Check if there is any plug and play decoder available. Plan in advance the capabilities you want – load compensation, lighting / sound effects. Measure the stall current. If the motor’s stall current exceeds the decoder’s rating you are sure to have problems down the road so, start by using the following procedure to check the stall current of your motor.
1. Put the locomotive without the shell on a regular DC track.
2. Attach a DC current meter (ammeter) in series with one of the track feeds. Some power packs that have ammeters are really ideal for this test.
3. Apply 12V DC power to the track for N or HO. (16V for G)
4. Hold the flywheel or drive shafts to stop the motor from rotating for a couple of seconds.
5. While the motor is stalled, measure the current that the unit is drawing from the power pack. Be sure that while you are taking the measurement that the power to the track remains at 12V to get an accurate measurement.
6. Use the manufacturers’ recommendations to choose the appropriate decoder for your application.
Generally speaking, N-scale engines with can motors draw about one amp, HO engines with can motors draw about one amp. Older Athearns with open frame motors and Bowers with Pittman motors draw around 1 3/4 amps. Large scale engines (O, S & G) vary in current draw and some even have two motors, those with can motors may draw less than 2 amps but each should be tested individually to determine which decoder to use.