Fresh Water Level Gauge Wiring Issue

[PNM]

I have a VDO Water Tank Level Gauge in my galley that is not working.

While checking the wiring I found the gauge powered by a 2 conductor (black, white) wire. The black wire went to an on-off switch, then to a junction wherein black wires continued to the gauge light, the gauge power, and the 5th blade connector, which I assume is the sender input???????

The white wire also went to a junction that connected it to the gauge light and the gauge power

The 3rd black wire was disconnected from the sender input, so I reconnected it.

When the on-off button is moved to the on position, the light comes on and the needle moves to the full position. (I know the tank is only about 1/2 full).

Unless VDO works in a way I do not understand, it looks to me like the sender wire is missing and the previous owner wired the sender blade connector to the bundle of wires connected to ground.

Any thoughts?

I am going to try to trace the sender wire from the water tank, but most of the bundles run behind the fuel tanks making them very hard to trace.

 

[PNM]

Trouble shooting a 1979 VDO Water Tank Level gauge and sender

Sorry: Brain F'rt! Corrected the resistance reading on the sender.

NOTE: For sake of this discussion, I am ignoring the black and white wires connected to the gauge light.

I was able to find the water level sender with two black wires attached. I cleaned the blade connectors on the sender and installed new female connectors on the two black wires. I tested the resistance across the two sender terminals with the wires disconnected and got 383.4 ohms.

I traced the two conductor (black & black) wire from the sender to behind the galley cabinetry then lost it.

I found a two conductor (black & white) wire coming from behind the cabinetry to the gauge. I could not find the junction where the black & black wires were connected to the black & white wires. But, I tested the resistance across the black & white wires, with the black wire disconnected from the gauge, and got 384.6 ohms. I also checked for voltage across the wires and got 0 volts. So, I am pretty sure they are the wires going to the sender.

I tested the voltage on a second black wire connected to the back of the gauge and it showed 13.5 volts.

The black wire coming from sender was connected directly to the gauge. The white wire from the sender was connected to a bundle of white wires that went to ground. A jumper went from the bundle of white wires to one of the three terminals on the back of the gauge.

When the wires were connected as found on first inspection and the instrument switch turned on, the gauge needle pegged beyond full.

I switched the two black wires - the one from the sender and the 13.5 volt black wire - and the needle moved slowly to the full position, but did not peg.

I have since found a wiring diagram for a 1980 era VDO water tank level gauge that has blade connectors on the back of the gauge arranged like mine. It appears that I now have the wires hooked up correctly: white to "-"; 13.5 vdc black to "+"; and sender black to "s".

Questions:
1. Is a reading of 384 ohms across the two water level sender connections reasonable? Or is my sending unit bad?
2. I don't think my tank is full, but if it is, would a 384 ohm resistance be consistent with a full tank?

Thanks for your help.