The value of the pot may be labeled on it, printed in ink or molded/stamped into the case.
The value of the pot may be labeled on it, printed in ink or molded/stamped into the case.
Yes I know what the value is supposed to be, but I suspect it might be faulty, that's why I want to check it with my meter (in the guitar if possible).
Unfortunately the relatively low resistance of the pickup will dominate the pot when measuring the total circuit. You will have to lift one wire, from either the pot or the switched in pickup to get a valid reading. For example, a 7K pickup and a 250K pot in parallel will read 6.8K, telling you more about the pickup than the pot. However, if you know already the DC resistance of the pickup accurately, you can solve for the pot value by 1/Rpot=1/Rtotal-1/Rpickup.
Unfortunately the relatively low resistance of the pickup will dominate the pot when measuring the total circuit. You will have to lift one wire, from either the pot or the switched in pickup to get a valid reading. For example, a 7K pickup and a 250K pot in parallel will read 6.8K, telling you more about the pickup than the pot. However, if you know already the DC resistance of the pickup accurately, you can solve for the pot value by 1/Rpot=1/Rtotal-1/Rpickup.
kinda correct , except thats only is true if the pot is in series with the pickup , on full volume it's in parallel , R1 +R2=Rt is a series equation , as you stated the best way, is to measure the pot with wires lifted , I've got t o say though , I've been down that path with little or no difference .
I did this a few weeks ago. Connected a TS cable to the output and measured the resistance between the tip and the sleeve.
I calculated that when my pickup had a resistance of 6.2k and the pot 250k, the resistance over the tip and sleeve would look like this while turning the pot:
I checked with my measurements if this graph was correct. If not, I would have another pot resistance.
You have to know the resistance of one of the pickups though if you want to do this.
Edit: I wanted to it this way to avoid having to open up the guitar.
kinda correct , except thats only is true if the pot is in series with the pickup , on full volume it's in parallel , R1 +R2=Rt is a series equation , as you stated the best way, is to measure the pot with wires lifted , I've got t o say though , I've been down that path with little or no difference .
He DID use the parallel equation 
However, doesn't it depend on wiring?
Just looked up a random schematic online; I think this wiring setup would actually read correctly, wouldn't it?
sorry, old eyes , his formula is infact correct , still , I've not had much joy changing pots, really doesn't seem to make much or any difference (could be old ears as well )
1/Rpot=1/Rtotal-1/Rpickup.
Here a basic wiring diagram of a Strat wich is easier to read. It shows a parallel connection .
The formular of Klappy would work but the resistance value would be very close to the Value of PU. Even if a value its written at the PU the real value can vary due manufacture bandwidth.
Other guitars have other wiring (even the Strat can have some alternative wiring)
A really answer you get only by desoldering the root of the switch.
On my Tom Anderson Drop Top it is something called vintage voicing. Here is the diagram
[Blocked Image: http://i1054.photobucket.com/albums/s482/mortyblack/186058_zpsxdyo03vf.jpg]
Display MoreI did this a few weeks ago. Connected a TS cable to the output and measured the resistance between the tip and the sleeve.
I calculated that when my pickup had a resistance of 6.2k and the pot 250k, the resistance over the tip and sleeve would look like this while turning the pot:
I checked with my measurements if this graph was correct. If not, I would have another pot resistance.
You have to know the resistance of one of the pickups though if you want to do this.
Edit: I wanted to it this way to avoid having to open up the guitar.
That's an excellent way of checking the pot along it's travel. I whole- heartedly endorse this approach for general troubleshooting of a pot.
