Help with failed controller

[Kelso]

Hey,
I'm new to working on E-bikes but trying to improve to offer this service at my shop. I got a GoTrax electric fat bike in (cheap, Amazon special) that had quit working and was able to test and determine the hub motor was ok but the controller had shorted. It's a 48V system with a Tongsheng KZQW26-BRD-XF26C controller and I can't find ANYTHING for that part number. I opened up the controller and found the failed MOSFET so replaced it and cut off the old, melted bullet connectors and soldered on an XT60 and MT60 connectors and hoped for the best... well, it worked. Once, for a second, then failed again. Now I'm getting some odd resistance readings including momentary continuity between power leads which settles out close to 800kohms, not sure if it's just from the failed mosfet or something else on the board which is causing the failure but I feel like I'm getting a bit beyond my knowledge and comfort level diagnosing circuit boards.
Contacting GoTrax proved to be useless, the warranty period on this bike was 90 days and expired and they do not have or sell replacement controllers. They did provide some useful nuggets though, such as "if it still doesn't work, try turning it back on and see if it works." and suggesting that the exposed power connector from the picture I sent them (after totally disconnecting it) might be touching a wire causing the problem.

How should I go about replacing this? The controller is hooked up to an S700 display with a handlebar switch used to power on the controller. I could suggest upgrading to a replacement controller and display such as from Grin but besides not being available it would cost nearly the price of this bike. I'm just not sure about compatibility finding a cheap replacement, it has both pedal assist and a throttle and like I said this one needs the power turned on through the display (handlebar switch connected to display) but I don't even know what amperage this controller is.

I run a small bike repair shop, I know I am spending way more time and energy than the bike is worth, or that I can charge for, and if it were mid-spring and I was slammed I would probably have handed it back after the diagnosis but I want to take advantage of the learning opportunity and at least have some viable options to present to my customer. Whether those options are worth it on a bike this cheap is another story, but I don't want to say to go buy a better bike without at least having options to present.

Thanks for the guidance, sorry if I seem like a noob but I totally am still!

 

[calab]

Controllers are cheap, you should have a spare sensorless for just that task. If nothing is visually wrong, then something downstream of the mosfet got wrecked. You will need a controller the same as the display.

 

[docw009]

How many amps are listed on the present controller? The option to present to the customer is a replacement controller of similar current and a new display.

Sure, you could go with Grin. Good stuff. Expensive. This ought to work. I just put this controller into a bike this week, but I used a cheaper monochrome disply.

https://www.aliexpress.com/item/32648717333.html?spm=a2g0o.store_pc_groupList.8148356.7.730c31f28ampf5

https://pswpower.com/products/colourful-kt-lcd8h-lcd-meter-with-waterproof-plug-and-usb-mobile-charge-port-for-kt-series-controller-68

 

[Comrade]

well, it worked. Once, for a second, then failed again.

Of course. You didn't identify and rectify the reason for why the MOSFET blew initially. It's like replacing a tire tube, but then putting the tire back on without taking out the nail that punctured the tube.

I feel like I'm getting a bit beyond my knowledge and comfort level diagnosing circuit boards.

It's very difficult, technically, and almost always financially impossible to diagnose modern electronics. Especially ones you are unfamiliar with. Especially ones with no documentation. There is a microprocessor on the board. With a bunch of computer code. It's not a 1960's pocket transistor radio.

Replacing modules (the controller in this case) is the only workable solution.

 

[E-HP]

I got a GoTrax electric fat bike in (cheap, Amazon special) that had quit working and was able to test and determine the hub motor was ok but the controller had shorted.

Can you describe more details about how you determined the motor is OK and that the "controller had shorted". I'm not really sure what the controller had shorted means, and the worst case would be to offer a solution of a new controller and display, and have the new controller fail; so I'd make sure your diagnosis is correct before spending more money.