Early 300Es don't have the sensor that you are talking about.

I would check the overvoltage protection relay behind the battery. It has a 10amp fuse on top of it. Check the fuse and it would be great if you had another relay to plug in. Also, check the air flow bypass valve. It's right under the air filter housing.

Try disconnecting the two pole wire on the valve, your idle should go up.

Vin