Rover SD1 Sub-System #1 - Coolant Level Indicator
The Rover SD1 Coolant Level Indicator system is inherently unreliable yet rarely due to component failure.
First off, the sensors' job is to detect the presence of coolant by passing an electrical current through the fluid. There are two isolated sensors in the expansion tank , which is not earthed, and one in the radiator  which is earthed.
The Low Coolant Level Indicator  inside the binnacle cover is powered by 12 volts on the White [W] wire which also feeds the Warning Lamp , itself connected to the level indicator that also has an earth connection.
With low coolant in the expansion tank OR the radiator, the path to earth on the White/Light Green [WLG] wire from the level indicator via the three sensors is broken and the Level Indicator illuminates the Warning Lamp by electronic trickery.
When BOTH fluid levels are correct the Level Indicator senses a completed path to earth and extinguishes the lamp.
To prevent the lamp reacting to coolant splish-sploshing around the sensors intermittantly breaking the path to earth the Level Indicator has a built in delay of several seconds so the Warning Lamp does not blink in sympathy.
When the system will not extinquish the lamp, remember the sensors and level indicator rarely fail but their connectors are subject to corrosion/broken wires and the sensors suffer lime scale deposits which inhibit their function. Check those things first.
When suspicion falls on the Level Indicator, simply connect the WLG wire from the indicator to earth and the lamp should extinguish. If it does not the fault will be with the indicator or it local connections inside the binnacle cover.
Please feel free to contact me and correct any errors and omissions.
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