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.

what circuit are you testing?

I think you need to use the Sears 82149 MulitMeter to obtain a correct reading....

I'm testing the x11 circuit, but I think I found my mistake. I tested with the engine running. Should I test with the key on, engine off after it gets to op temp. I did and it reads 70%.

It sounds as if it is doing just what it should do, but needs adjusting. Duty cycle is tested with engine running. At cold, it will give a fixed reading. As the engine reaches operating temperature, it goes into closed-loop control. At that point, the duty cycle should fluctuate as you observed. However, it should be close to 50%, not 90%. That is adjusted by the allen-head adjuster in the air-flow meter / fuel distributor. Adjust by small increments and wait for it to settle down.

Look at this article in the DIY section of this site, under "Resources":

http://www.peachparts.com/Wikka/EngineControls

Andrew
1989 190e 2.6

I did the adjustment at the fuel/air mixture and now I get a reading that starts at 50.2%, fluctuates up to 99.1% then repeats starting again at 50%. Did I do it right?