The Vehicle:
2007 Silverado WT (NNBS/GMT900)
4.8L V8, 2WD
E38 ECM, T42 TCM
EBCM PN: 265 800 571

Some background (maybe relevant):
I've done a 4L80E conversion with 3200 stall. I used a 2008 express van T42 TCM for my trans OS and repined the harness per instructions I found on Nelson Performance (including making up an ISS connector). Both ISS and OSS have been replaced to ensure they are not the cause of my issue. The transmission works excellent and I have absolutely no problem with it nor DTCs of any kind. (speedo works fine and is +/- 1 mph according to GPS. Using a scanner, the ISS and OSS RPMs read perfectly and sync with lockup, etc.

The issue:
While driving around normally, no DTCs, no codes from ECM, TCM, EBCM...life is good. If I connect my scanner to the EBCM and log the front left, front right, and rear wheel speed, they all match (thanks to getting the pulses per mile settings right in the tune via hptuners software and a handy spreadsheet I made).
Come to a complete stop and take off with 1/4-1/2 pedal and hold it steady. At this point I observe the rear wheel speed drop to 0 mph and then a second or 2 later the EBCM throws a C0055 with reason code 5A. As soon as I let off the throttle the rear wheel speed signal comes back to normal. The entire time my speedo is fine. From that point on until the next stop, the signal is rock solid even at WOT.

I am using Autoenginuty's Giotto (latest version as of writing this) which displays the following:
What I've done thus far:
1.)Removed and cleaned EBCM ground (no luck)
2.)Removed EBCM connector and checked pin 31 for >10v (pass)
3.)Replaced both speed sensors on transmission (no luck)
4.)Replaced both front hubs+speed sensor assemblies (no luck)

Note: The truck is 2wd with drum brakes so does not employ rear wheel speed sensors, instead uses the VSS for rear wheel speed then converts to 128K pulses per mile for EBCM.

 

Still no luck with this...

By process of elimination, I believe I'm down to either a bad EBCM, an EBCM in need of updated code (not sure if this is possible), or some sort of wiring related issue between EBCM and PCM.

When scanning the PCM directly, the VSS never drops out
when scanning the TCM directly, the VSS never drops out
when scanning the EBCM directly, the Rear wheel speed drops out when doing a hard start from a stop as described in the original post.

This tells me that the VSS sensor and PCM interpretation of it is fine, but the converted 128K pulse/mile serial signal is somehow being lost during that hard launch scenario for enough time to set the code. I noticed if I stay on it during the hard launch the rear wheel speed in the EBCM shows 0 mph for as long as I hold the pedal. The moment I let off, the ABS light is on (no Brake light) and the speed comes back and is fine again till I make another stop and do another hard start.



C0055 5A datalog

 

Circuit 1827 ye/bk x1 pin 57 at ECM and pin 31 at EBCM is the source of your issue. Before condemning the EBCM I would look up PIT4815c and or Doc ID 3058094. This will provide better testing methods than the unloaded circuit test for more than 10 volts. If it was me a would make a temporary overlay and eliminate that circuit's wiring altogether. Based on the info you provided and what I can see in GM's service information that would be the quickest and most effective approach to determining the failed component. My gut tells me this is a wire or terminal being forced to move while under load. This may sound nuts but will the rear signal drop under the same load in reverse? The drive line should load up in the opposite direction and may not stress the circuit the same way. Just my thoughts.

 

Great idea on the reverse test. I performed that one today and observed the same exact behavior as shown in the graph above. (signal drops out long enough to throw the C0055 then comes back.

I also disconnected the EBCM harness and tried to figure out how to get the back of the connector cover off but did not want to break it. I do see a yel/blk wire as well as a yel, yel w/black twisted pair so I wanted to confirm which is going to pin 31. No luck just yet. I guess I could shave the insulation off the wire at that part and then find 57 on the ECM and do the same for an overlay/jumper wire. I just kills me to cut insulation off nice clean harness wires. I guess I need to get over it and just do it.

The problem just seems so programmatic in nature. I literally cannot make the issue repeat after I've started rolling. I can floor it and and jam the brakes and then hold the brakes while already floored and the signal is solid. Its only doing this when my stall speed is very high from a dead stop.

