2006 Trailblazer SS Loses Speedometer

This 'Blazer belongs to my shop owner. Last year, I pulled the engine to install a lumpy cam, headers, torque converter and some other racecar parts I can't remember. :silly: Tuned by a local shop.

Since the work was done, the owner has noticed that the speedometer will stick at 0 MPH during a WOT acceleration from a stop. The speedometer works perfectly under all other conditions. As soon as you get out of the throttle from a stop, the speedometer needle immediately picks up the speed you're at, and starts indicating correctly. To restate clearly - You can WOT the truck all you want once you're moving, and the speedometer will work correctly. ONLY from a stop is there an issue.

So there's gotta be some codes, right? Nope. :lol:



I decided to start at the ICM. Maybe there's something wrong with the gauge itself? :huh: Wouldn't be the first time on a GM.



The data doesn't paint a perfect picture, but you can get the idea. At about frame 1035 I got WOT. I'm rolling down the street until I let off around 1050. The speedometer didn't indicate anything because there was no speed signal to indicate. :blink:

Well ain't that interesting. Next I decide to kill two birds with one stone by pulling out VCM Scanner. I go this way because it'll allow me to graph the Vehicle Speed signal from the PCM and the wheel speed signals from the ABS simultaneously. It also allows for more graphing options, resulting in a better overall picture of the problem.



You can see the front left wheel trying to spin. :lol: But the vehicle speed signal doesn't catch up with reality until the throttle is lifted.

So, what would be your next move? I honestly can't decide if this is a genuine problem, a side effect of the programming, or just how these SS's behave. I haven't done much in the way of research on how/where the VSS comes from, so feel free to take me to school as needed.

Tyler wrote: So there's gotta be some codes, right? Nope. :lol:

Try a different scan tool. No way does a Failblazer only have two codes set :silly:

Tyler wrote: Next I decide to kill two birds with one stone by pulling out VCM Scanner. I go this way because it'll allow me to graph the Vehicle Speed signal from the PCM and the wheel speed signals from the ABS simultaneously. It also allows for more graphing options, resulting in a better overall picture of the problem.

That one is interesting. VSS does register at the start of the pull then drops out when the vehicle lifts off the throttle. Kinda looks like the PCM is reacting to implausibly fast acceleration caused by all the extra horsepower :lol: Lifts off a bit to see if that drops the accel into believable territory then ignores VSS because it's still too high.

If it's only from a dig, then it must be because the front wheels are coming off the ground when that thing launches.
He must be granny shifting and not double clutching like you should.
Try tuning the wheelie bars to get the front end back on the ground faster.

Not sure if your data is from pcm or cluster or if it matters but from what i see with it dropping on acceleration from a stop. I think the wiring between sensor and computer is getting pulled or is broken internally.
Now just need to determine which sensor sends the mph info for pcm to ipc.

The VSS looks like it's in the output shaft, 2 wire AC generator.
Feeds the ECM, which relays the information to the cluster over the network for speedometer and odometer operation.
Should be easy enough to scope the signal at the ECM and compare to data.

Maybe some obscure AC ripple interfering with the VSS?

Matt T wrote: Try a different scan tool. No way does a Failblazer only have two codes set :silly:

Right? Actually, it does have P0171 and P0174 set due to... I dunno? :silly: I wanna point the finger at whoever did the tune, but that seems like a cop out.

That one is interesting. VSS does register at the start of the pull then drops out when the vehicle lifts off the throttle. Kinda looks like the PCM is reacting to implausibly fast acceleration caused by all the extra horsepower :lol: Lifts off a bit to see if that drops the accel into believable territory then ignores VSS because it's still too high.

See, I keep coming back to this being a programmed response. I just can't find any documentation to prove it...

He must be granny shifting and not double clutching like you should.

You just couldn't resist. :silly:

The VSS looks like it's in the output shaft, 2 wire AC generator.
Feeds the ECM, which relays the information to the cluster over the network for speedometer and odometer operation.
Should be easy enough to scope the signal at the ECM and compare to data.

Maybe some obscure AC ripple interfering with the VSS?

I'm really not trying to call you out, but do you have anything that says the VSS is wired to the PCM? 'Cause I'm trying to get a straight answer out of service info on how this VSS is wired. I really hate how GM's vary their VSS signals based on engine/trans/T-case/day-of-the-week.

What I found so far says that the VSS is an input to the TCM:



The network diagram says the only way the PCM and TCM talk is over the HS-LAN network. So, I'm assuming that's how the VSS signal gets communicated:



THEN, the VSS is sent by the PCM to the Radio, ICM and Air Suspension compressor:



From what I understand, this isn't a data line, but a pulse width signal based directly on vehicle speed:



Does this seem like a needlessly complex system to anyone else? :silly:

Not sure if your data is from pcm or cluster or if it matters but from what i see with it dropping on acceleration from a stop. I think the wiring between sensor and computer is getting pulled or is broken internally.
Now just need to determine which sensor sends the mph info for pcm to ipc.

Sorry, I should have specified. :blush: The first picture is from the IPC. The top graph in the second picture is from the PCM. The bottom graph is from the ABS.

Here you go
www.tbssowners.com/threads/speedo-not-working.95202/

I'm assuming the transmission is also modified. Looks like you will need to have 2 ecms wired in to make it work.

ok after more checking you may also be able to use a speedometer interface from dakota digital and bypass the ecm in the speedometer signal.
Input ac voltage into the speedo interface. output digital signal to IPC.

gav09 wrote: Here you go
www.tbssowners.com/threads/speedo-not-working.95202/

I'm assuming the transmission is also modified. Looks like you will need to have 2 ecms wired in to make it work.

Many thanks, gav09. This thread was very enlightening. I Googled this problem early on? :huh: Probably right in front of me the whole time.

The trans isn't internally modified, only the replacement torque converter. I don't recall the brand/specs. But everything in that thread seems to suggest the TCM doesn't like the change in stall speed over stock, which somehow causes its VSS calculation to fail. Good stuff.

It was also comforting to see that my interpretation of the VSS signal system was correct.

I gave the options to the owner. Source another ECM and have me wire it in to generate the right signal for the IPC, or get a standalone box to do the same. He opted not to worry about it. :lol:

Thanks to everyone that replied!