I had a bad O2 sensor in my 89-300ce and no indicator lights showed at the dash. My symptoms where exactly as yours, specifically +13ma to EHA with no response to mixture changes (although the motor responded as expected).

After I spliced a 1990 Ford Mustang 302 O2 sensor ($70cdn) to the Mercedes connector of the original sensor my EHA signal would go to +13ma at start and then slowly drop to -3ma at idle when hot. The -3ma is set with the allen mixture screw and should result in slightly rich WOT (open loop) operation. When the system is working properly the signal to the ECU should oscillate between eg. -3.5 to -2.5 about each second.

Jorg