A multimeter is an inexpensive tool that has several great uses in model railroading, especially if you deal with used or vintage trains a lot. Knowing how to use one will save you a lot of time and frustration.
Measuring a transformer’s output voltage with a multimeter
Switching your multimeter to volts is useful for measuring your transformer or your power pack’s output. Be sure to check the transformer for safety before plugging it in. But once you’ve ensured the transformer is safe, plug it in, then measure the AC or DC output as appropriate. A vintage Lionel or American Flyer transformer ought to top out around 18-20 volts AC, typically. An HO or N scale power pack will top out somewhat lower, and will output DC on its track terminals. Its accessory terminals will typically be around 18 volts AC.
If you measure very low or no output, then you know the transformer or power pack isn’t working and needs to be replaced. At least you know it’s the transformer and not the train.
Checking track for shorts with a multimeter
When you buy a batch of used track, it’s always a good idea to check it out for shorts before you use it. Switch your multimeter to ohms, then place one probe on each rail. In the case of 2-rail track, you should measure infinite resistance between the two rails on every piece of track.
If you find a short in HO or N scale track, look for wires or anything else that might be bridging the two rails. Shorts on HO or N scale track should be pretty unusual since they have plastic ties.
If you find a short in American Flyer S gauge track, replace the insulators on the insulated rail.
In the case of Lionel and Lionel-style 3-rail track, you should measure continuity between the outer rails, since they are connected to each other. But you should measure infinite resistance between the center rail and both outer rails. If you don’t, you need to replace the insulators on the center rail.
Checking assembled track for continuity with a multimeter
If you lay down track and the train slows down or halts on a section of it, you can check pieces of track for continuity with adjacent sections with a multimeter. Place one probe on one piece of track, and the second probe on the same rail on the adjacent section. If you measure high resistance compared to a section that works fine, run a jumper wire between the bad section and the two adjacent sections to restore continuity.
Checking a commutator for shorts with a multimeter
If you have a locomotive that runs poorly, you can use a multimeter to check and make sure the motor’s commutator is OK. Drop the motor out of the train and then take off the motor’s brushplate to expose the commutator, which is usually three copper plates. Switch your multimeter to ohms. Place one probe on one of the plates, and the second probe on another. Sequence your way through the plates, checking the resistance. It should be pretty consistent, within .1 or .2 ohms or so, from plate to plate.
If the resistance isn’t consistent, you probably need to clean the motor. Use a toothpick to dig out any crud in between the commutator plates. Graphite dust is conductive, so it can short out the commutator if enough of it builds up. I also clean the commutator plates themselves using metal polish to remove any dirt and oxidation that may be present. Graphite dust on the commutator plates doesn’t hurt much, but cleaning it off the surface keeps it from finding its way in between the plates where it can cause trouble.
Usually, cleaning between the segments eliminates any inconsistencies in resistance. If it doesn’t, and cleaning again doesn’t help, there’s probably a short in the commutator.
Next, check for resistance between each plate and the motor shaft. You should measure infinite resistance there. If there’s continuity, there’s a short in the commutator. You can rewind the commutator, but it’s pretty tedious work. When faced with a bad commutator, most hobbyists either replace the commutator or replace the whole motor.