Oil Warning Light Theory, Diagnosis & Repair
(Classic 89-95)

Theory of Operation
Diagnosis and Repair


On late model Classics, the operation of the oil pressure and oil level warning light is rather confusing and can lead to false problem diagnosis. (For an overview see the oil light section on the Common Problems and Fixes page for the Classic Range Rover). To try and reduce the confusion, Bill Fishel kindly contributed this writeup explaining the different modes of operation of the oil warning light.

Theory of Operation
From the circuit diagram in the manual, it is evident that  +12 volts is switched on through the ignition load relay and is sent directly to the oil light in the binnacle instrument cluster. (If you check voltage from the bulb to battery negative with the ignition switch on, it should read +12 volts.) This is the white wire in the diagram. This is not a closed circuit yet hence the bulb should not be lit.

From the bulb the white/brown wire branches to three separate devices; a) oil pressure switch, b) low oil level logic unit and c) heated screen timer unit. These are described below.  Thus a path to ground through any of these three will cause the bulb to light. 

     a) The oil pressure switch is normally closed when there is no oil pressure. This provides a path to battery negative allowing the oil indicator bulb to light        constantly. After the engine is started and oil pressure rises this switch opens the path to battery negative allowing the light to extinguish. 

    b) The low oil level logic unit is an electronic device that can also provide a path to battery negative. This path is normally open when the oil level is okay. When the low oil level probe (in the oil sump) detects that the oil level is low the logic unit is activated switching a path to battery negative on and off. This pulses the oil light for ten to twenty seconds. After the ten to twenty seconds the unit times out and opens the path to battery negative allowing the light to extinguish.

   c) The heated screen timer unit doesn't provide a path to battery negative for the oil light. It detects that the oil pressure switch has opened allowing the heated screen timer to start, also allowing the heated screen load relay to energise. It uses the oil pressure switch as a convenient method of detecting that the engine is running okay before allowing the heated screen to energise. It should not have any effect on the oil warning light.

(Note that 1989 was the first year for the heated windshield/windscreen, and 1995 was the last year for the oil level logic unit).

Diagnosis and Repair
To distinguish between the different causes of your oil light flashing or staying on, Bill  offers the following advice: If it comes on or flashes for 10 to 20 seconds upon starting and then goes out, suspect the low oil probe (or the $7 oil pressure sensor)  in the sump pan. If it continues flashing more than 10 to 20 seconds, suspect the logic unit itself (or, the oil may actually be low!!).

On a left hand drive vehicle the logic unit is located under the lower left dash panel just under the left most demister hose. Bill notes: "It's a black plastic box about 3x3x1 with 5 wires about 6 inches long going into a 5 pin multiplug. If it is the culprit, a new one unfortunately costs about $200. One solution (suggested by a dealer mechanic) is simply to disconnect it and do without low oil level warnings -- that is what Bill did on his 89 Range Rover. Bill notes: "The unit is labeled Jaeger and Made in France on the front. There isn't much inside the box. The LM1030 IC appears to be the heart of the unit. The rest of it is resistors, diodes, caps and trimmer pots. Can't be more than 20 dollars in components". 





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Page revised February 2, 2012