Ok, I remember you are new, so I'll try to explain how this all works. If any of this confuses you, let me know.
An ESC has a built in battery eliminator circuit (BEC). This is basically a voltage regulator to deliver the correct voltage to your receiver (Rx) and servos. Say you had a 3s battery and no BEC. That would destroy the Rx and servos with too much voltage. So a BEC is there to fix that.
Servos also require a few amps when under load, so your BEC needs to supply enough amps. So with two servos, yeah, your BEC may struggle.
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To alleviate that, you can install an external BEC. A Castle 10a BEC should do nicely.
https://www.amainhobbies.com/castle-creations-10-amp-adjustable-bec-cse010-0004-00/p18210
That BEC is preset to output 5.1V, but can go up to 9v. If you have servos that can handle 7.4v or 8.4v, you would notice an improvement in servo speed and torque by simply setting the BEC to whatever your servos can handle. To change that you need the Castle Link.
https://www.amainhobbies.com/castle...usb-programmer-adapter-cse011-0119-00/p640839
You can of course just run your servos at the preset 5.1v, but I would personally want the BEC putting out as much power as my servos could take. What servos do you have in the truck? There should be a brand/model number on them.
To wire up the BEC, you will need to remove the positive wire from the ESC to the Rx. Here is the wiring diagram.
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And I know you don't have an external ESC like that diagrsm, but you still have s cable going to the Rx. That will be the cable you need to modify. You also need to solder the positive and negative wores for the BEC into your battery harness.
And just a heads up... You don't have to cut that wire from the ESC to the Rx. You can gently press down on this tab and pull the wire out of the connector. That way you can slways reinstall that pin into the connector later if needed. Just make sure to tape it up good so it doesn't short out on anything.
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