For Bashers, By Bashers!

Losing grip – Transforming the TRX-4 transmitter functions

As far as plans go, mine was as simple as they come. Even so, it failed miserably when faced with reality. It was like this:  1) Buy TRX-4 kit and 2021 Ford Bronco body, 2) build and paint TRX-4 kit and Bronco body, 3) drive and enjoy TRX-4, 4) upgrade and do pre and post comparisons one thing at a time.

What has happened is this: 1) bought TRX-4 kit and 2021 Ford Bronco body, 2) immediately replaced all electronics, then replaced bumpers and rock sliders with Club5Racing parts, then  rearranged all the servo connections, still hasn’t driven it more than a few feet to test functions. This is where I was a week ago, more than two months after I started building:

Jeremy would have called this column “Shop news”. If you know me, the nearest thing in my case would be “Kitchen table news”, but that just won’t do for a header of an RC blog, so I picked a different one.

As I was saying, a week ago I had about a mile of wires to get to work on. A mile of wires, and a plan that started to take shape:

I’ve mentioned the basic premise before: there’s too much going on in the TRX-4, what with locking differentials and 2-speed gearbox. Also, there’s too little going on regarding controlling lights via the transmitter. Third, the Traxxas advanced light kit cannot pick up signals for brake lights and such from a brushless motor. How to get to grips with all of this?

Let’s start with a look at the schematics of the TRX-4 electronics, green is for signal wire:

Nice and simple, and as I said the other week with a well defined path for every single wire on the chassis. Much appreciated, credits to Traxxas for this. However, as I see it there’s things that can be improved upon, ways to make the TQi transmitter do more work.

Enter the

Traxxas 2021 Ford Bronco Most Awesomest Master Plan

1) Connect the gearbox servo and front and rear diff servos to channel 3

2) Connect the auxiliary wire of the Mamba ESC to channel 3

3) Connect a dual output electronic cyclic switch to channel 4, with further connections to bumper lights and roof rack

4) Leave channel 5 alone

5) Find a way to make the Traxxas Light system work with a brushless motor

Schematically, the plan looks like this:

Great, right? Let me walk you through it, receiver top to bottom.

Maxx link: connected to the Traxxas light power module the way it should. Do note that the power module is supplied with 7.4V via a BEC. I first powered it via the receiver, hence 5.5V, but that way only the running lights worked, nothing else. It needs at least 7.4V to work properly, thank you for Stefano at Traxxas customer service for helping me figure this out – a couple of emails back and forth saved me a lot of trial and error.

Channel 1: steering, as before. I’ve changed the servo to a ProTek 370TBL, delivering 650 oz-in at 7.4V.  That should be more than enough, and water proof at that. In order to give it all the voltage and current it needs I’m powering it via a castle BEC. Note that while it could be run at 6V directly from the original Traxxas XL-5HV ESC, the internal BEC of that ESC can’t deliver the current needed. At 6V the servo has a maximum current draw of 6A and you will need either an external BEC or a high end ESC with a good internal BEC to deliver that kind of current.

Two further notes on this: 1) don’t try powering the servo with 7.4V via the receiver. As I told you last week, that will smoke the receiver. 2) Not all servos have the same dimensions. The ProTek servo does not fit on the Traxxas servo mount, it is slightly too wide, and other servos might have this issue as well. I solved this by buying a cleverly designed adjustable servo mount from Samix RC, this one, that lets you mount servos with different dimensions without modifications. All of the other servo mounts I looked at were just direct replacements, with the same dimensions as the original. Black was out of stock when I ordered, but it looks good in grey too:

Channel 2: throttle. ESC and motor have been changed to a Castle Creations Mamba Micro X and Holmes Hobbies Puller Pro stubby 2700kV respectively. That combo used to live in my SCX10, but with all the weight that rig has gained, it tends to overheat. This rig weighs a bit less, so I hope it’ll do fine here. With time, I expect my TRX-4 to gain about a pound’s worth of brass and fancy wheels, we’ll see how the motor copes with that in due time.

Channel 3: this is where the cool stuff happens, the big red button on the TQi transmitter. Both diff servos and the gearbox servo have been connected to this channel, so that they all shift simultaneously. After a bit of trial and error, I found that I needed to add a servo reverser to reverse the output to the front diff servo for both of the diffs to either lock or open in tandem. With the gearbox coupled to them, diffs are now locked in low gear, open in high. That’s everything I need. The six options offered in the original setting are four too many, which will only make for confusion amongst young drivers, and a lot of fiddling around with switches on the transmitter. The auxiliary wire from the Micro X ESC is connected here as well, so that in low gear drag brake engages instantly at 100% with immediate reverse; in high gear drag brake is engaging at 80% with a medium onset time, and throttle signal goes forward – brake – reverse. When I hit the rocks, I’ll have all the control I ever want, and when I throw roost in high gear I won’t be worried I’ll do a front flip when I let go of the throttle. All with the flip of a single switch.

Channel 4: now freed of its previous mission of controlling the front diff it’ll instead do service turning on and off lights. I have front and rear bumper lights, rock lights and rock sliders with lights. It seems a bit too much to have it all on all the time, but I don’t want to remove the body as soon as I want to turn something on and off. My solution is to connect a double output electronic cyclic switch to channel 4. One output goes to the bumper lights, the other to rock lights and rock slider lights. As I toggle the switch on top of the TQi transmitter back and forth, I’ll shift between the following options: (all lights off) / (bumper lights on, rock lights off) / (bumper lights off, rock lights on) / (all lights on). Nice and simple. Another way to do it would be to run one set of lights on channel 4 and the other set on channel 5, with a 2-position switch on each channel. If you have a roof rack that wants 12V, this might be the best way to do it. Roof rack on channel 4, rock lights (on 12V,if you need it) on channel 5.

Channel 5: still available! I can control both lights, drag brake, gearbox and differentials from the TQi transmitter, yet this channel is still available to me. Pretty awesome, methinks. Should I add a winch, I will control it using this channel. A winch would normally need either three positions (out, stop, in), or a linear control, but with a winch controller from HeyOK, two positions will suffice. And yes, thanks to the design of the cyclic switch on channel 4, that needs a toggle back and forth (click-click) to change setting, I will be able to do this without turning the lights on and off at the same time.

There’s a little bit more to do regarding organising wires, but this is where I am now:

This only leaves the last bit of the plan to be worked out: finding a way to make the Traxxas Light system work with a brushless motor. I have an idea, but I want to try it out before I reveal it. It’s not very elegant, but then it might also solve two problems for the price of one. We’ll see. Parts should arrive in a couple of weeks. I’ll keep you posted. If you have any ideas on how to do it, or comments on anything else for that matter, please drop me a line at


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Posted by in Columns, Loosing Grip, Projects, RC Rock Crawling, Traxxas on Monday, September 26th, 2022 at 2:18 pm