Hello, I’m new to this forum but I’m sharing my issue in case anyone on here has had a similar problem to me and can share something that can help with my issue.

I’m getting a corrupted data log (pid’s show 0, freeze or show wildly inaccurate readings) whenever I go full throttle and when rpm reaches just shy of 5000rpm. Those readings stay corrupted until I end the log. I am using an autocal v3 and the issue occurs with blackbox logging in standalone and using the v8 software in pass through mode. I’ve also tried logging less pid’s.

Car is a 2001 Holden VX Commodore LS1 A4.

I had a chat to EFILive directly, in which they pointed me in the direction of a voltage short or a bridge to the VPW data lines that gets triggered by high rpm.

I started my search by checking all of the grounds and the grounds for the pcm which are the black and red wires. All show 0-0.2 ohms of resistance to battery negative, chassis, firewall and engine block and heads. The ground strap from the firewall to the chassis had a resistance of 20 ohms which I have just rectified so I’m hoping that may have caused my issue but I’m still not convinced before I try again. Might even be the pcm itself?

Does anyone have any expertise on these cars specifically or any US equivalent that can help me look further into what EFILive told me? Specifically to do with a voltage short or bridge to the VPW lines. I’ve looked at the wiring diagrams extensively and so far I’ve checked grounds as mentioned and checked the grounds on the obd2 port. The data wire which is a yellow one goes to the obd2 port from the pcm (I’ll look more into this wire). What could be bridging to the VPW data lines? Could it be grounding out, bridging to another wire in the loom caused by the vibration etc? This one has really stumped me.

 

Classic VPW comms dropout. You're on the right track. Since you've fixed grounds, check the alternator for AC noise, especially under high RPM load. A bad diode can flood the system with interference. Also, inspect the entire loom, especially near the exhaust, for melted wires that could be shorting the data line to power or ground when the engine torques over. The PCM itself is a possibility, but rule out the wiring and alternator first.

 

Thank you Robbi, that’s some good info. I’ll check the diodes. Is there a specific way I should test them? Running voltage is around 13.8 and sometimes 13.6 going off the logs and my gauge. Battery voltage when car is off isn’t fantastic, 11-12 volts but starts and runs as expected. The alternator has copped a drenching from the power steering fluid that used to leak on it from above and is pretty gunked up. I have been meaning to replace it so it might serve me good to do so if testing the diodes shows me anything.

I’ve chucked in a spare pcm I had laying around and disconnected some aftermarket oil pressure + voltage gauges just in case there’s anything funky going on there.

I’ll also try to get a better look at some of the loom near the exhaust in case I can spot anything I’ve missed but I believe it’s got to be either the alternator/pcm as those are the things that stand out to me at this stage.

I might just load my last tune up on the new pcm and link it and swap the alternator for a new one and see if the issue persists, that way it narrows down the likely culprits drastically.

 

Another thing worth mentioning is the maf sensor plug. Now I’m running off a speed density tune so I have no use for the maf or the maf plug. The maf plug had issues initially when I got the car which is why I went mafless. Do I need to get rid of this plug in some way as it is just sitting in a secured spot not plugged into anything or is it fine to leave.

 

Definitely replace that alternator. The power steering fluid leak is a classic killer. The gunk and low battery voltage are huge red flags. A new alternator will likely solve your voltage drop issue.

For the diodes, the best way is to pull the alternator and have it bench-tested at the parts store. They can check the diode rectifier for AC ripple.

You're on the right track. Swap the alternator first, that's your most probable culprit.

 

I keep getting an unknown error with obx explorer that causes a shut down and loss of log, think that could be caused by similar problems?

 

Cheers Robbi I’ll swap a brand new alternator in and take it for a run. I’ll keep this thread updated