This eNewsletter is the second part of two on vehicle diagnostics by a Mercedes-Benz technician using the DAWN OBD Mini Loggger. His customer complained that while accelerating slightly on the freeway, the vehicle would demonstrate abnormally high vibration. Here the technician describes how he solved the problem.
We suspected that the friction coefficient of the clutch material degraded and was not able to stay engaged, or possibly the torque converter was losing actuation oil pressure for the clutch causing the high frequency shutter during acceleration. Comparing the two plots below, the turbine wheel shows RPM fluctuation, and at the same time the accelerometer on the logger picked up the vibration.
The RPM fluctuation of the turbine wheel indicated that the vibration is on the transmission side rather than the engine side. The plot of the accelerometer is just an additional value that correlates with the RPM fluctuation and confirms that the two measurements are related and visualizes the vibration. (As technicians….we need visuals.)
There were no drop outs on the torque converter actuation side and no antifreeze /coolant contamination in the transmission fluid. Also, the vehicle was steady in 7th gear so we can rule out any clutch packs responsible for the various gears since no gear shift took place. That left the torque converter clutch as the only culprit.
The replacement of the torque converter did restore normal operation.
Plot showing turbine RPM fluctuation and vehicle vibration
*Click image to enlarge*
The upper plot shows the problem of an intermittent vibration: high frequency shutter during light acceleration at freeway speeds using HEM Data’s DAWN Mini Logger. This definitely points the finger at the torque converter or its actuation circuit since engine RPM goes up during light acceleration but the turbine speed does not follow. The higher vibration level measured by the accelerometer corresponds to when the problem occurs. The lower plot shows the results of the torque converter repair.
