95 century OBD 1 trouble retriving the codes at diagnostic port under the dash

I have a 95 century 3.1 with OBD 1 their is only 3 wires for the diagnostic port under the dash ,my 89 bonneville has 4 2 are side by side none on the century are side by side to check the codes as normally done have checked the wiring from the port back & have revealed only 3 wires from the factory .Have jumped all & cannot get the computer to go into diagnostic mode is their any other way I can get the codes