No OBD2, Fuel Gauge, and RPD; No Start... halp


Goblin Guru
After my engine rebuild my RPD was turning on and off intermittently. I could still turn the vehicle on without any issues but decided it was time to look into it. I noticed when I pressed the passenger kickplate it would turn on, which is odd. Usually the car would start with it off, then turn on some time later. I paid for the wiring service so I'm fairly dense when it comes to this stuff.

During my due diligence (aka unplugging and re-plugging, moving stuff around) I somehow lost access to my OBD2 reading, my fuel gauge isn't working, and my RPD has a backlight but never turns on. This sounds just like what @Mayor West was going through (here and here).

All signs point to it being the Tan wire and or a data wire but I don't know how to approach testing this given the interplay between the BCM and ECM plugs and pinouts. Any ideas? Where do I start? Any help is appreciated.


Goblin Guru
Take off the BCM connectors that have the GM high speed LAN wires (tan & tan/black, brown & brown/black)
Blow them out with compressed air... maybe electrical connector wash/brake cleaner wash if it doesn't dissolve the plastic.
Put them back on.
Do the same at the power steering.
Probably the ECM will be good, as that connector is water proof/dust proof.


Goblin Guru
Thanks @Ross ! It seems like you'd focus more on the high speed wires (Tan/Black #42,Tan #55, Brown #34, and Brown/Black #?) as opposed to the low ref Tan ( #9 & #19 ) ? These wires are found on the larger of the two ECU plugs?

Can anyone help with how the power flows from the battery to and from the BCM plugs / ECU plugs / fuses / relays that impact the obd2 ? I'm stumped on where I should look first to test continuity and voltage. All of my fuses look good, the relays are clicking and I've swapped a few to test. The BCM plugs wires look fine, same with the ECU plugs. I did a key relearn even though it wasn't showing signs of needing it.

Could I have bricked my BCM?
Could it by the Fuel Level Sensor in the tank? #15 and #19 .... how would I test that circuit?


I tested the continuity between the Clutch Pedal position connector plug Tan and then Fuel Tank Pressure plug Blk/White and there was continuity.



Goblin Guru
Based on Rosses post (here) it looks like the signal goes from;

battery > starter > power post on fuse box > big black plug in fuse box > big plug on the ECU > Small plug on ECU (?) > Fuel Pump plug/switch > BCM via X1 plug (?) > Power Steering > OBD2 ?


Goblin Guru
You started with an intermittent issue. Pressing on the passenger kickplate would turn it on, which is pressing on the wires to the BCM.
So those clues lead me to think the issue is at the BCM.

With my GM high speed LAN data issue, I loose signal to the gauges (fuel, RPM, speed), but not the turn signals on the instrument cluster.
Sometimes I loose the power steering along with the instrument cluster, and the OBD2 port is after that, so it is intermittent too.

Are your turn signals working on the instrument cluster? If not, maybe it is a power issue. But if those work, it sounds like a data issue.


Goblin Guru
Great thanks for sticking with me. The RPD would turn on with the kickplate press, the gas gauge/starting issue is brand new. That gives me a good frame of reference with the turn signals as they do work and nothing changes if I'm in AUTO lights or not. I get power to the instrument cluster for the backlights and see the odometer/temp and 'Low Fuel' digital readouts. The cluster needles all rise a little to 0 (Fuel Gauge to 0 as well). The VATs light comes on and then drops off. The CEL is on but it has always been on - it's unfortunate I can't read it. I have tested the ECM and Cigarette Lighter fuses and they each get 12v to one side.

Your post here is exactly my issue, although it has never been intermittent at this point - and I don't get OBDII for HPTuners:

Copying from that post for easy reference:
OBDII pin 6tan/black stripHigh speed GMLAN serial data +Goes to Power Steering, C2, pin 1 (C2 is the grey, 6 pin connector with the small wires)
OBDII pin 14tanHigh speed GMLAN serial data -Goes to Power Steering, C2, pin 2
Power Steering, C2, pin 4brown/white stripHigh speed GMLAN serial data +Goes to BCM, J2, pin 1 (J2/C2 is the white & blue vertical connector closest to the center tunnel)
Power Steering, C2, pin 5brownHigh speed GMLAN serial data -Goes to BCM, J2/C2, pin 19
BCM, J2/C2, pin 2tan/black stripHigh speed GMLAN serial data +Goes to PCM, C1, pin 1 (C1 is the blue connector on the PCM)
BCM, J2/C2, pin 20tanHigh speed GMLAN serial data -Goes to PCM, C1, pin 2

