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Hi Folks,

I'm researching using a Tesla motor in my build and I was wondering if anyone had an itemized list of the differences between the Model S large drive unit and the Mercedes B-class Electric Drive unit.

I've scoured this forum for the term 'mercedes', and have read the great thread about the Tesla unit acquired from the Mercedes lab, but have come up pretty short. In Damien's thread about his Tesla control board, it was brought up that the electrical connector is a little bit different, and in the motor's stock configuration it doesn't use analog throttle/brake signals (favoring CAN messages).

Based on pricing I've seen from dismantlers, the B-class drive units seem to be going for lower prices. From a cost perspective, it feels like a no-brainer to use a Mercedes unit. Is there some gotcha that I'm missing here?

Scott
 

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I don't know if it is a "gotcha", but the B250e drive unit is turned around compared to a Model S/X large (rear) drive unit: the motor and inverter are ahead of the axle line (with the motor on the right-hand side), not behind it. That's not inherently a problem, and might be an advantage - depending on where you intend to install it - but it is very significant to packaging the unit in the vehicle.

Also on the subject of packaging - the charger and other electrical/electronic components are stacked directly on top of the motor, while in a Model S/X they are distributed around the car. This can be significant if using these components and the stock interconnecting cables.

Mercedes-Benz B-Class Electric Drive Under The Skin – Photos
 

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I don't know if it is a "gotcha", but the B250e drive unit is turned around compared to a Model S/X small rear drive unit: the motor and inverter are ahead of the axle line (with the motor on the right-hand side), not behind it. That's not inherently a problem, and might be an advantage - depending on where you intend to install it - but it is very significant to packaging the unit in the vehicle.

Also on the subject of packaging - the charger and other electrical/electronic components are stacked directly on top of the motor, while in a Model S/X they are distributed around the car. This can be significant if using these components and the stock interconnecting cables.

Mercedes-Benz B-Class Electric Drive Under The Skin – Photos

Great link! Thank you. This is the type of difference I was looking for that I wasn't picking up on from the diagrams. Most of the information I've been able to find about the Mercedes units has been from scouring diagrams on parts websites like this.

This is definitely a gotcha for where I would be mounting the motor as I have to work around a semi-trailing arm suspension. Is there any (gearing, bearing, oil pickup, coolant flow, etc.) reason why I couldn't mount the motor as it is mounted in a Tesla and run it in the "reverse" direction from how it's run in the Mercedes application?

And for clarification: In your example you mentioned the Model S/X "small rear drive unit". I was under the impression that the Mercedes unit was based on the large rear drive unit. Is this not the case, or is it more nuanced than that?

Thanks again :)
Scott
 

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The other difference that I've seen / heard of is the addition of a parking brake element on the Mercedes build. There was a video from Johannes Huebner somewhere where he talks about being confused about not being able to get the motor to spin and it was due to the physical parking brake being applied.
 

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Sorry I missed this way back when this question was posted...
This is definitely a gotcha for where I would be mounting the motor as I have to work around a semi-trailing arm suspension.
Most transverse drivetrains (with an engine or an electric motor) place the engine/motor ahead of the axle line, and that's a bad combination with a semi-trailing arm suspension. I don't think you'll find any production car with this configuration and suspension combination. It can be done, if the suspension arms are narrow enough, the drive unit is narrow enough, and the vehicle is wide enough; in this forum, CanadaLT28 is doing this in the rear of a truck.

Is there any (gearing, bearing, oil pickup, coolant flow, etc.) reason why I couldn't mount the motor as it is mounted in a Tesla and run it in the "reverse" direction from how it's run in the Mercedes application?
Yes - the pump for the gear oil, which is also used for cooling, will turn the wrong way. If you want the motor behind the gearbox, I strongly suggest buying a drive unit which is built that way (like the Model S/X, which is much more common than the B-Class electric anyway).

And for clarification: In your example you mentioned the Model S/X "small rear drive unit". I was under the impression that the Mercedes unit was based on the large rear drive unit. Is this not the case, or is it more nuanced than that?
"Small" versus "large" is mostly motor size; I have no idea if the B-Class unit uses the same motor or any other components as the Model S/X large unit.

In Model S/X drive units:
  • the large rear and small rear units are configured the same way, and are very similar other than component sizes (the motor, inverter, and likely gears); however, they are different in details of lubrication and cooling
  • the small rear and small front (which is the only size for the front) units are nearly identical, except that the front unit places the motor higher, to clear the front of the battery pack
With time, other Tesla drive units will become available, from the Model 3 and some day the new Roadster and even the Semi. "Tesla drive unit" - even qualified by "large", "small, "front", or "rear" - will not be specific enough.
 
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