Guitar and bass pickups should be kept stock when ever possible. I refrain from rewinding old pickups and try to fix them as a first resort. Many pickups have been taken out of their instrument and either lost or broken while in a drawer. I’ve always suggested that when removing pickups from a guitar or bass, wrap them up in paper, label and put in a safe place. If you sell the instrument, the buyer will most likely want the stock pickups. Having the stock pickups will give you a higher selling price even though you may have a better sounding pickup installed on the instrument. Always remember that: modifying the pickup should only be done when the coil is completely damaged. Even though a pickup doesn’t work, it doesn’t mean it can’t be fixed.

Single coil pickups:

If there is no physical damage to the coils you could have ICPC (Inner coil pole corrosion) and the pickup will have to be completely rewound. Qualified pickup winders can re-use the existing magnet wire from the broken pickup by de-reeling it to the break and re-winding it back on the bobbin.
When the pickup has a damaged outer coil or broken connection to the eyelet the coil can generally be repaired.
When the magnet wire breaks from the beginning eyelet to the internal winds of the coil, skillfulness is needed to repair that kind of coil damage. Especially when the magnet wire is broken at the point it enters the coil.
Humbucking pickups:
Damage to humbucker coils usually occurs when trying to remove pickup cover without the proper soldering iron or tools. Years ago when it the was the “in-thing” to remove the covers, I received many humbuckers because the coils were gouged with screwdrivers or the solder left on the cover gouged through the tape and damaged the coil. Repair can usually be done by removing outer turns on the coil to you get to the break. If several turns were taken off, I would splice the magnet wire back onto the coil and add the desired number of turns.
With pickup covers removed, strings could get caught on the under-edge of the humbucking bobbin. Especially when using a tremolo as the string tension was loosened the string could get snagged under the edge of the bobbin, cutting into the coil when the tremolo was released.
Cold solder joints could occur between the magnet wire (coil) and hookup wire and occasionally at the connection between the hookup and lead wires to the controls. Carefully removing the insulation tape, the connections between the magnet wire and hookup wire can be re-soldered giving the pickup continuity again.
Make sure all switches and controls are working properly. I heard many stories of pickups being removed and still no sound out of the instrument, later finding out it was a faulty control, switch, or terminal connection. Check the pickup at the lead wires and don’t always rely on continuity at the jack.
Pickups with two or more magnets like Gibson P-90s, make sure the magnets are put in with like poles facing each other or it will give little sound.
Before modifying your instrument there are a few things to consider.
The model and age of the instrument. Due to the rarity and value of older instruments I would really question whether any modifications should be done at all. I would only change anything that can be put back stock. Avoid routing, drilling, cutting, soldering anything in the valued instrument. If pickups need to be done, find someone to either custom build or wind a pickup that meets your specifications. Don’t rewind your 53′ Tele just to make it louder and leave those Patent Applied For Humbuckers alone!
Collectibility and value of the instrument should be a factor to keep from modifying a particular instrument. Supply and demand and especially who is using a particular instrument determines the value.
The sound you are looking for. I played and collected Fender Esquires because Jeff Beck used them. I played Telecasters because James Burton, Albert Collins and Roy Buchanan used them. Many players bought Gibson ES-335’s because of Larry Carlton. It seems what player is popular determines what sound a player is looking for. I was always happy with a one (1) pickup Fender Esquire as I could learn to do more with less.
The sound of an instrument is determined by many factors: Body and neck wood type, wood density, wood weight, frets, fret scale, string gauge, string tension, bridge material and dimensions, pickups include bobbin shape, coil shape, coil turns, magnet wire specifications, insulation’s, coil pitch, pickup position under strings, pickup height, magnet material, magnet shape, magnet calibration (aging) magnet placement in pickup, controls such as volume and tone specifications, and guitar cable to the amplifier. There are many basic components that determine the total sound of a pickup but the # 1 is the player. I’ve seen Jeff Beck, Eric Clapton and Jimmy Page all playing the same instrument (Strat) and Fender Princeton Amplifier, each using the same settings, they each sounded like Jeff Beck, Eric Clapton and Jimmy Page. This opened my eyes and I realized that the sound comes from…their left, right hand and music from the heart.
At all means, please don’t modify a vintage or collectible instrument for any pickup modifications. In other words don’t cut an old Strat and put a humbucker in the bridge. The older instruments are getting far and few and will be more valuable if left alone. Read through VGM and look at the many great dealers that see stock instruments and the prices. When you see instruments modified you see a drastic price difference.
When modifying your sound, try and make replacement pickguards and leave the original one stock. There are many replacements pickups available with a wide variety of sounds. Experiment with the sound you are looking for. Even though an artist may endorse a particular model, it doesn’t always mean you will sound like them. Try other models and read articles about their equipment. You may see the artist perform live with a certain instrument but he is actually recording with another. Don’t be afraid to experiment but only do it when you can put the instrument back to stock and not destroy the value of that great piece of work.
When removing pickups don’t cut the leads in the middle just to get them out, un-solder them at the switch or controls. I’ve seen pickups removed by cutting the wires flush with the bottom plate on humbuckers or at the eyelet’s of a Fender style pickup. It’s better to do proper soldering and wiring as it will give your playing perpetuity.