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Thread: [Help] Transmission Switching Gears No Input

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Old 04-09-2019, 02:49 PM   #1
Rock Stacker
Join Date: May 2018
Location: North Carolina
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Default [Help] Transmission Switching Gears No Input

Hey guys,

I'm sure there is a thread somewhere but I've been looking almost 30 minutes and can't find anything so I guess I gotta make a new thread...

I got my TRX-4 Bronco in July of last year. its probably at about 10 hours of use. I definitely don't maintain it nearly as seriously as most of you guys do, and that is probably the source of my problem here.

The last two times I've taken the truck out, the 2-speed transmission seems to be beginning to show some signs of problems. Generally, when in high gear position on the transmitter and both while underway or while resting, the truck will just switch to low gear arbitrarily for a little while. Flipping the switch to low gear and back to high will generally fix it, and sometimes if I just keep running it with the switch on "high" but the truck actually in low, it will decide to kick back into high gear.

Anyone know anything about this? Is there a thread or troubleshooting guide I've missed? Could it be caused by low battery or something? Thanks for any help!
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Old 04-09-2019, 03:23 PM   #2
I wanna be Dave
Join Date: Aug 2004
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Default Re: [Help] Transmission Switching Gears No Input

Sounds like the shift servo is either getting wonky or the servo saver itself is weak. Easy enough to swap with a locker servo to test.
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Old 04-09-2019, 03:23 PM   #3
Join Date: Oct 2018
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Default Re: [Help] Transmission Switching Gears No Input

Never experienced this with easily 10x as many hours on my Bronco. Including several runs in deep water and a lot of runs in snow. Five things come to mind that could be the source of your problem.

1) The shift servo is either damaged or has moisture in it. Open, inspect, let dry, reassemble with silicone grease on the seals.

2) The receiver is either damaged or has moisture in it. Open, inspect, let dry, reassemble with silicone grease on the seals and the foam for wire sealing.

3) The signal wire for your shift servo is loose, damaged, or has a corroded connection into your receiver. When you have your receiver cover open, pull off the signal cable, make sure the connection is clean, maybe add some die electric grease to the connection and reassemble. Inspect the servo wire for damage.

4) You could have a bug that has been fixed via firmware update to your transmitter and receiver. If you have the Traxxas link installed and use the app, this will force you to update firmware. If you don't have it installed, consider buying one, or if you have a friend that would let you use theirs for an hour, you could update.

5) You could have a bad transmitter or bad switch in the transmitter. Try borrowing a friend's to test with, if that solves the problem, consider buying a different transmitter or trying to fix yours.

Good Luck

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Old 04-09-2019, 04:07 PM   #4
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Default Re: [Help] Transmission Switching Gears No Input

One important tip if you decide to swap servos is to be very careful when unscrewing the servo saver, you WILL strip the gears inside the servo in an instant if you just unscrew the servo saver. What you need to do to avoid this is use some needle nose pliers to grab onto the top of the servo saver (theres a tiny ridge) as you unscrew the horn.

I was warned about this but failed to heed the warning and had to rebuild my diff servo. It takes so little force to strip these that is worth warning others.

Looks like most of the bases have been covered above. The only other option I see is that the switch on the remote is at fault but I kind of doubt that would happen.
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Old 04-10-2019, 01:02 PM   #5
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Join Date: May 2018
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Default Re: [Help] Transmission Switching Gears No Input

Thanks a million for the replies so far guys. Just in case some other human runs into this problem I'll my own thoughts after reading what you guys suggested and update in a few days when I've had a chance to do some tinkering.

1. Almost 100% sure my transmitter had at least 1 completely dead battery in it when this was occurring, so I'll try sticking some fresh ones in there first.

2. At one point I opened up the ESC to replace the steering servo and wasn't very meticulous about making sure I got it snug when I put it back together. I also didn't have any grease at that time. Since that seems the most likely place that water intrusion has occurred (and its already been opened up once) I'll start by opening that one up and cleaning it.

3. Generally when it does respond, it shifts between gears snappily and just as smoothly as always, which to me reinforces that its more likely an issue somewhere between my transmitter and the diff servo, not the servo itself.

Thanks again folks.
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