Stratocaster Scratchplate Pickguard Troubleshooting:
Brilliant design as the Stratocaster is, having to de-string and remove the scratchplate / pickguard to work on the electrics can be a real frustration. So this Stratocaster scratchplate pickguard troubleshooting guide focuses on what you can do before you even lift the plate! Whilst focussing on the Strat, many of these tips apply to all guitar types.
Things that can go wrong. Let's start by considering what might go wrong. Guitar pickups themselves have 2 likely failure modes; open and short circuit. By far the more common of the two is open circuit, typically a break in the hair thickness winding wire. This renders that particular pickup dead, but does not prevent the operation of other pickups. Less common is the pickup going short-circuit (i.e. measuring zero Ohms with a multi-meter) to the guitar's earth. This could be caused by a stray whisker of wire shorting between the two pickup connection eyelets for example. Then there is the actual wiring within the guitar itself. Again simplest described, poor construction could lead to wires breaking or losing electrical contact, or shorting across other contacts. Whilst things like a faulty tone capacitor can of course also occur, the steps below will capture the majority of Stratocaster scratchplate troubleshooting issue.
Tests without a Multimeter. We can begin with nothing more than a screw driver - 'tap testing'. Connect your guitar to an amplifier / speaker and set the volume on the guitar and amp lower than normal. Select (say) the Bridge pickup and gently tap its pole pieces with the screwdriver. A distinct 'thump' should be audible. Just as importantly, no sound at all should come from the other two pickups with just the Bridge pup selected. No thump shows a problem with that pickup. Repeat this for all 5 way switch positions. This is also a very useful test to do on Strat after wiring work but before the effort of re-stringing. In this way, even tone controls can be checked with no strings in place, as the tone of the 'thump' will be noticeably different moving from full treble cut to no cut.
Tests with a Multimeter So here is where it gets interesting. With the scratchplate still in place, connect a normal guitar lead to the guitar. Connect a Multimeter on a resistance range appropriate for your pickups resistance (20kOhm is a common range) and attach the 2 meter leads to the open jack plug; one connection to the jack tip, one to the jack 'body'. Now get the guitar volume and tone controls to maximum '10'. Select the Bridge pickup on the 5 way switch. Correct to a couple of hundred Ohms, you will now see the Bridge pickups resistance whilst it is connected 'in-circuit'. The same will be true when you select the Middle or Neck pickups.
The in-between 5 way switch positions 2 and 4 put two Strat pickups in parallel, so you would expect to see resistance readings of approximately half the 2 pickups being combined (e.g. for standard 6kOhm pickups a combined reading of 3kOhm). If a single pickup has gone open circuit, you will get an open circuit reading for it, and its related position 2 or 4 reading will be the normal reading of the adjacent working pickup (e.g. 6kOhm in our examples). If its gone short circuit (zero resistance) that is what you see for the pickup itself and the related position 2 or 4. You can even 'see' from its resistance a pickup in operation. Connect a Multimeter as described and select a pickup. Now do the 'tap test' from earlier on the selected pickup but without the amplifier etc. You will see the resistance reading of the pickup jump around by plus or minus several kOhm.
So without even having taken any screws out (or even without strings in place), its possible to get a lot of information about the health and specification of your pickups and general wiring. Stratocaster scratchplate pickguard troubleshooting has never been so simple! Because you are measuring effectively the resistance between the guitar's ground and the output 'wiper' of the volume potentiometer, its even possible to get a sense of any problems there. Regardless of its value (e.g. 250 / 500kOhm), sweeping the volume pot from full volume to no volume (10 to 1 on a Strat) will result in a resistance reading from the value of the pickup(s) selected at full volume to zero Ohms at lowest volume. Note, the resistance seen in the middle of the volume control's travel will be a rather odd seeming value (60kOhm or so) as the low pickup resistance and (relatively) high volume potentiometer resistance interact. Its worth also saying that a faulty 5 way switch (pretty rare in our experience), may give similar results to a faulty pickup(s). Sadly there is no way of getting a definitive answer without physically disconnecting the pickups and measuring them individually.
Conclusion. Stratocaster scratchplate pickguard troubleshooting naturally may need to go beyond what I have outlined already, and of course once you find a problem, physically repairing it will need the scratchplate to be removed. But at least the diagnosis can be done simply and quickly. Earthing problems are of course another topic altogether, and I have put some other links below to start to address those.