Tow Starting a W123
I read a lot about differnt techniques to "Tow Starting a W123"
Here is the straight story from the manual of my 1978 300D
I did image and text so it could be searchable.
Emergency-starting and Towing the Vehicle
Towing eyes are situated underneath the front and rear end on the RH side. Use a tow- bar or long cable. Tow-start vehicle only with the battery connected.
Caution: Remember, however, that until the engine is running, the power steering and power brakes do not offer assistance and considerable additional effort is required to steer and stop the car.
Emergency Starting of an Engine (Tow-starting) in a Vehicle with Automatic Trans¬mission
Turn idle speed adjusting knob counterclockwise to the stop. Shift selector lever to "N" and turn key in steering lock to position "2". Have vehicle towed. Having attained a speed of 18 mph (30 km/h) — cold transmission — or 30 mph (50 km/h) — warm transmission — keep on driving at this speed for approximately 2 minutes to ensure sufficient fluid pressure in the transmission.
To crank the engine, shift selector lever to "L" (300 D, 300 CD - "S"). Touch the accelerator only when the engine starts firing. As soon as the engine has started, immediately return selector lever to "N". Adjust idle speed. It is important to allow the engine to idle for at least 3 minutes before starting off because the preglowing process starts when the key is in steering lock position "2" and is not immediately disrupted after the engine has been tow-started. During this time the preglowing process is cut out automatically. If the engine has not fired after a few seconds, shift the selector lever from "L" to "N" to protect the transmission from damage.
For a new starting attempt, tow- start the vehicle for some time again with the selector lever in position "N" and repeat the starting procedure. The same method can be used to start the engine in emergencies when rolling downhill.