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1966 Crown Coupe, 2016 200 S AWD, 1962 Lark Daytona V8.
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Welcome to Allpar. Any fault codes? I would also supect a fuel level sender or its wiring to the cluster or the cluster circuit board itself.
It is interesting that the service manual text mentions a BCM as there is none and the wiring diagrams show the sender going to the instrument cluster, then bussed to the PCM. It comes into the cluster on a DB wire. The other end of the level sender must go to a good ground. I would believe the wiring diagrams. If your neighbor is going to pursue the diagnosis, I recommend a factory service manual.
I don't know how the previous owner's technician diagnosed this, but it sounds like the diagnosis should start again from the beginning.
There was a Chrysler TSB # 14-001-01A for PTs built before 11-04-00 for fuel pumps that were indexed wrong and could cause the gauge not to read full, but the gauge wouldn't drop to empty either.
There is an instrument cluster self-test that is shown on YouTube. Hold the trip odometer button in while rolling the key to on and then release the button. Do this while the gauge isn't working.
The gauges should all move to various points on their scales and the odometer display may show fault codes. Have a pen and paper ready to write them down. I wouldn't say that this is a common problem with PTs. It can happen on any car.

FUEL LEVEL SENSOR DESCRIPTION
The level sensor is attached to the side of the fuel pump module Fuel Level Sensor The level sensor consists of a float, an arm, and a variable resistor
OPERATION
As the fuel level increases, the float and arm move up. This decreases the sending unit resistance, causing the fuel gauge on the instrument panel to read full. The fuel level sensor (fuel gauge sending unit) sends a signal to the BCM and the BCM sends the signal over the PCI bus circuit to the PCM to indicate fuel level. The purpose of this feature is to prevent a false setting of misfire and fuel system monitor trouble codes if the fuel level is less than approximately 15 percent of its rated capacity. It is also used to send a signal for fuel gauge operation via the PCI bus circuits.

P0462-FUEL LEVEL SENDING UNIT VOLTS TOO LOW
When Monitored: Ignition on.
Set Condition: The fuel level sensor signal voltage at the PCM goes below 0.1 volt.

POSSIBLE CAUSES
WARM UP CYCLE = 0
FUEL PUMP MODULE WIRING HARNESS INTERMITTENT

P0463-FUEL LEVEL SENDING UNIT VOLTS TOO HIGH
When Monitored: Ignition on.
Set Condition: The fuel level sensor signal voltage at the PCM goes above 4.5 volts.

POSSILE CAUSES
FUEL PUMP MODULE WIRING HARNESS INTERMITTENT
WARM UP CYCLE = 0
 
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