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Your car has a smart charging system that saves gas by limiting battery charge.
Once a while the voltage control quietly disfunctions and then toast the prefuse module with (90A + 25A x 14.9V =) 1700.Watts.
Once that keeps repeating the tinned bras is oxidized into a poor conduction that drops more or less voltage based on system load.
None of the Mercedes specialist deliver cheap repairs. So DIY'ers do their own maintenance when possible. You should collect data to see why your engine is has bad days.
Last edited by CaliBenzDriver; Apr 25, 2026 at 03:53 AM.
Your car has a smart charging system that saves gas by limiting battery charge.
Once a while the voltage control quietly disfunctions and then toast the prefuse module with (90A + 25A x 14.9V =) 1700.Watts.
Once that keeps repeating the tinned bras is oxidized into a poor conduction that drops more or less voltage based on system load.
None of the Mercedes specialist deliver cheap repairs. So DIY'ers do their own maintenance when possible. You should collect data to see why your engine is has bad days.
Ok scanners cost more than a tank of gas. Luckily it is free to display 'battery sensor" data.
Battery sensor displays both voltage & current
See youtube for video on how to enter "shop menu" before starting your engine.
> Test drive steps...
set display to show battery voltage
drive conditions: non-stop 30mn daytime
watch normal voltage steps: 14.9... 13.7... 12.6V
Pay attention that running voltage never gets below 12.5V with high Amps greater than 25Amps
Report testdrive : pass/fail.
Did you see low voltage below 12.5V... 12.3V.... 11.9V.... 11.7V drains both of your batteries?
High currents oxidize the prefuse pathways with high heat from drop voltage.
Last edited by CaliBenzDriver; Apr 25, 2026 at 02:02 PM.
Ive discovered that foot placement on the accelerator makes a difference, higher up on the pedal during hard acceleration does not buck, but lower on the pedal it bucks.
Im curious if anyone else has had the intermittent problem with their w212 bucking between 1st and 2nd gear when hard accelerating. When i bought the car two years ago the dealer said the transmission was brand new a year prjor to me buying. The bucking only happens between 1st and 2nd gear, doesnt do it every time, maybe only with the 1st start of the car for the day, sometimes it doesnt do it at all.
Gear between 1st and 2nd mean you are going very slow.
These cars start on the 2nd gear in ECO (OR COMFORT) mode and if you step on it hard in slow speed the car will change to 1st gear and it may not be that smooth operation especially with cold transmission.
Easy test.
With cold tranny set it in SPORT mode that should set it to the 1st gear and step on it to see if the "bucking" still happens?
Gear between 1st and 2nd mean you are going very slow.
These cars start on the 2nd gear in ECO (OR COMFORT) mode and if you step on it hard in slow speed the car will change to 1st gear and it may not be that smooth operation especially with cold transmission.
Easy test.
With cold tranny set it in SPORT mode that should set it to the 1st gear and step on it to see if the "bucking" still happens?
cold tranny in sports mode did not buck during hard acceleration, first start of the day.
My old 2010 E550 and 2012 S550 both start with second gear in Comfort mode. Has to use Sports to use first gear.
Obviously MB has changed this for newer cars.
My old 2014 E350 used to always start in D2 in Comfort mode as well for 50kMi.
I thought that was the normal way of ECO... 5th gear at 20mph then better use the S-sport mode for city course.
Until one day engine started to run well and now Comfort tranny always starts in D1 by itself without any firmware change then automatically shifts the right gears around 2000.Rpm.
Last edited by CaliBenzDriver; May 7, 2026 at 03:17 PM.
Mercedes SLR McLaren 722 S Is Extremely Rare Example Modified by McLaren
Slideshow: A one-of-one U.S.-spec Mercedes-Benz SLR McLaren Roadster became even rarer after a factory-backed transformation at McLaren's headquarters.
