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I am replacing the main fuel pump on my 1984 BMW 633 CSi, and as part of the project, I removed the in-tank fuel pump and fuel level sending unit, too.
When I turn the latter upside down, I feel and hear something sliding inside. And, when I turn it right side up, it slides in the opposite direction. That makes sense to me. In the process, it’s probably changing some or other internal resistance, and that’s why the fuel gauge works as it does.
The unit has three wires, and I made a little test harness to see how the resistance changes between the wires. I gave the test harness with insulated male plugs at the end of each wire so I could easily make contact with my ohmmeter.
BMW grounds the fuel sending unit via wiring that runs through the unit and out again, as opposed to grounding to the body of the car (via the body of the fuel tank) such as the starter would do.
The test harness I made comes from wires off an E28, which I’m told is similar to an E24.
I saw three wires in the test harness, with colors being respectively:
- Brown (ground)
- Brown and yellow, mixed (fuel level)
- Brown and black, mixed (fuel warning light)
The brown-and-yellow wire is positive. The current flows in from it, and through the unit via its variable internal resistor, and out to ground.
The brown-and-black wire is positive too. The current flows in from it, and through the unit via its internal switch, and out to ground (if the switch is on).
The connector is a three-wire plug shaped like the letter “D” with the totally-flat part of the “D” uppermost. Looking at the plug, not at the sending unit:
- The brown-and-black wire is to the left,
- The brown-and-yellow wire is in the center, and
- The brown wire is to the right.
Measuring the resistance between the brown-and-yellow wire and the brown wire, I found:
- When the float level simulated an empty tank, the resistance was 75 ohms.
- When the float level simulated a neither-full-nor-empty tank, the resistance was 26 ohms.
- When the float level simulated an empty tank, the resistance was 7 ohms.
I rounded the values to the nearest integer, but you get the idea.
I could get an “open circuit” or “closed circuit” reading as to the brown-and-black wire, relative to the brown wire.