FAQ:Avoiding Locomotive Runaways

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A runaway is a situation where the multifunction decoder loses the DCC signal and reverts to Analog mode, immediately accelerating to full speed.

Good trackwork, proper wiring following the recommended best practices, proper power management and eliminating the cause of shorts goes a long way to prevent runaways from occurring. A common cause of runaways is a distorted DCC signal causing the vehicle's multifunction decoder to switch to analog operation, accelerating to full speed in the process.

A runaway is scary, and the only way to stop it is a total shutdown of track power.

Also read the section on Compatibility Issues for technical details on multifunction decoder addressing to avoid a runaway.


Runaways can be caused by several factors.

When a multifunction decoder powers up, several things happen.

  1. There must be enough voltage to power the microcontroller and other circuits
  2. Like any computer the microcontroller will boot up and execute its operating system.
  3. The memory will be tested to ensure it is valid. A checksum will be calculated and compared to one stored in memory. If the two match, everything is good.
    1. If the two checksums do not match, the operating system will clear the memory and reboot, returning the decoder its default state.
  4. The decoder will, after all these checks have been done, examine the track signal to determine if it is a valid DCC signal or not.
  5. If it determines it sees an analog DC voltage, it will fall back to Analog Conversion mode.
    1. This is fine if that track is powered by an analog control system.
    2. Since the DCC signal is always at full voltage, a distorted DCC signal will cause the vehicle to immediately accelerate to full speed.

Avoiding Runaway Locomotives

One approach to avoiding runaways is to disable Analog Conversion in CV29. Setting bitswitch #2 to 0 (zero) accomplishes this.

When enabled, Automatic Power Conversion allows a multifunction decoder to switch to analog operation if a valid DCC signal is not received. When disabled, the Packet Timeout controls the decoder, which will stop after a set period of time has elasped without receiving a valid DCC Packet.

Proper Wiring

Proper wiring goes a long way to avoiding runaways or loss of control.

Improper wire gauge, excessive run lengths, and poor wiring practices contribute to this issue. Proper selection of wire gauge for the planned run, and keeping the bus wires together helps manage the impedance of the bus. Excessive impedance results in ringing on the bus, which distorts the DCC signal. A distorted signal can result in a runaway.

Short Circuits

A typical cause is a turnout. Often by an operator running into a turnout which is set against the direction of the train. Or by incorrectly gauged track and/or wheels creating a condition for shorts to occur.

If a multifunction decoder's start up procedure is interrupted by an event such as a short circuit, it can lead to memory corruption, which results in a reset of the decoder's parameters. This will reactivate Analog Conversion, which when coupled with a distorted track signal, can result in a runaway.

Loss of Control

Some decoders may remember their last state. If layout power was removed at for example, the end of an operating station, everything stops.

When layout power is restored, the locomotive will immediately resume what it was doing previously.

This can also occur if the locomotive was removed from the layout and placed on another layout later.

Packet Timeout

The NMRA DCC Standard includes the optional CV11, which contains the Packet Timeout Value. This value determines how long a decoder will continue to follow the last throttle commands in the absence of a valid DCC signal. When that time has elapsed without the resumption of a valid DCC signal, the vehicle will stop.

The default is 20 seconds.

This CV is optional.

Ghost Throttles

A Ghost or Phantom Throttle means there are two throttles controlling the same address. The operators may not be aware their throttle is controlling another address. The throttle may not be in use, adding additional confusion

Wireless Throttles

A common issue is a wireless throttle going to sleep. Control will not be re-established until the throttle awakes and begins transmitting commands.

One option is to configure the throttle's sleep mode to its longest period. Many will go to sleep after a brief period to conserve battery power. Another solution is to make subtle speed adjustments or activate a function to keep the throttle active.

NCE Wireless Systems

If NCE Wireless Cabs are not version 1.5B or the RB02 base station has not been upgraded to version 2.0 or higher, a situation can occur where one throttle suddenly goes to speed step 128. Upgrading the RB02 should fix this issue.

Contact NCE for further information.