"Just replace the part" was not good enough for me!
I recently had the chance to go through my old tablet from my former job (long story) I had a bunch of cool scope captures on it that I did not take with me when I left. Some of these captures have some cool stories behind them, I would like to share one of them with you today.
The vehicle is a 2004 M3 that is in almost pristine condition. This vehicle is the customer's baby. It came in for a check engine light and complaint of major hesitation when first taking off when cold. After the first or second shift, the vehicle cleared up and was all better.
When I scanned the vehicle for codes, I had a code for tps 1 and 2 correlation (apologies, this was over a year ago and I did not save the code scan). Intermittent throttle problems are the bane of career. I usually have trouble seeing any issues, no matter how hard I look. I wasn't going to let this stop me though. I was on a mission. It was time to break out the scope and get to work.
First I would need a diagram.
At the throttle actuator, pins 4 and 1 are the signal return wires. I probed into those wires, and found a suitable ground. I was using 2 channels on my scope on a long time base of 1 sec per division. I operated the throttle full open to close many times and saved my recording
For the most part, it looked OK. The voltages were mirror images of each other and I had no major dropouts, However I did notice something odd...
The Red trace had some deeper humps in it than the Blue trace. Could this be anything? I needed to find out.
I decided to run a math channel and invert the Red trace.
OK, now we have a Brownish-red trace that is the mirror image of the Red trace. I don't like too many things on my screen at one time, I decided to shut off the red trace to clean things up...
Now I can lay the traces on top of one another. In theory, these should be identical.
When I zoom into where the humps were, you can definitely see a difference.
Let's put a little filtering on there to clean this up
Wow!!! OK, there is without a doubt a difference now
Now, we can measure the difference...
We have 80.3mv of difference between tps 1 and 2. For this vehicle, this was enough to turn on the light and freak out the DME causing a hesitation.
I replaced the throttle actuator, scoped it again, and it was perfect. A cold soak and test drive in the morning confirmed the fix. I did not save the captures of the fix (sorry), but it was perfect.
I had my boss, coworkers, and M3 "experts" telling me to "just replace the actuator" but could not tell me why. I'm not that type of guy, I need to see a failure before I recommend a repair. I'm glad I took the time to verify the issue.
Thank you for reading.
*wiring diagram snip courtesy of Alldata
Great story, Chris! If you still have psdata file, I can show a cool math trick with this dataset.
Nice capture. It would be nice to know what the code set criteria was for the code.
It wouldve been. I took this a year ago and forget the exact code. All I remember is the symptom and basics of the codes. I just figured it would be a good story and a cool technique
Any time I start trying to diag a problem from a code, I always try to find the code set criteria. GM is great for giving that information, other manufacturers, no so much. With that information, it makes the use of waveforms and test data a little easier, and it also makes it easier to put the vehicle in the operating conditions to make the code set. An example, if the code only sets at idle…
diag.net/file/f6c4gmdgy… The DME was comparing bank 1 &2 and looking for either a greater than 4% difference on throttle closing, or greater than 10% difference on throttle opening. In this case as little as 20mV difference could have set the fault. Nice work, thanks for sharing
Thanks!! I wish I had that chart while writing up the case study. I was sweating bullets before I pressed the post button. I was assuming I was going to get eviscerated for lack of information.
Thank you for sharing Chris, great case study and technique! To the less experienced (myself included) scoping the signal wires and getting that mirror image, they look great. 80mv will make the difference in this circuit. Again thanks for sharing.
Great use of math channels and filtering. So many awesome ways to use that tool.
This was a great case study. Thank you. Helps me stay On the right path
Chris what I want to know besides the great case study here is what did you tell these coworkers of you when they told you just replace it? I go through this same deal.