Recently completed a 30-year set aside Dumas 44' Lifeboat with Dumas running gear, including speed control. Using a Hitec 3-ch Lynx 3D Tx/Rx. When the throttle is engaged, the rudder control won't(?) I've checked the servo settings on the Tx and they appear OK (100% trims). Any help on this? Thanks.

Jim, If you post this in the radio and car forums you will get additional exposure. Does it work alright with either one disconnected? Dan.

Is the speed control mechanical or electronic?
Is there a separate battery for the RX and the motors?

Thanks, Dan, but I'm not clear on disconnect. I only have 2 channels operational, throttle and rudder. I failed to mention that when throttle is in reverse, rudder control works (?). How's that for a weird action? .

It's a Dumas speed control (servo mechanical action). The motors (two 6V) and the speed control are powered by its own battery, while the Rx and servos are powered by a separate battery.

First thing is to make sure the transmitter and receiver are working properly.
Reverse the plugs in the receiver for throttle and steering . Rudder servo should now work via the transmitters throttle control and the motors should work via the transmitters steering control.
Report back on how it operates after you do the swap.

Jim, I meant to use one channel at a time with no servo or speed control connected the other. Use your sticks as you usually would. This will tell you if the transmitter is the problem. If your transmitter has a mix function make sure it is not active. Dan.

No difference on switching channels at Rx. Still can't control rudder when props/speed control on forward.

Did you try to program this on a clean model location in the TX?
One more thing I would try to rule out the TX is to put the motor control servo back on the throttle channel and the steering servo back on the rudder channel. Then reverse the throttle channel and see if the problem persists with the trigger reversed.
If so, I would then disconnect the motors and try it with only one motor connected at a time and then try the other motor to rule out a problem motor.
Check the wiring and make sure the motor and speed control battery wiring is isolated from the servo /receiver wiring.

Try it with a servo at each channel. Dan.

Trying to keep it as simple as possible, with nothing but RX power plugged in and the steering servo, what happens when operating the rudder stick, and what happens when you operate the throttle stick and then both?

The only thing I can think of is the RX is your problem, check the wiring pins make sure the grounds and signal wires are correct and maybe try another RX to see if that could be the problem. Also make sure the servos and speed controls have only one power source for control that in itself could be your problem.

Solution found. I tried all the above suggestions, with no success. Took the boat and radio to my LHS. After checking with other Tx/Rx and servos, they felt it was likely a variable in the Tx program. It turned out to be the ABS parameter had been activated. Once the default was set, the speed control - rudder problem disappeared. The boat had its successful maiden voyage in the town local pool. Thanks to all for your help.