I do not have power steering, and have connected the wires as follows, which has been working for a very long time now. The Brown = Tan sees 1.5v with the key ON and the Brown/White = Tan/Black sees 3v with the key ON

brown/white = tan/black
brown = tan
red = nothing

Based on this post (here) I should be able to test Pin 1 & 19 (signal in) and Pin 2 & 20 (signal out) on the BCM and Pin 42 & 55 on the ECM plug. Any idea on the voltage or resistance? I read a previous post from Lonny where he said there should be continuity between Tan/Black OBD2 and Tan/Black ECM as well as Tan OBD2 to Tan/Black ECM. I'll look into this as well

Thank you so much @ctuinstra for laying this out!
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Goblin Guru
My GMLAN wiring map is correct for my 2006 SS, but might be a bit different for your 2010 SS.
The GM high speed LAN is passing data at 500Kb/second, and even when my instrument cluster isn't working, I still see data flowing on the GMLAN. Probably the BCM requesting data from the ECM.

I'm not sure what voltage or resistance you would get with a meter. Measuring voltage with a meter doesn't tell us what data is being requested and answered. You could buy some hacking tools that read CANBUS, and convert it to USB, then start learning what the data is... but there is probably an easier solution to our electrical issues.


Goblin Guru
Yea seems like a lot of unnecessary work and yep i'm finding the pinouts and plug names to be different from LSJ and LNF's.

The vertical J2/C2 plug on the LNF is X2. I was reading dc 2.4v on pin 20 Tan with the ignition on and ~1.4v with it off,. Pins 1, 2, 19 were moving up and down which I assume is the transfer of data? - it did this with the key ON or off. I don't know how to make sense of all this.

I tested continuity between the OBD2 Tan / Tan & Brown wires and the larger ECM plug #42 and #55 and there was no continuity.

Next up is testing continuity between ECM Plug Pins #42 / #55 on BCM X2 Pin 1 and Pin 19 - this should tell me if there's a fault between the two plugs.


Goblin Guru
While I doubt this is your problem as things we working well before, but you removed the 120 Ohm termination resistor when you removed the power steering unit. You essentially created an impedance mismatch which can cause strange things to happen.



Goblin Guru
Brian, are we suppose to add back a 120 ohm resistor? I haven't done this.
If the stock 120 ohm resistor is in the power steering unit, then it will still be in my car.
Did Optimus remove the stock power steering?


Goblin Guru
I would assume that he removed the entire power steering unit. Normally we don't mess with BN/WH and BN wires that come out of the power steering unit and run over to the BCM. So either he removed the entire unit or is making an extra connection with BN/WH and BN wires to the TN/BK and TN wires. That could cause a problem too.



Goblin Guru
Thank you gents, I really appreciate the help - I got it sorted!

The RPD has a pigtail that plugs directly into the screen and then into the harness and completes the circuit. Well, I was only checking (and rechecking) the harness plug side of the pigtail shown at the bottom of the pic, when it was the plug that goes into the screen that was the culprit. I assume me pressing on the kickplate was juuuust enough to move the rpd plug and make me believe it was something else. Feeling slightly dumb but what else is new...

I blew out every connector and taped off the large opening on the BCM plugs around the blue caps, same with the ECM plugs. The BCM plugs were unexpectedly dirty for being what and where they are. Ross I know this doesn't help your situation, but after going through this I'd put the focus on the 2500 & 2501 circuits (sorry if that's obvious and you've already beat it to death).

I did totally remove my power steering and rewired as shown above, I will come back through and put a 120ohm resistor on. Thanks @Briann1177 !

Here's the path btw

OBDII port
EPS (electronic power steering)
BCM (body control module)
RPD (realtime performance display) turbo only option
In and out of the dash harness to body harness connector
TCM (transmission control module) auto donors only
ECM (engine control module)

This was helpful to visualize what's going on - pins 1 & 19 on X2 in, 1 & 20 out to the RPD input pins E & F and out on A & B to complete the circuit