If you already use DumboRC CH4 from Project 24M
DumboRC CH4 is not the same as the stock FCX24M G3 transmitter (radio) gear switch. It behaves like a normal servo channel, so this setup is low gear and high gear only. Do not use the stock neutral-center method here.
- Enter X6PM-350 settings mode by holding the wheel and trigger fully back while powering on, then pressing power again within 5 seconds.
- Select CH4, start around 60% travel, then double-press power to save.
- Keep the servo arm disconnected before radio power so the servo can rest first.
- Hold the gearbox shift shaft at the low gear end, then connect the linkage to the resting servo arm.
- Test low gear first, then tap CH4 again for high gear.
- If one end does not lock cleanly, raise travel by about 10%, save again, and retest until both gears lock.
For the live DumboRC build path, go back to CH4 gear shift setup in Project 24M.
Why the FCX24M stops shifting correctly after a motor change
The problem usually starts after the motor and gearbox come out of the center mount and the shift rod gets disconnected from the servo saver arm. Once that linkage is off, most people lose track of what state the gearbox was in before the teardown.
Then the servo goes back on with no reference. One gear may half-engage, neutral disappears, and the truck feels broken even though the gearbox itself may still be fine.
Why neutral-center is the only sane reference point on the stock FCX24M G3 transmitter
The FCX24M transmission has three states. Move the shift shaft toward the visible motor can side and the gearbox goes into low gear. Move it to the opposite side and the gearbox goes into high gear. The free-moving middle between those two ends is neutral.
Low and high are risky places to refit the linkage because the gear shift servo moves left and right from center under the FCX24M G3 transmitter. If you start from an end-stop, one side of the servo EPA is already wrong before you even power the receiver. That is how beginners turn a working truck into a confusing one.
Before powering anything on
- Remove the driveshaft links from the gearbox output shaft using the in-box PH00 screwdriver.
- Do this before testing. If the driveshafts stay connected, the gearbox output can bind and break the shaft while you are diagnosing it.
- Keep the servo arm disconnected from the gear shift servo.
- Move the transmission shift shaft by hand to the free-moving middle. That is neutral.
- Put the FCX24M G3 gear switch in the center neutral position before the transmitter comes on.
How to reset EPA (End Point Adjustment) from neutral on the stock FCX24M G3 transmitter
This is the part many tutorials make illogical. EPA (End Point Adjustment) has to be adjusted from center because the servo moves both directions from center. If you set EPA from low or high gear, one end gets even more mis-set once the receiver powers up.
- Keep the receiver off. Turn on the transmitter first, with the gear switch already centered.
- Double-press BIND while the receiver is still off. The green LED starts blinking, which means you are in EPA (End Point Adjustment) mode.
- Press TH+ until you reach the full-beep reference, then press TH- twice. My tested recovery point is about 50% EPA to start safely from center.
- Double-press BIND again to save the setting. The receiver is still off at this point.
- Only now turn on the receiver, with the shift linkage still disconnected. Always radio first, receiver second.
- Let the gear shift servo settle on its own first. This is exactly why the horn and linkage must stay disconnected until now.
- Once the servo has centered itself, reconnect the shift linkage to the servo arm.
Do not reconnect the linkage first and hope the radio fixes it later. Let the radio establish center, then attach the linkage.
Bench test: low gear, neutral, high gear
This is where the setup becomes obvious. With the driveshafts off, you can read the gearbox output cleanly instead of guessing through the rest of the driveline.
Low Gear
Shift to low gear and apply throttle. The gearbox output shaft should rotate slowly.
Neutral
Return the switch to center. The output shaft should not rotate at all.
High Gear
Shift to high gear and apply throttle again. The same shaft should rotate faster than it did in low gear.
Cycle low, neutral, and high several times. If it works once and then loses neutral again, the setup is still not stable.
If low gear or high gear still does not lock
Do not jump straight to maximum EPA. Increase it one step and re-test from center again.
- Put the FCX24M G3 gear switch back to center.
- Double-press BIND again.
- Increase EPA one step. My next test point is about 75%.
- Restart radio first and receiver second.
- Repeat the same bench test: low gear must rotate the shaft slowly, neutral must not rotate it, high gear must rotate it faster.
If you tune from an end-stop or jump too far, you are back in guesswork again.
Weak stock servo or bad setup?
Rule out setup first. If the sequence above is correct and the truck still will not hold neutral, then the stock servo itself may be too weak.
- If low, neutral, and high work repeatedly after the reset, the issue was setup, not hardware.
- If neutral drifts or one gear drops out even after the center reset and second-pass EPA test, the stock servo is the likely weak point.
- In my video, the stock gear shift servo looks almost correct at first, but neutral still fails to disengage the shaft consistently. After switching to the new servo, the shift behavior becomes much smoother and neutral holds properly.
Direct replacement servo that worked
If the setup is correct and the stock servo still will not hold neutral, the replacement that worked for me was the DSPower 5g Plastic DS-S003. It is a direct fit for this use case because it comes with the JST plug that matches the stock receiver side.
My bench video shows the difference clearly: the stock servo would not hold neutral correctly, the replacement did.
DSPower DS-S003 5g Plastic Servo
Select the plastic variant. For shift duty it is quiet, has enough power, and avoids unnecessary overkill. Choose JST plug if you still use the stock orange receiver. Choose JR plug if you followed the main brushless guide and now run the DumboRC P6FP receiver.
FAQ
Because the shift linkage often goes back on with no reference. Most of the time the gearbox is still fine. The actual mistake is reconnecting the horn and linkage without restoring neutral-center first.
Because the gear shift servo moves left and right from center. If you start from low gear or high gear, one side is already wrong before you even tune EPA. Neutral-center is the only position that lets both directions stay logical.
With the gear switch centered and the horn disconnected, double-press BIND, set EPA (End Point Adjustment) conservatively first, save it, then power the receiver so the servo can settle at center before reconnecting the linkage.
Do not use the stock FCX24M G3 neutral-center method. DumboRC CH4 is a normal two-position servo setup for low gear and high gear. Enter settings mode, select CH4, start around 60% travel, save it, keep the servo arm disconnected, reconnect the linkage at the low-gear end, then raise travel in small steps until both gears lock cleanly.
Because the gearbox output can bind against the rest of the driveline during testing. Removing the driveshafts lets you read low gear, neutral, and high gear cleanly without stressing the shaft.
If neutral-center is correct, EPA (End Point Adjustment) is conservative, and the servo still cannot hold neutral or engage both gears consistently on the bench, the stock gear shift servo is likely the weak link.
No. First do the neutral-center recovery and bench test with the driveshafts removed. Open the gearbox only if the shift shaft itself is out of place or the linkage cannot be recovered externally.