1992 RD690s Electrical problems

My Mack when I turn the heater blower or any heavy draw on the voltmeter will turn off like the ignition has been turned off and none of accessories that need the ignition in the on position
will work. I have replaced the ignition switch same problem sometimes it will come back to to normal but I need to figure this out. Any and all help would be great thanks in advance
John

1992 RD690s Electrical problems