Check the fuses for power to that relay.

There should be an A/C compressor speed sensor which is connected from the compressor to the A/C relay - be sure that is connected.

Then check engine coolant temperature sensors to be sure they are connected, one of those is an input to the A/C relay. (If engine is too hot, the A/C will shut off.)

And there should be an engine RPM input to the A/C relay - that would tell the relay that the engine RPMs are fast enough to turn on the A/C compressor.

I got the above from here...
http://www.startekinfo.com/StarTek/outside/12253/program/ETM/ACCfrom86.pdf