Support - Multimeter Troubleshooting

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NOTE: This section requires the use of a digital or analog multimeter to perform electrical checks.

NOTE: Continuity checks that include the rails must be performed with one lead connected or touched to the rail.

NOTE: ALL resistance measurements including continuity MUST be performed with the power OFF.


Continuity & Resistance Checks

Check resistance/continuity for the following:

  • Left Rail to Right Rail
  • Orange to Grey (motor connected)*
  • Orange to Grey (motor disconnected)**
  • Left Rail to Orange
  • Left Rail to Grey
  • Right Rail to Orange
  • Right Rail to Grey
  • Speaker contacts (speaker disconnected from decoder)***

WARNING: NONE of the above tests should read 0 or near-zero. If any of these tests show 0 Ohms or near-zero readings, you have identified a point of failure.


*Without a decoder connected, resistance measurements of a typical HO-scale motor will read upwards of 200 Ohms, and will vary from manufacturer to manufacturer. Similarly, motors within different model scales will vary, but generally will remain within a certain range. A motor which reads 0 or infinite resistance between its terminals is absolutely bad in all circumstances. Resistance values below 50 Ohms will typically indicate a bad motor, with notable exceptions being "coreless" motors. NOTE: We recommend performing this test while rotating the flywheel. If you see large spikes in resistance upwards or downwards, this indicates poor brush connections or bad brushes, or a failed internal winding. Both of these circumstances can damage a decoder over time as they will often result in spikes of current outside of the maximum safe level.

**With the decoder disconnected from the motor, the resistance between the Orange and Grey wires should be exactly 20K Ohms (with about 100 Ohms tolerance one way or the other). A value in the single-digit KOhms, or MegOhms is unacceptable, and indicates component failure on the decoder.  A value of 0 Ohms is also unacceptable.

***Only applies to SOUND DECODERS - An 8 Ohm speaker itself should read Between 7.2 and 8.8 Ohms. If the value you read is less than or greater than the rated impedance by more than ~1 Ohm, the speaker has failed or is failing and should be replaced. A reading of infinite Ohms (open circuit) could indicate either a bad wire or a failed speaker.


Place the locomotive on a spare, isolated, unpowered piece of track. Check resistance/continuity for the following:

  • Left Rail to (Black) Power Pickup on decoder
  • Right Rail to (Red) Power Pickup on decoder  

NOTE: These readings should be near-zero and not fluctuate. Use one lead of your meter to touch the rail; use the other lead to touch the pickup for that rail as close to the decoder's connection as possible.

Voltage Tests

NOTE: Voltage measurements are to be performed with power applied to the engine.

Check DC Voltage for the following:

  • Blue [wire] to Decoder Ground Only perform this test if you are skillful with electronics and know what you are doing. Carelessness can lead to damage to the decoder. If you can identify these two points on the decoder, a DC voltage measurement between these two points should show a reading approximately 1-2 VDC less than the nominal track voltage.
  • Orange to Grey (This test should be done with the motor disconnected or with the locomotive on rollers) Gradually increase the speed step. You should see the DC voltage increase from 0 to ~12V. Reversing direction will also reverse the polarity of this reading.

   

Check AC Voltage for the following:

  • Left Rail to Right Rail NOTE: Should read approximately the input track voltage. Most multimeters are not intended to accurately read the Voltage of a DCC signal, so you can expect a margin of error of ~2VAC. You should only suspect a legitimate issue if you see only a fraction of the expected track voltage.