Last week I removed the 4T Trackstar motor in my '13 Xray T4 and replaced it with a 21.5T motor to run in the Sportsman class. The car ran fine but the gearing was too slow, so I swapped the 18T/84T combo with a 35T/70T combo. Car spun up OK on the stand.

Put it on the track, hit the throttle and the car literally did a 180 and stopped dead, servo stuck at full lock, no throttle, no steering, no nothing. Fan doesn't even spin on the ESC anymore.

I tested the servo on the bench and that was working. Turning the ESC on I got a single red flash, then nothing. If I press the calibrate button it beeped and the motor would turn slightly "tik-tik-tik-tik" but I couldnt calibrate the Tx. But I can still program the ESC via the USB plug too. Weird.

Then I checked the ESC --> receiver wire with a multimeter and I'm not reading any power at the connecter, so I presume the ESC has shorted.

I don't have a spare ESC for it and wondering if there is any more troubleshooting I can do to try and fix the ESC? Is there any point? Or am I up for a replacement ESC? It's an X-Car 120A ESC and I was assured by HobbyKing it was compatible with the 21.5T motor.
Thanks

If you've tested the voltage output to the RX and it's dead, there's probably very little else you can do. With anything HobbyKing, it's generally disposable, not repairable.

That ESC is more than capable for a 21.5 turn motor sounds to me like the bec circuit gave out for some reason I had one that smoked on the same esc after some soldering on the motor wires as well I have 4 of these esc's and always keep a spare as well considering the price of them it is not that hard lol. if you bought it from hobbyking it does have a 1 year warranty on it just need to do a short video with the esc on and maybe a volt meter showing the output and than follow the warranty / return instructions on hobbyking you don't have to send in the defective esc they watch the video and than issue a credit took about 2-3 weeks for mine but still that is faster and easier thjan sending one back to the manufacturer.

You could try an external bec the esc might still work.

I went and got myself a decent soldering iron, removed the motor wires and then pulled the ESC apart over the weekend.

The solder where the red wire connects to the Rx was splashed all over another little component at the board, looked like it melted actually. And it was all brown/blown at the board.

I decided the ESC was toast, so am now looking for a replacement. Might just go a SpeedPassion combo or something and be done with it.

Thanks for the ideas/advice.