This is weird....anyone help?

The coolant level dashboard light came on recently and won't go off. I've checked and topped-up the radiator, and replaced the sensor. Still the light stays on.

I think the sensor has a magnetic reed-switch in it as I can hear it "tick" when a magnet is placed nearby. Both the old and the new sensors give the same (apparently working) resistances when tested with an Ohm-meter and the magnet.

I figured it might be a seized magnet-float in the radiator, but I'm sure it's not that. This is because I tested tried the magnet near the sensor whilst it was connected, but not housed in the radiator. Still the light stays on.

So it must be between the sensor and the dashboard (I think!). Any ideas anyone??

Don't know your circuitry-- but in general -- The sensor is either open or shorted. So what I would do is pull the plug off the sensor and see if the light is one (probably will be - open circuit) then I would take a short wire jumper and short the two leads together at the plug which should simulate proper antifreeze level. If the dashlight stays on - you have a wiring problem - most likely a short which is causing the light to stay on. Good luck

What YEAR car?? MB re-wired this system in 1996 & the coolant light can come on due to the W/S washer reservoir sending unit going bad!! They are in series & both are variable sensors instead of on-off switches.

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Thanks for the comments. Although the car's a 1996 model it seems to have simple reed switches in both the w/s washer sensor and the coolant level sensor. (However both the od and replacement coolant level sensors flips between 0 and 500ohms with level changes whereas the w/s washer sensor flips between 0 and open circuit. )

I've also noticed that one of the wires to each sensor has the same colour & band.

Does this mean they're wired in series and there's a circuit somewhere that looks at the total resistance and works out which light to illuminate?

Cool site BTW. Shall pass url on to other MB owners.

IF the W/S washer sending unit has part number 210-540-00-45 then IT is supposed to be a variable sensor instead of a switch. & they are hooked in series,

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

M.B.DOC - as you predicted, the W/S sensor is part 210-540-00-45 and the radiator sensor is 210-545-00-24. Looks like I need to replace the W/S sensor then!.... Thanks for your help. Never would have guessed that!

chap_C200

Just to let anyone interested know.....

I took the Windshield Washer Level Sensor apart to discover that a 180 Ohm resistor had corroded through due to water ingress. Simple enough job to replace the resistor and reassemble the sensor ensuring it was watertight. Low coolant light now out and problem fixed! Both sensors now work as they should.

Many thanks to M.B.DOC for the help given. Never would have worked it out without that!