Mark

Give me a couple days to get the manual scanned.

The bussing plugs are mounted to the forward wing spar under the Krueger Flaps.

Those flaps can close quickly and take a person's head off.

Any time someone is working behind the flaps, hydraulics should be locked out. Safety lockouts (if you have them) should also be used.
  
     
     
 

Please reference: rotate.aero/forum/test-equipment/224-fuel-quantity-test#385 for some basic information on fuel quantity systems.

LoZ - Excitation to all of the probes.
HiZ - Resultant output capacitance of individual or group of probes.
Comp - A special probe used to measure fuel density. It receives the same LoZ excitation as all the other probes. It is located in the lowest point in the tank. It can actually be a part of one of the other probes (Mounted at the bottom, but considered a completely separate probe).

You have a Smiths Industries fuel quantity system.

Behind the indicator is a DCTU (Digital Cal Trim Unit). This unit is calibrated for the actual tank it is connected to.

On the wing spar is a bussing plug. It combines all the probes. By combine I mean add the capaitances. Capacitors in parallel are additive.

LoZ excitation is fed to all probes. HiZ is one output of the combined probe reading. The other output is the Compensator capacitance by itself.

On your model 737, only three wires connect the DCTU to the tank. LoZ, HiZ, and Comp. The HiZ signal wire is a shielded wire.

You've got a ton of failures. I would suspect the Bussing Plug or Tank Harness issue.

The DCTU's "can" be swapped for troubleshooting, but being calibrated for a specific tank might introduce more failures.

The bussing plugs can be swapped for troubleshooting. More often than not, they have moisture behind them which is causing all the problems. If you pull it off and see water droplets.... clean it off of both surfaces. Reinstall the BP, you might be surprised when all the faults go away.

If not, all the probes can be checked from the bussing plug. A capacitance and meg check should be accomplished. If you're showing shorts between HiZ, LoZ, Comp, Shield, or Ground.... you've got a bad harness.

I did a ton of troubleshooting on this system during my first days in commercial aviation.

I have a manual that could be scanned and sent to you.
 

Location of sensor.....
  
  
 
  
   
 
  
  
Location of filters (Left sidewall directly above stabilizer).....
  
  
 
  
  
Filter part number (TRN01-010099-01).....
  
  
 
  
  
There is a complete assembly with all four filters (Part number  284T0869-1).....
  
  
 
  
 

First of all, do you have an aircraft with a problem?

If so, exactly what have you done to troubleshoot it?

I’m not going to deal with hypothetical problems.

This is a engineering level issue. I will not be able to answer your question.

I'm not suggesting anything.
I have no clue why Boeing puts certain items in their manuals and excludes others.

I don't know why certain equipment links are not included in Boeing manuals.

As previously stated, either the system works or it doesn't.
If you're wearing tires unevenly.... it doesn't work.
The Fault Isolation Manual should be consulted as to what area might be causing you problems.
Any one of the areas you listed could be causing issues.
The "BIT" or Built In Test on the front of the unit would be the first place to go.

I'm sorry, I can't help you.
I neither work on 777's, nor would I be at liberty to supply any related manuals.