Does the relay work OK if you use a separate power source with the pump already connected?
If not, the relay seems to be faulty.
Do you have the control side negative wired directly back to the control system? If it's sharing a ground return with the power circuit at any point, there could be interference or voltage offsets messing it up.
A DC motor will pull several times its rated current when first started, so an apparently low resistance shared connection can have quite a voltage spike on it during the motor start.