When I look up 70% duty cycle for a 91 300E I get Starter signal (circuit 50) recognized.
You should first check that your hook up is correct by checking the duty cycle key on engine off. BTW was that the way you got the 70%, because all federal cars give 70% key on engine off. To get real values the engine must be running.
Also if you do the key on engine off test and you get 85% you have a California car.
Bosch Master, ASE Master, L1
33 years MB technician