I'd also like to explain why this works for those who might not see it:
As with my other post, it applies only to a single pickup in parallel with a volume pot. Most guitars can do this by simply selecting a single pickup. Tone controls don't matter because of the DC blocking capacitor. At volume =10, the equation I gave earlier for two resistors in parallel will apply. The measured Rtotal will be close to that of the pickup itself, but slightly lower, like I showed before. This is the left side of Daan's plot. As you lower the volume pot, you are adding series resistance to the pickup and removing it from the pot. Essentially, you still have two resistors in parallel, but now the resistors are R1 = (Rpickup + Rseries) and R2 = (Rpot-Rseries), where Rseries is the resistance between the "top" of the pot and the wiper. If you keep lowering the volume, you will eventually have a situation where R1=R2, this is the maximum of Daan's plot. If you keep reducing volume, then R2 will become less then R1 and will dominate the measurement again, making the Rtotal decrease until R2=0 where Rtotal=0. It should be noted that the shape of the curve will depend on the taper of the pot, but this won't affect the maximum value. The maximum will occur where R1=R2=1/2*Rpickup+Rpot.
So doing some simple algebra:
1/Rmax=1/(1/2(Rpickup+Rpot)+1/2(Rpickup+Rpot))
1/Rmax=4/(Rpickup+Rpot)
Rpot=4Rmax-Rpickup
If you play around with the numbers to determine the errors you get from guessing the wrong pickup resistance, you'll see that the errors are small. If your pot is a true 250K and your pickup is a true 7K, guessing 6.5K gives a calculated pot of 249K and guessing 7.5K gives 251K. Definitely close enough.
So to measure your pot in circuit:
Here are some plots for different tapers:
[Blocked Image: https://s18.postimg.org/9lpir5iyx/pottaper.jpg]
[Blocked Image: https://s22.postimg.org/sgjxvous1/insitu.jpg]
Note that with a typical log volume pot, the maximum resistance measures should be somewhere between 5 and 10 and not at 5. Also most pots aren't true log pots, but rather linear pots with two different slopes that approximate a log pot (a "linear log" pot).
On my Tom Anderson Drop Top it is something called vintage voicing. Here is the diagram
[Blocked Image: http://i1054.photobucket.com/albums/s482/mortyblack/186058_zpsxdyo03vf.jpg]
That's one case where Daan's approach and my equation won't work unless you know what you are looking for, but this is fairly uncommon. This wiring puts another resistor in parallel with the pot when the single-coils are switched in. It warms up the single coils a bit by reducing the load down to about 250K. The load is still just the 500K pot load for the humbucker though. If you were to measure the pot on a single-coil setting, you would get around 250K with the 470K load resistor. You would still measure 500K for the pot when the bridge humbucker was switched in.
You guys hurt my brain.
I'd just buy a damned new one and solder it in for all this theory and wondering, lol. (well, I DO like excuses to solder tho)
Yeah, but all the hard work is done for you 
.
Meter your resistance as you sweep your volume pot. This will tell you if it's log or linear. Use the max resistance to calculate the value in seconds. Done!
Like cenzo said, it usually ISN'T the pot that's the problem (unless it's scratchy).
You guys hurt my brain.
I'd just buy a damned new one and solder it in for all this theory and wondering, lol. (well, I DO like excuses to solder tho)
Yeah, but all the hard work is done for you
Meter your resistance as you sweep your volume pot. This will tell you if it's log or linear. Use the max resistance to calculate the value in seconds. Done!
Like cenzo said, it usually ISN'T the pot that's the problem (unless it's scratchy).
Good to know it's probably not the Pot that would affect the tone. I've just had so many problems with this project that I need to start from scratch and check every component.
I'm a geek so THANKS for all the schematics and stuff. I have 2 humbuckers, 1 volume, 1 tone and 3 way switch in this guitar, I was wondering if I put a thin piece of paper in the switch contacts, Would it disable the connection to the pickup and I could check the Pot?
Rock On,
Jen 
Good to know it's probably not the Pot that would affect the tone. I've just had so many problems with this project that I need to start from scratch and check every component.
I'm a geek so THANKS for all the schematics and stuff. I have 2 humbuckers, 1 volume, 1 tone and 3 way switch in this guitar, I was wondering if I put a thin piece of paper in the switch contacts, Would it disable the connection to the pickup and I could check the Pot?
Rock On,
Jen
You're probably in luck since you just have one vol and tone. They probably come after the pickup selector in this case. A thin piece of plastic is ideal, but paper should be fine too. Maybe stick a piece of scotch tape on the paper first if you want to go crazy.
