49 CFR §236.556
Verified against eCFR.gov as of June 20, 2026View official text on eCFR.gov ↗
Change in adjustment of relay shall be made only in a shop equipped for that purpose except when receiver coils, electro-pneumatic valve, or other essential part of the equipment is replaced. Irregularities in power-supply voltage or other variable factors in the circuit shall not be compensated for by adjustment of the relay.