Can stock 2016+ be programmed for 35mph?

GEM Forum
1

I have a 2018 GEM e4 with 12.4kW Lithium Battery, 3kW Level 1 Charger with Clipper Creek PCS-15 Level 1 EVSE, 6.5kW Motor, and 14" Aluminum Wheels

Without replacing the battery, charger, motor or wheels, can the GEM electronics be reprogrammed to achieve 35mph on level ground with an accurate speedometer reading? Range is not a factor.

San Diego roads marked for 25mph are actually fiercer than those marked for 35mph. I’m installing a panoramic recording system with a blinking red light on the rear camera to notify tailgaters but a bit more speed would help too.

2

Not that I know of. I can reprogram Sevcon, but the speed limit is set in the Polaris vcm.
As of now a gear change gets you the speed. The car will run without the dash display, so you can put a gps speedometer in the opening.
I’m sure someone will have a hack eventually.

3

Just thinking. The speedometer is servo driven. It may be possible to re-calibrate the analog portion to correct reading, after a gear swap.
That is, if it can be opened easily.
Anyone have deep enough pockets to risk destroying an oem speedometer?
I have an extra, but am not inclined to rip it apart. :slight_smile:
@MikeKC do you have one?

4

I do have one, It is still in the 16’ Gem Dash. I have not looked at it much at all. In the back of my mind I guess I was hoping to sell it some day to help recover some of the cost of getting the 16 back on the road. I will pull it out on Monday when I am at the house and see what tearing it apart might entail. I might be willing to donate to the cause.

So when you say servo driven? I noticed on the 5kw stock motor there is a magnetic or optical pickup inside the motor going to what looks like would be the encoder wires. If that gear spins faster (IE 8000RPM instead of 5000rpm) does that feed the speedo? and if so, why would the speedo just just show the higher speed.

Now if we put the 8k motor in, now I can see where we now have issues as the pickup is now gone.

5

The 5kw and 8kw motor use the same encoder. The sevcon sends a can speed signal to the Gem vcm.
It may go directly to speedometer or be scaled by the Gem.
I haven’t looked for scaler in the oem dcf. Might it be that simple to correct speedometer?
Same when we replace the controller. If we find and copy, the same can output, will the oem speedometer read it?
That’s why I need help. I never seem to get anything done by myself! :slight_smile:

6

Speed is calculated from two or three factors in a Sevcon. Lookup Velocity in the manual. There is RPM and ratio and I think factor. All three of those thing determine what the Sevcon thinks is speed and distance…

7

I think I’m following this.

There is an RPM sensor in the AC traction motor that is used to detect motor speed directly and road speed indirectly.

The RPM sensor, via 4-pin connector, goes to the Sevcon Motor Controller (MC).

The Throttle Position Sensor, via 6-pin connector, goes to the Bosch Vehicle Control Module (VCM).

The Speedometer needle is driven by a digital stepper motor (servo) with speed data from the CAN bus.

The MC and VCM are both connected to the CAN bus.

Since the Dashboard Display has multiple indicators besides the Speedometer, it is likely the road speed signal is provided by the Motor Controller and simply decoded by the Dashboard Display while providing no limiting function. Therefore I conclude the Speedometer will read to 35mph with no programming needed.

The MC vice VCM relationship is a bit more complicated because the motor/road speed sensor is connected to the MC and the throttle position sensor is connected to the VCM. Where is the speed limiter?

As I will explain, I conclude the speed limiter is programmed into the VCM.

GEM offers a Speed Key (https://gem.polaris.com/en-us/shop/accessories/additional-accessories/2883054/) for which “Dealer installation and re-flash required.” The key’s only connection is to the CAN bus for both signal and power.

Therefore the speed limit is communicated on the CAN bus. Since the throttle position sensor is connected to the VCM, it seems most likely the VCM commands the MC according to throttle position and road speed within the programmed speed limit. It is possible the VCM simply passes the throttle position to the MC which provides the speed limiting function but this is unlikely because Polaris seems to have centralized the GEM’s “brains” within the VCM.

The other two limits on speed are maximum motor RPM and reserve duty cycle (ie, AC phases).

If 25mph is at maximum motor RPM, then a gearbox change is required (not an option for me).

Using the 80/20 rule as an example, if 25mph on level ground is 80% duty cycle then that leaves 20% for additional speed. I haven’t tested this possibility by driving uphill at 25mph so this is just hypothetical now.

From what I’ve read at this forum, it appears the VCM programming hasn’t been cracked so changing its 25mph speed limit is currently not possible. And apparently that programming has been obfuscated to some extent so it might not ever be possible to change it.

8

The sevcon is configured as a slave to the vcm
Upping speed limit in Sevcon has no effect.
Rpm at 25mph is about 6,000 rpm. I’ve spun the motor to 10,000rpm, which leaves a little for speed increase. To be practical it will depend on noise.
The sevcon does send speed information either raw or scaled to Gem. If it’s raw encoder data, it will be a tough nut to crack. If scaled, we should be able to change that in dcf.
Gem may have taken the time to make the car shut down if checksums don’t match.