In truck bench-like harness at a flick of a switch

[Tahoe03Nea]

Hi all,

I am a newbie on tuning and here is a quick sum up of my concerns and ideas with questions for more knowledgeable people here.
Sorry for my English, I am not a native speaker.

I have a 2003 Chevy Tahoe LT, stock, LM7 engine with P59 PCM.
I have an aftermarket radio of course, with a PAC AUDIO RP4-GM11 module (includes steering wheel buttons and chimes...)

I previously had a diablo CRM tune loaded with an Intune device.
I decided I wanted to see more inside and be able to tweak myself, so bought an MPVI2.

I first connected HP Tuners, did a successful PCM read as is and backup this file.
I then used the Diablo Intune to revert back to stock : OK.
I then connected HP Tuners, did a successful PCM read and backup this file in 3 different spots, as this is my precious reference stock tune !!

I then made a renamed copy of this file, and changed 3 minor things like disable the rear O2 sensors error reporting, changed the tire size to meet my wheels and disabled the speed limiter.
Saved the file, then write to the PCM with no problem.

Switched everything off, started the car... all good.

Then decided I wanted to make a read and save the file as the next base to modify as next step... and this is where it went bad.
I had no way to read. Scanner OK, gathered info ok, but read failed everytime I tried, if my memory is correct on something like boot loader timeout ???

As I have no spare PCM and there is no easy way to find a spare PCM around here... all stayed like this since then as the car was running and I needed it.

Time has passed since then, and I should have tried to read again later on maybe ... maybe now it works, but anyhow, I am afraid of bricking my PCM when doing my next mod and trying to write PCM...

So, I have been reading and thinking, and I am pretty convinced that the problem has a good chance to come from the aftermarket radio module, or maybe even another module on the serial bus...
I saw you could try to remove the radio fuse, or even disconnect the radio module... but that's a pain each time you want to flash, and you even loose all the radio settings each time... and that might even not be sufficient and I could ruin the PCM.
I saw that generally, bench flashing seems to work flawlessly... like the safest way maybe of flashing... but need a bench harness and a pain to remove the PCM each time you want to change a parameter !!

As I do not want to take the chance to write PCM and create a problem, I though :
by the use of a 3PDT ON-ON toggle switch, why don't I recreate a "bench like connection in car"... please see below the drawing base on data line wiring diagram

in car bench like harness-POS B.JPG

In switch POS A, everything is wired stock

In switch POS B, The PIN 2 of DLC goes straight to PIN 59 of PCM, so there is no other modules on the bus than the MPVI and the PCM...
with 3PDT switch, even the secondary link between BCM and PCM can be opened (yellow wire) while flashing.

What do you guys think of this idea ?? Anyone tried this ?

Would that make for a safer setup to flash (if car battery voltage is good, laptop is good and USB connection with MPVI2 is good) ?

And from your experience :

=> would the powering KOEO of the car with the PCM missing on the class2 serial bus cause DTC everywhere ? and/or be a problem then ??

=> In my "b" scenario, is it a good or a bad idea to open the serial line between (yellow wire) PCM and BCM when flashing ??

Your valuable knowledge on this idea would be highly appreciated

Oups... another silly idea if the above is crap... how about using a real bench harness, but with the PCM still in the engine bay : pop plastic cover off, remove blue plug, plug in the bench harness blue plug, flash, and then back to normal... still not as convenient as flicking a switch... but would this work ?
what about the power supply difference inside PCM between blue plug from the bench harness (with its mains power adapter) and maybe some battery power still present on the green plug from the car (have not checked this) ?

Thanks in advance for your valuable feedbacks

Sincerely

Erik.

[2xLS1]

I see no reason why #1 wouldn't work.

[TheMechanic]

Yes that will work. We always had issues with Corvette programming at the dealer so Kent Moore made a great tool I have that plugs into the star connector and you select the module you want to "stand alone" program without removal. Works great. All it takes is one of the modules to "wake up" and you have the potential to brick the PCM/ECM. I agree with 2xLS1 in that I believe that would work just fine and you would have zero issues.

[Tahoe03Nea]

Hi gents,

thank you all for your answers.

I indeed thought it would be a "good idea", but was lacking experience and confidence to do it before asking here )

I had read the "switch on the radio adapter module wire" hack, which gave me the initial idea, but was worried that if any other module was causing a problem (I read on forums sometime the transfer case module or driver door module... as TheMechanic tells), it would still be trouble...

Then I discovered the splice pack, which I believe TheMechanic is plugging in the tool that lets him select the module you want to program as "stand alone"... and as it is my own car, came to the idea of wiring in a permanent switch that would allow to program the PCM as stand alone.
So looks great !

Still, I am wondering, TheMechanic, when you do so... are all the other modules, which are powered during the stand-alone flash, but not on the programming bus, so seeing that the PCM is not present, maybe triggering Uxxx DTCS everywhere ? or not?

Also, I saw the serial data wire, "in parallel" of the serial data bus, which just connects directly the PCM (pin 59) and the BCM...
For flashing as intended, should I indeed cut this link while flashing or leave it connected ?

Thanks for the valuable feedbacks.

Cheers
Erik.

[2xLS1]

If you do decide to do a bench harness directly to the PCM don't use any power supply that comes with it. Power it with alligator clips connected directly to the vehicle battery. That is what I will do if I can't get a 03-07 classic to read or write through the ALDL if an aftermarket radio/chime adapter interferes and pulling the radio and radio amp fuse doesn't work. And you only have to unplug the blue connector. You can leave the green one connected. Although sometimes I think there are times pulling the radio can be easier.
I also will check for something aftermarket tapped into the purple data wire on the ALDL.

[Tahoe03Nea]

Hi 2xLS1,
thanks for your message.
Yes indeed, silly me, you are right, for the bench harness plugged "on site"... just power it from the car battery !! It's so obvious I am kicking myself !
Thanks for the valuable info about leaving the green one connected.
Cheers
Erik.