Duty Cycle Help

[handsworth]

While trying to do a duty cycle measure, I got some eratic readings and need help translating what's going on. The car is a 1991 420SEL federal. Using a Sears 82139 multimeter, it read 70% with cold engine, key on, engine off. At about 60 degrees it read 50%. When it finally got to operating temp (80 degrees) it started to increase. Then it started fluctuating from 89% to 99% then starting again at 89% and repeating this pattern. Does anyone know what's happening here. Any replies appreciated.