Four of my PF L motors don't work. How could I make them work again? I didn't find any information about L motors, only M and XL.
Since the basic architecture of the motors in a line (in this case PF M-L-XL) are the same, the tips you have found for the other models would most likely apply for your case as well.
Otherwise I'd recommend some basic troubleshooting steps:
- Ensure you have a working battery box and cable by setting up your functioning motor. Once you have confirmed that it runs OK, you can be sure that if after swapping the motor for a broken one, the problem must be in that motor.
- Check if you can turn the motor's axle manually by inserting a Technic axle and turning it by hand. It may be more comfortable if you add a gear or a wheel to get a better grip. If you can't turn it at all or only considerably harder than the working one, you should suspect some mechanical failure (e.g. broken gears inside).
- If you have a multimeter, you could check if the electrical characteristics of the broken motors match those of the working one. A significantly lower resistance between the contacts is a sign of a short circuit, while a significantly higher one indicates a broken wire. With a bit of jerry-rigging you could also measure the current flowing through a motor that should be running from a battery box and comparing it to a normal motor's to see if there is too little or too much current.
- You could also try opening up the motor and checking for broken or burned parts, foreign objects or debris, etc. Note, the above guide is for the M motor, but the same principles would apply for the L motor as well.
If none of the above helps, try contacting LEGO, as they would be able to provide replacement parts or further advice.
Based on the information given in the question and comments, it sounds like an all too typical pinch-point wire failure. On large electronics this is fairly easy to fix, but smaller things like PF motors are tedious and require some experience and skill. Because of the design, the wire will be constantly bending against the motor case, or connector plug, in the exact same place over and over again. Because the wire insulation is more flexible then the metal wire, it will stay intact while the wire inside of it has broken. When you wiggle or push the wire to get it working again, you are smashing the broken wires back together again inside the insulation. You might be able to determine whether this breakage is on the motor side or plug side of the wire by isolating one end while pushing/bending the other end, while plugged into battery box.
Since you have new ones coming, and they're already broken, this is a great learning opportunity if you really want to attempt a fix. You are going to need a soldering gun/iron and some solder at the very least.
The basic wiring setup for all the PF motors is very similar. Here are two examples of PF motors with the pinch-point area circled:
Because this is more or a raw electronics question (vs. Lego) and requires a lengthy description, I have placed a link to YouTube listing several videos on how to do this. Any of these videos should help you because all of the insides and disassembling techniques are very similar to your L-motor.
Here are some pictures of the tools recommended by Zovits and myself: