I'm trying to troubleshoot my 1992 190E 2.6L. Following directions from other posts, I've measured duty cycle at pins 3 and at the x11 connector.
engine not running, I get 30% initially, and 10% when actuating throttle off the stop. That's using the Mercedes definition of time at low voltage being the duty cycle. So voltage is actually high most of the time. I confirmed this using DC voltage measurements, getting 8.3 volts initially and 10.6 volts upon actuating throttle. To make sure I didn't just have leads reversed, I used chassis ground in place of pin 2 and got the same values.
I'm baffled. What could make these readings exactly opposite what they should be? The symptoms of the car are that it runs fairly normally when at mid rpm and driving, but goes rich at idle and will stumble, then clear itself when taking off from a prolonged stop.

Where should I check next? Is it possible someone made a wrong connection when working on the car? It was in a shop a long time with unsuccessful results.