Oil light won't go out after engine rebuild 😩

[Jlo]

Evening all.
I have a Smart 450 petrol which at 70,000 miles melted a piston. following the guide on fq101 and information from other sites I have rebuilt the engine with new pistons, valves and big end shells. I also replaced the timing chain and tensioner.
It now starts and runs without any rattles but the oil light won't go out. I've tested the switch with a tyre pump & pressure gauge and it seems OK In any case it would be a bit coincidental for the switch to fail as it was OK before the engine gave up.
I've had the sump & rocker cover off again and the oil is getting around and everything looks OK.
Any ideas as to what could cause this?

[tolsen]

Check oil pressure with a gauge.

[mottofreee]

If it was without oil pressure the engine runs noisy (hydraulic tapet).
But even so, what is the measured oil pressure at idle?

[mottofreee]

Remove the thread from the oil bulb and touch the thread at the table. If the oil lamp goes off then the oil bulb is defective (contaminated and repaired by replacement).
If the oil bulb does not go out, it must be diagnosed with MB Star - if MB Star says nothing, it means the thread is broken.

Another method is to reset the computers by disconnecting the battery (12V) and reconnecting after at least 10 minutes. Chances of success are small but it's worth trying because it's a simple procedure.

[Jlo]

Remove the thread from the oil bulb and touch the thread at the table. If the oil lamp goes off then the oil bulb is defective (contaminated and repaired by replacement).
If the oil bulb does not go out, it must be diagnosed with MB Star - if MB Star says nothing, it means the thread is broken.

Another method is to reset the computers by disconnecting the battery (12V) and reconnecting after at least 10 minutes. Chances of success are small but it's worth trying because it's a simple procedure.

Hi Mottofreee, Thanks for your reply. I'm not sure I understand what you mean about removing the thread and touching it at the table. Are you talking about removing the instrument cluster to check the bulb or are you referring to the sensor?

[Thrumbleux]

I think he means grounding/earthing the wire to the oil pressure switch.
If the system works as per older cars, if this wire is short circuited to earth anywhere along its length, the light will remain lit. This is probably what has happened.
(The light extinguishes when the continuity to earth by the closed switch is interrupted when the rising oil pressure opens the switch).

[mottofreee]

Thank you.

[mikerj]

If you have a multimeter, check resistance between the oil pressure wire and chassis ground with the wire disconnected from the pressure switch and ignition off. If you get a low value (e.g. a few ohms or less) check to see if the wire has been trapped/pinched during engine installation.

[mottofreee]

So:
Disconnect the thread from the oil sensor - located on the engine block near the oil filter.
If the light stays on it means it's a problem in the wires (the electrical system)
If it goes out, it means the sensor is faulty.

[Jlo]

So:
Disconnect the thread from the oil sensor - located on the engine block near the oil filter.
If the light stays on it means it's a problem in the wires (the electrical system)
If it goes out, it means the sensor is faulty.

Spot on! Thanks - it was the sensor