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 DTCU 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 DTCU'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.