Car starts then dies on 1990 Mercedes-Benz 560SEC

As soon as I let go of the ignition key, the engine will die. As long as I keep the key cranked the vehicle will run, any suggestions??

Asked by for the 1990 Mercedes-Benz 560SEC
Sounds like the electrical part of your ignition switch is bad. probably a bad terminal or maybe a broken wire, to the ignition position of the switch.
2 more answers
Ignition switch needs to be inspected fixed or replaced.
It wound up being a shorted tachometer.