So, my three year old Uro oil level sender died. Replaced it today with a new OE part ($$), worth every penny not to have that pesky yellow light all the time. I took the old one apart to see how they work, and as usual, an insanely complex design. My number one question was to see how they are assembled, as it's not obvious. Never figured it out. It's like it was carved from solid aluminum, no seams or seals. Odd.

The mechanism itself was unexpected. I thought I would find a simple float which grounded the circuit when oil was low, like the fuel sender. I guess maybe the oil would tend to insulate a copper contact. The sensor is magnetic. There is a ring magnet embedded in the nylon float, which rides on a guide tube. Inside the guide tube is a magnetic reed switch. When the float reaches a level, the light turns on.

But there was another surprise. A thermodisc is riveted to the lower portion of the switch. This may be an unfamiliar component, but every car has several of these. You'll find one inside fan thermoswitches, for example. The disc is a sandwich of two metal plates, each with a different expansion rate. At room temperature, it has a slight "dish", which is impossible to capture in a photograph. When it heats, the plates expand at different rates, and the dish relaxes. As mentioned, these are usually used to drive the on/off function of fan switches and glow plug timers. In this case, the disc seals the drain of the oil sender. When the oil reaches operating temperature, it flattens out, allowing the oil to drain. At that point, the sender can report operating oil level to the dash light. Why? I don't know, but it's clever.

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