a guess at this point.

I’m curious if versatune could be used to monitor voltages at all of the sensors individually, to see if I can trace something specific seeing a lower voltage sent by the PCM… or if the software could even be used to somehow modify the variance of voltage the PCM is willing to see to it (likely the suction control valve) before triggering this limp mode and turning off.



Second thing I’m wondering about the software, is if there is a limit to how high it will allow you to set your idle? I’d love to not have to sit inside the car, feathering the gas pedal to increase alternator output while playing for people at shows. Assuming the software can set the idle to ~2000rpm, how easy is it to load this when I’m parked at the show, and then load back stock right there to drive home with it back to normal?

[Ugnius @ VersaTune]

VersaTuner can data log the "Control module voltage", which is the voltage received by the PCM. It can also log the voltages returned by some sensors, such as the fuel pressure sensor, MAP and MAF sensors. You can log that even with the free version.
The ECU should have no problem running at as low as 12V. I would check your circuit with an oscilloscope to see if there are large voltage drops or transients.
You can probably disable the error codes to prevent the CEL from illuminating, but the internal logic that puts the car into limp mode will still be active. It is not possible to change voltage sensing logic with VersaTuner.
Technically you can set your idle as high as you wish, but we have not tested with crazy values. Most customers just need to raise it by a few hundred RPM. Flashing a tune takes 4-5 minutes, you just need to click "Next" multiple times and turn the ignition on and off when instructed.