i think i screwed up.....

[kewljay223]

so i think i may have fried a circuit on the pcm.... if anybody knows how the internal circuits work, i would like a confirmation if i did infact fry it...

i did not realize at the time that GM changed the resistance values in the sending units from 0-90 to the new 0-240... and on top of that, that the new style used a dedicated 5v circuit with reference vs the old 12v to ground style my truck originally came with.

i knew i had to feed the pcm the signal but i wanted to keep my gauge working so i spliced in the new 5v+ wire to the old purple wire (signal to gauge) thinking that would feed the signal to the pcm. and then took the old ground wire from sending unit directly to the gnd on the pcm for the fuel ckt. WAH-LAH, pcm gets signal and gauge should work right??? WRROOONNNGGG!!!! well sort of. the gauge would read fine when key on but would peg full when key off (since the pcm stopped supplying the ground). i knew something wasnt right but let it ride since the gauge worked when i needed it to.

well, i went to the ref and he told me i had a P063 code, which is fuel level ckt high voltage. my scanner would give me most codes but this is aparantly a gm specific which my software couldnt recognize so i didnt even know about it. well i did some research and found out about the differences in the resistance AND voltage and thats when it hit me that i was supplying a constant 12V to the pcm's 5v output!!!!

YES im a dumb ass for not researching that first and im pissed at myself for it. so i went ahead and rewired everything. old gauge is hooked up like original. i bought a sending unit for the newer trucks, gutted it for the sensor, made a braket for it, long story short, i now have two floats in the tank. one for the gauge and one for the pcm to see fuel level. i also took the time to put a second pump in there since i had it on hand and tank was out. thought i was all good to go.

well i upgraded my scaning software and i can now view all gms parameters including the fuel level and its at a constant 5v/100% tank full about 95% of the time. every once in a while i will see it drop to about 1.9v/35-40% tank full then jump back up to 5v. and its throwing p063 again. i measured the voltage between the 5v and pcm gnd and sure enough 5v. i measured resistance of the sender at 1/8th tank which gave me 47ohms and i filled up some, not full, and it reads 151ohms.

this tells me the float is working properly and the pcm is not sensing the change... which leads me to belive i fried the sensing part of the circuit with the 12v i had going to it. but it seems the 5v feed is still there...

so what you guys think? did i screw it up? if so, is there any way to open it up and fix it or is it pretty well shot? the rest of the pcm still works great. but i need this ckt functioning in order to pass emissions....

[kewljay223]

all these views and nobody has an idea how the pcm circuits work? tuners? electronics experts? im kinda needing to know if i need to pick up a new pcm...

[jeast]

as far as the level indication to the pcm i wouldn't worry about it too much, as long as it stays within the right parameters. U said u have the old float rigged up to run ur gauge right? Sounds like you fixed the big problem, good job on that! I wouldnt get to concerned about the pcm giving strange readings if u already have a guage solution. Who ever tuned ur pcm should be able to turn off that code so it don't keep coming back.

[kewljay223]

hey man thanks for the reply. your right it normally wouldnt be that big of a concern since i have my gauge working but i live in cali. i need the fuel level circuit working properly for the emissions stuff. specifically the evap tests. they will only run if the fuel level is between 15-85% or so. so if the pcm thinks its full all the time, then the test will never run. ive been monitoring the tests while driving and it keeps getting aborted. im assuming this is part of the reason...

im pretty sure everything works but i will be doing some more testing in a bit to confirm that everything is working properly.

[kewljay223]

ok well i found the damn problem... i am utilizing the connector just under the brake booster for the two wires to go through. so this is where i was taking my measurements and i decided to try putting a 100ohm resistor in the little sockets to see if i could get anything other than the 5v, and i noticed that the one for the ground was spread WAY open. i remember shoving a test meter lead in there a while back when i was doing some testing and was getting pissed off... LOL so i took the connector apart, pinched the lil socket closed again and plugged it in. BAM, scanner shows 1.6v with the sender at 128ohms... RIGHT within specs!!! that explains why i would get a reading every once in a while. the damn thing wasnt making a connection to ground so the pcm saw the full 5v all the time.

so YES my new float set up works and NO i did not fry the pcm... answered my own questions. sorry for the useless post. its been proven time and time again, when you cant figure it out, SLEEP on it!!! it will eventually work out.

while im on it though, scanner says 33% fuel which is obviously incorrect. it should be at least 50% but this is just the calibrations in the tune right?? im assuming the pcm still thinks its in an escalade with a bigger tank?

[jeast]

good find man! Im sure u will sleep better now.