Unfortunately I dont have any access to the PIT4815c or DOC ID 3058094 and not sure how to look for that info. I will have to reach out to a buddy of mine who works at a GM parts counter to see if he knows how.

 

I'll post them up later today.

 

You had to add the iss right? Can you kick this code while sitting still or does it need to see the front wheel speed signal to kick the code?

 

Yes i had to add the ISS since the 4L60e didnt have one.

I can only get the code to kick when moving (front wheel speed > 0mph)

 

Well GR8TALK, you were right about the problem NOT being with the EBCM. I replaced it with an identical part number from another truck (junk yard) and I still have the same exact problem.

If you are able to help me locate any of the documentation on how to do a more thorough testing, I'd greatly appreciate it. I suppose my next step should be the circuit overlay you suggested in your second post. I'll report back what I find once I get a chance to do that.

 

My bad man. I'll get it posted tonight. I'm at the track right now and need my laptop. You might be able to google the #s. That might show you something. I'm still of the opinion you should overlay that wire to eliminate it. I have seen the some crazy stuff happen with Lin bus wires acting stupid.

 

The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.
<a id="d717e34">Condition/ConcernWhile diagnosing a Vehicle Speed Sensor (VSS) concern you may encounter one or more of the following issues:

1. Difficult to follow the speed signal path between the components. Shown below are the two typical VSS paths depending on the vehicle configuration.
2. ECM showing correct MPH in the data list, however it is setting a DTC for the VSS (example P0502). The ECM can get MPH data from the TCM over GMLAN which may mislead your diagnostics. The VSS hard wire inputs (821, 822 and/or 6399 circuits) will need to be checked using the Fluke 87 meter-DO NOT rely on the Tech 2 MPH data as an indication the ECM is getting the hard wire VSS inputs.
3. IPC speedo working correctly when there are ECM or ABS DTCs for the VSS (example P0502, C0055, ext.). The ECM can get MPH data from the TCM over GMLAN and pass it along to the BCM, and the BCM to the IPC. DO NOT rely on the IPC speedo working as an indication the VSS hard wire circuits are good. The VSS hard wire inputs (1827, 821,822 and/or 6399 circuits) will need to be checked using the Fluke 87 meter.
4. ABS MIL on with EBCM DTC C0055 and no other customer complaints. Since the MPH data can be sent over GMLAN, using the TECH 2 to help narrow down the area of concern can be misleading. The VSS hard wire inputs (1827, 821, 822 and/or 6399 circuits) will need to be checked using the Fluke 87 meter.
   
   
   

<a id="d717e67">Recommendation/InstructionsTypical Fluke 87 meter reads for each of the VSS hard wire circuits:

• 821 and 822 from the VSS. This is an analog A/C signal with a HZ reading of approximately 25 HZ per one MPH (example at 10 MPH typical read may be 250 Hz). Connect the Fluke meter leads between circuits 821 and 822 and set the meter to A/C voltage and then press the "HZ" button.
• 6399 from the TCM. This is a digital signal with a typical HZ reading of approximately 25 HZ per one MPH (example at 10 MPH typical read may be 250 Hz). The TCM is sending out approximately 12 volts on this circuit and also toggles it to ground to create the signal. Connect the Fluke meter red lead to circuit 6399 and the black lead to ground. Then set the meter to D/C voltage and press the "HZ" button.
• 1827 from the ECM to EBCM. This is a digital signal with a typical HZ reading of approximately 25 HZ per one MPH (example at 10 MPH typical read may be 250 Hz). The ECM is sending out approximately 12 volts and it also toggles it to ground to create the signal. Connect the Fluke meter's red lead to circuit 1827 and the black lead to ground. Then set the meter to D/C voltage and press the "HZ" button.
• 817 is a digital signal with a typical HZ reading of approximately 1.1 HZ per one MPH (example at 10 MPH typical read may be 11 Hz). The EBCM toggles the voltage on this circuit to ground creating the signal. Connect the Fluke meter's red lead to circuit 817 and the black lead to ground. Then set the meter to D/C voltage and press the "HZ" button.

<a id="d717e95">Warranty InformationFor vehicles repaired under warranty, please use the appropriate warranty labor operation based on the actual cause and repair. The latest version of bulletin 10-00-89-005: "Warranty Administration - Revised Wiring Repair Labor Operations and Required Additional Information" can be used as a reference.
Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.