1990 Volvo 740 idiot lights

The idiot llights (test mode) should be on when the ignition is on and no AC (yes, AC not DC) is detected from the alternator. This voltage is contected by a single wire using a push-on spade connector to the back of the alternator. Try disconnecting this and re-connecting in case of simply a corroded connection. It happened to me. Note: if you have a tac, it uses the AC to calculate the RPM. If this AC connection fails, the tac drops to "0".