Reverse a switch – upated brake fuild level warning circuit

On a 1988 car the brake level sensor works the opposite way around to a 2011. On the 1988 when the brake level gets low the switch connects the circuit and turns the light on. On the 2011 it is the other way around, a reed switch is close so there is power through the circuit until the fluid gets low and opens the circuit.

This presents a problem when you use a 2011 level sensor on a 1988 circuit. Fortunately there is a solution in the form of a relay. It needs to be a normally close type. Or one with open and closed contacts, but connect to the NC terminal. Normally closed means if no power is applied to the relay it is close.

The normally close part of the relay replaces the old level switch. This means when power is applied to the relay the circuit is open and the light is off. When power is cut the circuit closes and turns the light on.

Power is taken from ignition runs through the relay and the new level sensor. This means the relay is power as long as ignition is on and the brake fluid is full. 

The advantage of the new brake sensor circuit is that it will pick up low fluid but also any brakes in the wiring to the sensor.

Shiny new brake level sensor


New relay, normally closed. Pin 30 and 87a take the place of the old sensor, Pin 30 and 86 take power from ignition and are connected to the new sensor.

Used a ratcheting crimper to – highly recommend getting one of these as it makes crimping terminals really easy.

Couple more terminals, these go into the car’s E30 1988 circuit.

This handy relay holder/fuse holder combo makes it easy to change should the relay ever fail, or if the fuse blows.

Using a power probe to test the relay, it simulates the new level sensor circuit, with power applied the brake warning light is off, this would be the normal state when the car is on.

Power disconnected from the relay the warning light turns on – if the fuse blows, the brake level is low or a wiring breaks it will show the warning light.

All the wires added to the fuse and relay holder.

Zipped tied up into place under the dash where it’s easy to get to and service if needed.


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