"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