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Bolt EV: Drive Unit Deep Dive

While batteries get a lot of attention, drive units in electric vehicles don’t get as much love. Why? Because many think they’re simple and all the same. An electric motor turns a reduction gear of some kind, which turns the CV axles, which turn the wheels and makes us go. It’s just a few moving parts, right?

While batteries get a lot of attention, drive units in electric vehicles don’t get as much love. Why? Because many think they’re simple and all the same. An electric motor turns a reduction gear of some kind, which turns the CV axles, which turn the wheels and makes us go. It’s just a few moving parts, right?

But in our quest to look at things that make us go, we shouldn’t overlook the drive unit. They’re actually not all the same. By learning about the differences between different manufacturers’ approaches here, we can learn useful things. As a driver, knowing what makes the car tick gives you more control. As an EV enthusiast, you can learn more about what makes them tick. Finally, if you’re an investor, it can pay to know what the different companies’ philosophical approaches and supply chains are.

All of this makes what Weber State University does a real treat. In the past, they’ve released several EV drive unit disassembly and deep dives on their YouTube channel. Now, they’ve done the Chevy Bolt EV, for the 2017-19 model years. This video is part of a series on a Bolt that they’re adding DC fast charging to (why GM sells any without this is anybody’s guess). You can see the rest of the videos on the project here.

Here’s the video, but scroll further down to read about some of the highlights, context, and things to look for.

One of the first things the professor points out is that the drive unit is built in one piece like a Tesla, as opposed to the Nissan LEAF, where the electric motor can be unbolted from the gear reduction and differential.


The Bolt EV’s electric motor makes less torque than the Spark EV (the compliance car predecessor to the Bolt), but has a much lower final drive. The result is that the Bolt EV has much more torque at the wheels than the old Spark did.


The Bolt EV pulls a lot less amperage from the batteries than the Spark did, but with better performance. This means it’s much more efficient. GM is continuing to improve. Also, the Bolt has a higher “redline” speed for the electric motor than the Spark did. This allows the lower gear ratio to not result in a car with a super low top speed.


This drive unit fits in well with the rest of the GM lineup.

The drive unit uses common fluids with other GM vehicles. It uses typical GM automatic transmission fluid, and uses the same coolant as GM gas cars do. (4:45) The drive unit is numbered like other GM transmissions. In this case, it’s the 1ET25.

  • The 1 represents the number of forward gear ratios
  • E is for electric drive
  • T identifies it as a transaxle (longitudinal transmissions for rear-drive gas cars use an L here)
  • 25 is the relative torque rating.

This system has been in use for decades, with only one digit added for the electric drive.


ATF drain/fill procedure:

The transmission fluid fill plug is near the top by a blue electrical connector. Two drain plugs are on the bottom of the drive unit, on the driver and passenger side (motor and transmission sides). To check levels, there’s a fluid level check plug on the opposite side from the fill plug. To get the amount right when replacing fluid (after closing the drain plugs, of course), add fluid through the fill plug until fluid begins to seep from the smaller check plug hole. Then, close everything up. GM recommends warming the unit up a bit with driving and then re-checking the fluid level.

As more Bolt EVs get up in miles, this will be an important DIY skill to have for extended ownership. Also, given that this uses ATF, the fluid should be changed every 50k miles or so to maximize the life of the unit.


There’s an electric motor/pump that circulates fluids within the unit when driving. It pulls filtered ATF from the bottom and puts it in a sump tank on the top that drips fluid to cool and lube the internal parts. This pump is only active in drive and reverse. It appears to be removable when still installed in the vehicle, so GM may anticipate failure of this unit prior to vehicle end-of-life.


There are two coolant pipes (intake and outlet) toward the bottom. These feed to the vehicle’s radiator and cool internal components.


Disassembly of the unit begins.


One axle shaft is much longer than the other. While the CV shafts are of equal length, having uneven axle shafts like this does contribute to torque steer in unavoidable ways. In the Bolts I’ve tested, this is quite noticeable.


He removes the shift actuator assembly. What makes little sense to me here is why they went through the trouble of having an electronic shifter that actuates a cable mover. Call me old school, but a cable-actuated shifter in the cabin would have been much simpler, no? Yeah, I know. Several of you are saying “OK, Boomer,” and it fits, despite my upper-Millennial age.

On the upside, this may be an opening for future EV swaps into older vehicles. Better hackers than me will eventually know this.


He “splits” the case on the differential/transmission side.


This drive unit is supposedly serviceable in the vehicle. Both ends of the case can be removed while the electric motor and center portion of the drive unit remain bolted in. This may be one of the times when mechanics are swearing at the engineers while they work.

For a simple fluid and filter change, it might not be that bad, though. In most cases, that’s all a person would be doing.


He splits the case on the driver’s side of the unit. Once open, we can see the filter, several sensors, and the electric motor itself. The resolver, or electric motor sensor, is serviceable (unlike some Toyota hybrids).


Motor stator is ready to pull out, but it’s going to be humanly impossible (and somewhat dangerous) to just unbolt and pull out. The magnets are simply too strong. A special $1000 GM guide and pulling tool is required. Despite the price, the tool may need some sanding to properly fit (see the video for details).

However, this should be a very rare thing for any Bolt EV to ever need. The motor should outlast the rest of the vehicle by a wide margin.


You can see the rotor alone, finally removed from the DU.


Stator is removed. Windings are deeper on this motor than the Volt, which makes it more efficient at high RPM.


We can see the three main moving parts of the drivetrain, and the total of 13 pieces that move with them (bearings, etc). For this reason (simplicity), the drivetrain should last a very long time.

While we at CleanTechnica are still down on GM right now for backing the Trump administration in their statist fight against state clean air standards that exceed federal standards, it is good to see that GM did put some serious effort at designing a good EV system for the Bolt EV. Hopefully they’ll eventually come around and build some more EVs.

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Written By

Jennifer Sensiba is a long time efficient vehicle enthusiast, writer, and photographer. She grew up around a transmission shop, and has been experimenting with vehicle efficiency since she was 16 and drove a Pontiac Fiero. She likes to explore the Southwest US with her partner, kids, and animals. Follow her on Twitter for her latest articles and other random things: Do you think I've been helpful in your understanding of Tesla, clean energy, etc? Feel free to use my Tesla referral code to get yourself (and me) some small perks and discounts on their cars and solar products.


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