SDNomad wrote:

Will it harm a 24 volt motor to run it on 12 volts just to make sure
the brushes and everything are good? I know it will run slow but will
it have any harmful effects?

If you mean to run the motor for a moment in air (no load) just to check
that it turns over then you won't hurt it. You may not learn much about
its condition either but if the motor runs smoothly on 12V then it is
probably OK.

Who knows what the speed control will do though.

Rick