Oil level sender, dissected

[Mxfrank]

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.

[Mxfrank]

Here's a close up of the thermodisc. I think the numbers are the closed/open temperature: 23/80. These discs can be made with great precision by selecting the right combination of metals, degree of dish, and thickness of plates.

[spock505]

Did you figure out which bit caused the fault Frank?

[mach4]

Originally Posted by Mxfrank

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.

My theory would be that when the oil is cold it is quite thick. The oil pump is close to a positive displacement pump which would mean that the amount of oil being pumped would be relatively constant but the oil would tend to drain slower and stay in the engine longer than when warm. Without the temperature sensor it would tend to give false positives which would have the average driver overfilling the oil.

...very clever.

If I were designing it I'd just have a thermo switch that would disable the light until at operating temperature.

[Mxfrank]

Originally Posted by spock505

Did you figure out which bit caused the fault Frank?

Probably caused by the float absorbing oil.

[spock505]

..ah thanks, mine does it when cold for 2/3 minutes, I thought they might be serviceable - same thoughts as you, simple fuel tank type operation but maybe not.

[jay_bob]

I had to replace my sensor in the W210's OM606. It uses the exact same part number as the W124's OM603. Except on the 210, there is logic in the front SAM that does not produce the warning until the contact has remained open (the switch is fail safe, it closes on good oil level and opens for low oil level), and the engine has gotten to 60C, and run for a certain number of minutes. The end result was that I was getting the warning every time I passed the grocery store 3 miles from my house +/- 500 feet from a cold start. Almost kind of spooky until I figured out why.

The 124 cluster I believe just has a simple inverter circuit that turns the light on when the contact is open (or the wire gets broken or disconnected).