Nostalgia & History > Speed indicator help

Hello,
We have a old pulse electronics speed indicator in the locomotive. It wired to the transition control box and does not read steady, it bounces all over the scale. The bach Simpsons axle alt is mounted on the truck and spring and paddle are intact.
Is there any way of testing the axle alt for a signal.

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Stick the tip of the probe into an electric hand drill. The speedo should read about 30 mph when the drill is running at full speed.

Check terminals AD1 & AD2 on the transition panel for clean and tight.

If the axle allternator is mounted on a Hyatt box make sure oil has not leaked into the alternator body.  Make sure the pickup is clean and connections are tight.  Also the cable from the axle alternator to the panel may have high resistance..

There are issues that occur when you hang multiple inputs like TCP, speedo, event recorder, etc. on a single axle alternator.  We always drove the Barco or Bach-Simpson TCP from its own axle alternator on locomotives that were not equipped with an isolation amplifier.