Step 6. Cover the slow cooker. Cook on high for 4 to 5 hours, or on low for 6 to 8 hours. Insert an instant-read thermometer into the center of the roast.If the temperature is not at least 145 degrees Fahrenheit, cook the roast for another 15 minutes on high, or 30 minutes on low, then check the temperature again.Place the rump roast on a platter.tep 6. Cover the slow cooker. Cook on high for 4 to 5 hours, or on low for 6 to 8 hours. Insert an instant-read thermometer into the center of the roast.