92 s15 4wd inop

You've probably answered this one before, but I couldn't find an answer so here goes: My 92 Jimmy, 150,000 miles. Runs pretty good. When I start car, 4wd lights both come on, then go out, which is normal operation. But if I go to ingage 4wd, the lights do not come on, I hear no actuators or noise. If I disconnect battery, wait a while and reconnect it, the 4wd will work one time. So what's my problem? I've checked the vaccuum actuator below battery and it's good. I think it's electrical.......... Can anyone help? Thanks in advance.

Does anyone have a schmatic of the electrical circuits for this 4wd system?