Got some old Lego electric connectors with the black insulation flaking off?
Don't throw them away! RECYCLE!
You can reuse them by carefully taking them apart, putting new wires in, and closing them back up!
2 Small flat-head screw drivers (jewlers) or similar tools
Digital Multimeter or ohm-meter
You need to be comfortable working with small plastic thingies, careful not to stress the plastic too much, and be able to solder carefully.
If you are really good, you will find a usable match for the wire and avoid the soldering, but I would recommend soldering as these will be "reused" connectors and the contacts will be weaker than new without solder.
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Step 1: Open Connector
Notice there are two tabs on both sides of the connector. These are holding the back plate onto the connector. By carefully putting pressure on the housing (where the tabs are going in) and separating them from the backing plate, you can pop off the backing plate and replace the wires.
The way I did it, was to take two small jewelers flat-head screw drivers and pry open the wire side of the connector, one side at a time. Don't worry if you break off the back plate tab that sticks up, as it mostly cosmetic. Out of about 40 connectors back plates I removed, I broke off several, but the connector and back plate tabs are what is important.
Step 2: Remove Old Wires
After removing the back plate carefully, you will see the old wires running over a plastic tab and onto two contacts that bite into the wires.
Pull out the wires and clean off rubber insulation.
Notice on the back plate there are two indentations that were molded there to provide a housing for the contacts to set into.
inspect the back plate to make sure the tabs are still intact. If you happened to break off the cosmetic, vertical tab on the back plate, you can rotate the back plate so the other tab is on the wire end before closing.
Step 3: Solder New Wires Onto Contacts
Since you are re-using the connector, it is best to solder new wires onto the contacts. You will probably find the contacts that bit into the original wires have moved slightly and you wire may not fit the contacts like the original wires did.
There are two metal plates, one for each side of the connector. Do NOT short them together. Also, you MUST make sure the metal tongs are straight up, otherwise they will collapse when the cover is closed on them. There are guide grooves in the cover plate that these go into.
To ensure a good connection, insert the wires into the contacts and solder them to the tabs as shown.
Use a multimeter to verify that the two sides of the connector are not shorted from soldering, and that each wire has continuity.
Step 4: Insert Back Plate
After the wires have been inserted and soldered, make sure the wires are routed over the plastic tab and straight back through the original opening the old wires were routed to.
You may want to cut the center plastic tab off the housing, if you wire insulation is thicker than the original wires. If you wire is small enough, you don't have to worry about this. Notice the back plate has a cavity to allow the wire to curve over the plastic tab on the housing.
After you wires are routed straight back, insert one end of the back plate back into the Lego connector and push down on the other side.
You should feel the satisfying "SNAP" as all four tabs lock back into place.
Super glue may be optional, but I have not found that necessary. Maybe some back plates with weakened tabs might require this.