Diodes are usually put in line to keep the voltage from flowing in the wrong direction, or to keep it flowing in the right direction (DC). You sure it wasn't a resistor (to limit current)?. If it was a resistor, you can expect the led lamps to blow pretty soon with that resistor out of the circuit.
Are the exit signs rated for the supply voltage you are using on this application? If not, would explain why the part was making the exit sign not work.
Good Luck!
