Even the 737 Jurassic have this door. But only the NG have the micro-switch.
When you want to lower the L/G on the 737-100 to 500 manuell the L/G lever must be in the OFF-position to connect the UP and DOWN side of the actuators with the return line. If you donīt do this, the trapped hydraulic fluid prevent the correct lowering.
In the NG it does not matter in which position the lever is because the opening of the manual extension door actuate the micro-switch.
Now you have the manual extend solenoid valve in the landing gear selector valve powered and the L/G goes down as if lever is in OFF.
With this in mind you can do a little trouble shooting if L/G did not go Up after lift off:
If you pull the CB D6 (LANDING GEAR ALTN EXTEND SOL) and then retraction is possible, you have a faulty activated micro-switch.
Or simply the access door is open
