Fuel Quantity Indicator

According to troubleshooting section of AMM If the fuel quantity indicator displays zero kilograms and presents Error Code 2, this signifies the presence of a short circuit within the compensator unit(HI-Z to COMP LO-Z). An Error Code 4 indicates either an open circuit or a short circuit within the TANK LO-Z line.Should Error Code 5 be displayed, it denotes a short circuit in tank unit (HI-Z to TANK LO-Z).Finally, the appearance of Error Code 10 implies the existence of either an open circuit or a short circuit within the HI-Z line itself.
Could you please clarify the technical meaning of the terms 'HI-Z to COMP LO-Z', 'TANK LO-Z line', 'HI-Z to TANK LO-Z', and 'HI-Z line' within the context of the fuel quantity measurement system?

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.
 

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.
  
     
     
 

Thank you kindly for your detailed explanation. I would be most grateful if you could forward me a scanned copy of the manual.

Give me a couple days to get the manual scanned.

Are continuity measurements performed with a standard multimeter, and insulation resistance tests(megger test) carried out with a conventional megohmmeter, acceptable for verifying the following conditions, or is the use of a specialised test instrument required? 1-A short circuit within the compensator unit between the HI-Z and COMP LO-Z terminals. 2-An open circuit or short circuit in the TANK LO-Z conductor. 3-A short circuit within the tank unit between the HI-Z and TANK LO-Z terminals. 4-An open circuit or short circuit in the HI-Z conductor.

Ref:  rotate.aero/forum/test-equipment/224-fuel-quantity-test
You need a PSD-60 or equivalent fuel quantity test unit. You need the appropriate break-out test cable. You need the test adapter to check individual probes at the bussing plug.

NEVER USE A CONVENTIONAL MEGGER FOR ANYTHING INSIDE THE TANK!!!

 

This is the adapter to check individual probes at the removed bussing unit at the wing spar.
  
     
 

Did you check the things I suggested earlier?
Wet bussing plug to tank harness connection.
Did you try swapping bussing plugs?