Not a good week...

Tell us your ongoing tales and experiences with your French car here. Post pictures of your car here as well.

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CitroJim
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Re: Not a good week...

Post by CitroJim »

Gosh! That's a clever way of doing it...

Makes perfect sense why it's not happy in some sorts of oils now...

Thanks Mike, really appreciate you posting the above :)
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white exec
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Re: Not a good week...

Post by white exec »

Thanks, Mike, for pulling those posts across - couldn't find anything about the sensor there when I searched yesterday.

That's a clever way of operating, and I wondered whether it explained why the gauge shows Full when the IGN is switched on and the engine is cold, but always shows a much lower reading on a hot engine. I had always assumed that was because the sump level had dropped a tad with a recently-run engine, and that the sensor was sensitive to this. A more rapid rise in resistance gets interpreted as low level (heat taken away more quickly by cold oil than the air above it), but hot oil accelerates the temp rise.

Really learned something there. Will run some checks on ours.
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xantia_v6
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Re: Not a good week...

Post by xantia_v6 »

Those quoted posts were made today, so it depends when you searched.

Note that the described operating principles do not quite agree. My subsequent guess of the actual operation is that the current through the probe is switched on for a fixed period, 2 or 3 seconds, and the current measured at the beginning and end of the period. From the difference in those 2 measurements, the oil level can be estimated. The initial temperature should cancel out in the calculation.

On my XM the level guage reads more or less the same, cold or hot, so maybe yours has been failing for some time.
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white exec
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Re: Not a good week...

Post by white exec »

The lower reading when hot has been like that since new. When cold, gauge can read Full; after driving and re-starting, reading is pretty variable - usually about half or a third. It is responsive to oil level though.
What you say about before-and-after measurements makes sense.
The sensor must get pretty hot to still be able to make its heat-loss calculation in scaldingly hot engine oil.
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CitroJim
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Re: Not a good week...

Post by CitroJim »

white exec wrote:
30 Nov 2017, 10:54
The lower reading when hot has been like that since new. When cold, gauge can read Full; after driving and re-starting, reading is pretty variable - usually about half or a third. It is responsive to oil level though.
What you say about before-and-after measurements makes sense.
The sensor must get pretty hot to still be able to make its heat-loss calculation in scaldingly hot engine oil.


And on that basis, it makes sense now what the thing, in my experience on my Activa, would be OK when cold but would often cry wolf on a hot restart - hot oil leading to an erroneous reading/interpretation...

Excellent :) We are now getting to the bottom of an issue that has long vexed some of us...
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white exec
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Re: Not a good week...

Post by white exec »

XM owners' handbook (1996) also says that the gauge should be read (a) when cold, and (b) after leaving the engine switched off for at least 10 minutes. Guess that the latter is to allow oil level to return to 'stopped', rather than the oil to cool down, which takes an age.