Bringing the Sunstar S03 mid drive motors back from the dead

Electrifying the Dummy part 1

So a few weeks ago we received an email from Antony and Celia who had a very interesting proposal.

They had 4 broken SunStar S03 Mid drive motors that they had been donated. They all looked to be in good physical condition but none of the electronics worked and all had not shown any sign of life. Did we want them and could we get any life out of them?

Always up for a challenge we agreed that yes we would indeed be very interest and if we could get more than one working would be happy to help Antony and Celia in electrifying there Surly ‘Big Dummy’ Cargo bike with one.

Anthony had already done some background investigation into the 24V motors, and he had one controller board de-potted and come up against a brick wall. The likelihood is that some of the mosfets were blown and it might be possible to replace them, but as there is very little information on the electronics used in these old but well build drives a different approach might be the best option. Anthony had done what I would have to be honest and purchased a cheap 24/36v generic ebike controller with the hope to wire this up and get some life out of the unit. But this is where things got a little complex. He could wire up the motor phase cables but whatever combination he tried he just got a ’03 error’ on the screen.

So the driver clearly has three high power phase windings for the motor and connecting these was not tricky, however that’s not all that’s needed to get the motor to turn, or more accurately to tell the controller how to turn the motor.

Antony contacted us at this point to see if we could help them figure this out.

So I took the 4 motors and started to look in more detail at the wiring.

Now almost all ebike motors I have worked with use hall sensors within the motor stator to detect the speed and direction of rotation. Its these hall sensors that tell the driver, when the motor is running, and simply pulse a single voltage every time a magnet passes them, so a hall sensor needs supply voltage, a ground and gives an output signal. Without this vital feedback the controller would very quickly refuse to output any power. In fact its so quick you don’t really even notice the motor move before it stops. So I was confident the missing link here was just how to wire up the hall sensors.

Easy!, well not quite…

You see the hall sensor cable on the controller side was logical and the pinout was as follows…

Red – 5v
Green – Phase 1
Blue – Phase 2
Yellow – Phase 3
Black – Gnd

Simple enough. but at the motor end I had 6 pins not 5 and the colours were…

Brown, Red ,Orange ,Yellow ,Green ,Black


So what could this pinout be, what’s the extra pin for and which one is it.

Well thankfully after removing the motor and gaining access to the Hall sensor printed circuit board I could clearly see the tracks and work out the pinout.

So colour coding the pins you can see the heavier tracks top and bottom highlighted as Brown and Black look to be logical power and ground rails. and brown and black would be logical colours to use, however my first thought was that red would be 5v power 😉

Ok so lets assume those are the power rails what’s left?

Well you can see the next three wires , red, orange and yellow go to the three hall sensors via a inline resistor and they are even labelled R1, R2 and R3. I think its a safe guess these are our three Hall Sensors!

Ok so what’s the remaining green wire?

Well if we follow that track it goes right over to the far left of the board and hidden under the blob of white silicon you can just see it says ‘TH1’ could this maybe be a thermistor? sure enough the other side of the thermistor goes to ground with a filter capacitor to the 5v rail.

So I think we have found our pinout!

Motor side – Driver side

Brown – Red 5v
Red – Green Phase 1 GREEN
Orange – Blue Phase 2 BLUE
Yellow – Yellow Phase 3 YELLOW
Green – TH1 Temp sensor
Black – Black GND

Bingo, cable it up and we no longer get an error and the motor now rotates!

I could even measure the resistance of the green wire pin as the motor heated up, we may even be able to calibrate a thermometer to this value to get a temperature reading.

After this proof of concept on the bench, and showing Anthony and Celia the good news, we ordered a cable to better connect the motor to the controller and used a separate controller housing on the bike to make it a tidy solution that would be able to cope with some wet weather!

Anthony helps me solder up the new cable when it arrived.

and fitted the motor onto his bike for the initial throttle tests.

Shortly afterwards, he wired up the pedal assist sensor

Hope you like pedalling fast Anthony!

We would like to test the motor on 36v as the power generated is not as much as it could be and with such a heavy bike when fully loaded and the steep hills of Calderdale every bit would help!

That thermistor may come in very handy if we go down that route to keep track of how hot the motor is getting.

So keep posted for future updates on our experiments with this motor.


Hi, I am one of the trustees of Empowered people. As well as looking after this website, I am also an ebike builder, modifier, hacker, and general bike mechanic. I have been involved with the Charity since 2015 and participate in as many rides as I can with my wife Catherine on the 'Bike Train' if you look around the website you are sure to find me in the background somewhere.

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