I need a wiring diagram for my 1991 peavey koa bass guitar. It has 2 split coil, Alinco humbucking passive pickups. 2 volume and 1 master tone knobs. Thank you!!!

Broken cable inside, possibly at the output jack?
Or a failed potentiometer (volume or tone)
Pull the back plate off, and have a look at the wiring, Trace it with an ohmmeter to check for continuity at each step.
https://www.premierguitar.com/articles/23496-guitar-shop---five-ways-to-use-a-digital-multimeter

Standard J-Bass wiring is parallel. Not series.
http://www.mojotone.com/support/Schematics-for-Pickups-and-Guitars/Jazz-Bass-Wiring-Diagram

To run the pickups in series, the ground of one pickup would feed into the hot of the next. That's not what comes from the factory.

What you describe sounds like pickups fighting against each other meaning, does this occur when using pickups individually or together? This is normal when using more than one pickup together, if it occurs in single mode, something is wired wrong. Another thing is, If using single, are you splitting coils? Individual coils wired wrong can do this, a bad pickup selector switch can also. A loose wire or fiber touching the tone pot capacitor or ground could also. A dead coil on a humbucker can as well. I'm guessing that you are using these in combo meaning a possible pickup selector switch may not be switching properly. The player's level of experience is also in my mind since, all of my guitars do the exact same ONLY when I use more than one pickup. I hardly ever use more than one at a time unless I'm playing a strat.

> There are plenty of ways to wire this guitar...check out wiring diagram 2 humbuckers 2 volume 1 tone 3 way switch Google Search

That guitar features two single coil pickups when stock. If, however, you or someone else installed a 4 conductor humbucker in the neck and you want to access other tonal options, check out the Seymour Duncan web site- they have tons of easy to understand diagrams- you just need the soldering skills to make use of them!
Make your selections on this page and good luck!
http://www.seymourduncan.com/support/wiring-diagrams/

If it is the socket on the bass, it should only be 2 wires, on the 2 ears of the socket. He must have soldered it to the wrong place. The wire from the pickup switch(if more than 1) or the volume(if only 1) should go to the part of the socket that touches the tip on the guitar cord when plugged in. The other is ground and connects to the backs of controls and switches throughout the instrument, even to the bridge and tailpiece.

Here is the wiring diagram for a 2 humbucker, 2 volume, 2 tone, 3-way switch setup. The wires off the pickup may be a little different. Just attach the lead wire to the arm of the pot and the ground to the ground on the top of the pot. Should be very strait forward.

http://www.seymourduncan.com/pdfs/support/schematics/2h_2v_2t_3w.pdf

Match your resources of your guitar at this site to find typical schematic:

http://www.guitarelectronics.com/c=t6JiCQzmaFtGBjJUnATWv1XQ2/category/wiring_resources_guitar_wiring_diagrams/

Sorry but your post did not provide enough info. Are you having a problem with the volume control. Do you need a wiring diagram. What model Peavey bass do you have. For instance, the Peavey bass I have has active electronics ans requires a 9 volt transistor radio battery to power them.

Find appropriate diagram at this site:

http://www.guitarelectronics.com/category/wiringresources/