Gen IV E38 with crazy P0690 fault code-----A LONG READ, BUT PLEASE BARE WITH ME

[G.I. Turbo]

Good day people.

I have an interesting issue with a P0690 fault code in my 08 Tahoe.
I did a 6L Turbo (OLSD) build which I have been driving since last December without this or anything this severe ever happening. I have around 8000 miles on the build.

Original OS was 12617631, but changed to 1250216- for Speed Density

I jumped in it after lunch to flash an updated tune (which I've done many a time) and went through the routine key on, shut everything off, verify some 11-12volts, open the correct tune file and write accordingly. This was only a write calibration so nothing serious was being changed.

The write went as I thought it should. Shut off the key, open the door for 30 seconds, key on for 30 seconds, key back off once more just to be safe, open VCMScanner and start logging before start up. Went to start up engine and it turned over for a few seconds than quit. Nothing out of the norm after a reflash. But when I went to start it again, I got a click click, SERVICE AIR BAG, SERVICE 4 WHEEL DRIVE, P0690 fault, and some very unexplainable electrical issues. The 4WD selector light was no longer lit up, my AEM wideband (which is wired into the CANbus) now only reads "batt," and my Vacuum/Boost gauge light flickers like crazy. The strangest part being that I still have power to the boost gauge light (which flickers constantly) even after I key off and open up the drivers door. Which has never happened before.

I started by taking my multimeter and testing all the fuses in the dash and under the hood. All are good and have been double checked a few extra times. I have swapped relays into and out of the CRANK/RUN, POWERTRAIN, and STARTER CIRCUITS with no luck at all. I have checked the ground from the battery to the engine on both sides verifying a good engine ground (GOOD), checked several plugs throughout the engine against the battery ground to verify ground continuity (GOOD), checked all ecm/tcm connectors for anything unusual, pulled the entire under hood fuse pane out of the keeper and blew all of the connectors out (NO CHANGE), pulled starter out in case it got stuck in the "out" position and was causing a voltage or amperage issue (NO CHANGE), looked for loose, burnt, or cracked grounds elsewhere without luck. I checked the constant power at the CRANK/RUN relay as well as the ground which were both good. I also have an Apollo D8 scan tool that allows me to run tests and other functions, as well as check the theft deterrent status, fueling status, and all the other things you can check.The battery is an OPTIMA Yellow and is only a year and half old. And I charged it for an hour or so before diving further into the diagnosis.

Now for more confusion, randomly when I an fiddling around with the key off or on, it will cycle through the normal start up with no "service this or that" on the dash and the 4wd indicator will be lit back up. Try to start it and nothing. Right back to the previous situation.

I am unable to clear the p0690 code at all. I have read into the "permanent" code thing and have labeled that as such. May be wrong but I don't know. The scan tool indicates no "theft deterrent system" based fault codes being the cause of the NO CRANK/NO START issue. The key and security programming (as far as I can tell thru scan tool) all show system as good to go. I have done a Throttle sweep test, idle position relearn, checked the key and programming status as I was turning it and pulling it out for security issues, key fob test to verify no security issue. I tried to run a STARTER RELAY test thru scan tool, but it says FUEL ENABLE FLAG NOT SET FOR THEFT DETERRENT SYSTEM. Which is what led me down the THEFT/SECURITY SYSTEM checks. I have tried a "WRITE ENTIRE" back to the stock tune to see if that helped. And before that, I did a write calibration to the last known working tune with no luck, and right now I am at a loss for what to do.

I have heard the Fuse block, ECM, Battery and the battery cables could still be an issue too. I had even done the 30 minutes (key on for 10 minutes then off for 10 seconds and repeat) just in case. I am obviously trying not to just throw money at it, but I am about to start the purchasing game.

If any of you have any incite into this I would love to hear it.


Thanks and sorry for long read
**DISCLAIMER**
I KNOW THIS IS A LONG READ, BUT I WANTED YOU ALL TO BE INFORMED OF HOW THIS HAS
PLAYED OUT AND WHAT I HAVE DONE TO TRY AND REMEDY THIS**

[kevin87turbot]

The Service Manual indicates a bad battery or bad ground are likely culprits for the P0690. I've seen several issues caused by a bad ground cable. I would start there.

[G.I. Turbo]

I will give that a shot and see what happens.

Thank you

[G.I. Turbo]

Just a quick bit of closure. I went out this morning to check for voltage at starter (constant-which was good), and then starter solenoid (which clicked then failed to start), then checked the starter ground all with the test light. I had the test light glowing while checking the starter ground.

Indicated there was a faulty engine ground.

I went to the main engine ground from battery to passenger side front of head and tested for voltage. I got 8.8-9.3 volts testing from battery negative (negative lead from multimeter to battery negative) to the engine block by the ground stud on the cylinder head (red lead of multimeter to area near ground stud).

They were really bad looking or corroded, but I removed them, brushed the eyelets off the 2 grounds located there, then reinstalled them.

The gauges and trucked fired up immediately without issue. Reconfirmed with a few restarts and she was up and running.

I know this was a long read with only a few views, but this is a good lesson learned on the importance of your engine block grounds, and proper diagnosis. The only thing invested was time. And fortunately I had some.

Thanks for the help and I hope this may help someone.

[pcmcobra]

glad it was that easy