What the ??????

This weekend I was working with a friends truck several hours away from home. He was thinking about lettting me do a tune on it so I read the thing and then we took the thing for a drive. as i had never worked with a 5.3 before i decided to log it before I did anything with it. in the course of logging I plugged my charger in cause the laptop was low and my scan tool promptly crashed. Well ok restarted the scan tool and it worked fine, but then at the stop light the truck dies! It started fine but after then it wouldnt idle. He drove it around and after a while it would idle real slow and eventually it returned to normal. It works fine now but what the flip happened! He was sitting there looking at me like WTF did u do to my truck. I have a good charger so i didnt think that was the problem but im stumped.

Hi JeromeGTO5.7,

First, welcome to the forum...:cheers:


It sounds to me like your charger introduced a "ground loop"... a ground loop occurs when you connect grounds which are at different potentials together, resulting in a large current flow between the grounds...

Your PC/laptop already had a ground connection to the PCM/vehicle ground via FlashScan... then the charger introduced another ground connection between PC/laptop and vehicle ground, except the charger's ground is floating at some potential that is different than the ground via FlashScan... so you have a ground loop...

it seems the effect of a ground loop is that it causes the voltage drop across the PCM's power/ground inputs to be less than 12V, this effects how the PCM functions and even alters which rows/cols are used in the injector voltage offset table.

Joe

Thanks for the welcome I kinda figured the plugging in the laptop had something to do with it. Well at least now I know not to do that again. do you think that i messed anything up permanately? What weirded me out was that after i unpluggged everything it still acted that way for about an hour. And can i safely run a charger at all when i have flashscan hooked up? I read the stuff about the inverters and all but i thought a good charger was ok.

I don't know why the symptom persisted for an hour... maybe some circuit in the PCM latched up... the PCM is fairly robust, it has various forms protection on the in/out-puts (I don't know what exactly)...

If the vehicle now drives ok and you don't get any DTC's then the vehicle/PCM is probably ok.

Use laptop charger if and only if your laptop is not connected to vehicle...

Same thing applies with other test equipment... for example, avoid probing the 115VAC voltage at the wall outlet when the test instrument (DMM/scope) is being charged by 115VAC power... this makes smoke in a hurry.

Thanks Joe I guess i've learned a lesson. I tell you what there is nothing cool about making somebodys vehicle run like crap and having them sitting there wondering wat the flip you did.

Yeah, I know... "Hey, watch this..." and then you smartly proceed to get egg on your face... I know... :)