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Need help troubleshooting Razor Ground Force kart..

After the previous owner gave up troubleshooting, I received a non-functioning kart along with a pile of parts, all in unknown condition.  I'm hoping someone can help me make sense of things and give me some recommendations.

When I first got my hands on it, it was obvious that the battery pack was trashed.  One cell was showing 4V, the other cell showing 0V, tried to revive them with a stand-alone charger, no luck, so bought new batteries.

Batteries were initially charged independently @ 12V with external charger to ensure a full initial charge, each cell is showing about 12.6V standby voltage.

Hooking everything up, turning switch, lights are on, but depressing throttle has no effect.  Voltage across the batteries stays at 24.9V (no drop)

Using this helpful diagram as a reference:


When the switch is turned on, there is 4.25V on the Red/Black pair that indicates should have +5V.  With the throttle in neutral position there is about 0.75V on the white/black pair.  With the throttle fully depressed, there is about 4V on the white/black pair.

Checking the voltage output at the motor connector, as soon as the power switch is turned on there is approx 2.2V on the motor output, moving the throttle does not have an impact on the output voltage, squeezing the brake increases the voltage by about 0.5V to 2.7V

There was what looks to be an unused controller that was included in the pile of parts....The behavior of that controller is exactly the same.

Connecting the motor directly to the batteries causes it to run just fine.

There were also 2 battery chargers in the pile.  The behavior of both battery chargers is the same, when plugged into the wall with nothing connected, the red light on the charger flashes...testing the output voltage, there seems to be a voltage spike with the light flashes, but otherwise it appears to be 0V output.

I'm not sure how this is supposed to work, based on the troubleshooting, I'd be inclined to think that the controller is bad, but having 2 separate controllers behaving in the exact same way (one of which appears to be new) has me suspicious...Same with the battery chargers.

Before I start buying a bunch of unnecessary parts what should I inspect?

It seems like I can assume the motor and batteries to be good, but everything else is suspect.


The throttle can run on a 4.25 supply Voltage and the throttle's 0.75 through 4V output signal is within specifications for a working throttle so the throttle is good.

This leaves the problem as being the brake lever switch, battery pack, controller, motor, or any combination of these parts.

The brake lever switch can be unplugged from the controller to see if it is causing the problem. If unplugging the brake lever from the controller does not help then it should be left unplugged from the controller for the remaining testing procedures just to make sure it is not causing a problem with the tests. 

The motor obviously runs since it spun when you tested it however it could have a burned out winding which is overloading and burning out the controllers. If the motor has a burned out winding then it will consume a significant amount of current when being tested and its wires will start to get warm which is an obvious sign of a problem with the motor. The chances of the motor having a burned out winding are not very high however it is a possibility to consider.

The battery pack can be load tested by connecting it directly to the motor and reading the Voltage level of the battery pack while applying the brake to put mechanical resistance on the motor. If during this load test the Voltage of the battery pack drops under 23 Volts then it has failed the load test and needs to be replaced.

If the battery pack passes the load test then it can be used to test the controller. If at this point the controller does not run the motor then the throttle, battery pack, and motor have all been tested and the controller can be deduced as being the faulty part by the process of elimination. 

If the battery chargers have a blinking red light that is a code which indicates an internal failure with them. The battery chargers should provide a steady output Voltage and not a pulsed output.

Please let us know how it goes.

Thanks for the fast response....you confirmed what I was suspecting....

I found this youtube video that turned out to be helpful:

It is probably not advisable to disassemble the controller, but when I did, I found that I had a variable voltage of 0.8V - 22V across the output terminals of the relay.  As a test, I bridged the relay output and then had 0V - 22V on the motor output terminal.  I hooked that up to the motor and it spun right up.

Out of curiosity, I checked the coil voltage on the relay and it does NOT look like the relay is being energized, but I'm not sure whether the relay should be normally open or normally closed.   I wish I could understand what the issue was with the controller and ideally fix it by replacing the failed component, I'll just go ahead and get a replacement controller.

I had been thinking about using 40V Li ion battery from a lawnmower....I'm assuming that would work with a 36V controller.  Do you have any controllers that will handle 40V of voltage, but still operate @ stock 24V for now?

Thanks for the help.

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