Mazda3 Reverse Parking With Ultrasonic Sensors

Ultrasonic sensors are economical, about $30 for a control unit with 4 sensors, but installation typically requires removal of the rear bumper and drilling holes. Even when color matched, these sensors are visible and spoil the smooth lines of the vehicle.

Ultrasonic sensor I developed an installation procedure for the 2013 Mazda3 sedan which does not require drilling holes anywhere in the car. I attach two sensors in the rear licence plate location which typically has an overhang making the sensors less visible. I attach using double-sided adhesive tabs which come with picture hooks. They are strong but easily removed when a better idea occurs.

Here the white tab supports an angle bracket, hacked from a plastic box, to hold the sensor.

Not shown, are the dabs of thermal glue which prevents the sensor from slipping out of the bracket.

The Shiny Bumper

ASCII���User comments This is the rear bumper of the 2013 Mazda3 Sedan. I attached a sensor on each side of the licence plate holder. I find this view less disturbing than holes drilled in the bumper but perhaps it could be improved by someone with skills and tools for car eyecandy.

The 2014 Mazda3 Sedan has the licence plate recess moved up to the trunk lid which also has the reverse lamps. So wiring the ultrasonic sensors should be much easier.

Attachment Options

ASCII���User comments My first experiment attached the sensor with a cable-tie mount. However, the white adhesive mount and the cable tie contrasted too much with the graphite mica color of my M3.

But it was an easy and useful test.

Bumper Wiring

The bumper has two metal plates behind the plastic bumper which (fortunately) have large holes. Guide the sensor cables through these holes by taping the ends to a coat hanger or some plastic rod. Now remove the holding pins of the trunk trims and put the trims away. Trace the license plate lamp wires and find the rubber grommet feeding into the trunk. Pry out the grommet, and pull the sensor wires though using the coat hanger trick. Cut a notch in the grommet and replace it so that there is no opening into the trunk.

Routing the sensor cable with a connector on one end may be difficult. If so, cut the sensor cable in half. But be prepared to join the severed ends. Inside the cable is white wire with a stranded shield.

Reverse Lamp Connection

ASCII���User comments The M3 reverse lamps are in the trunk lid and there are 8 grey wires going to the trunk lid. The correct connections can only be made near the lamp which has two wires. Green is about +12v and black is common. I prefer to strip the insulation at non-adjacent points, solder the connections, and then apply electrical tape. There is a 15amp fuse protecting the car wiring harness so I include a 3amp inline fuse to minimize the risk of fire in the additional wiring for the sensors. (Disclaimer: I am not an electrical engineer)

The control unit power wires are very short so you need to use about 2 meters of 18-guage twisted pair wire to go from the reverse lamp to your chosen location of the control unit, which should be somewhere in the trunk.

Trunk Wire Routing

ASCII���User comments The trunk lid wires are protected by a very flexible rubber hose. I tied the power wires to a plastic tie wrap and pulled it through the hose. Then joined it to the control unit power cable which I placed in the spare tire location under the trunk carpet.

This photo shows the wires being guided upto the trunk lid before soldering to the reverse lamp.


shift-display2.jpg Finally, there is the display-beeper cable which is routed under carpet to driver’s seat. I attached it to the console next to the shifter. It is only active in reverse gear and the beeping gets more intense when you reverse less than 2 meters from an object behind your car.


The 2-sensor installation works well for straight reversing into a parking space. It helps you judge the actual distance to a car or wall which you see in your mirrors. It is not dependable for parking between cars on the street since the bumper corner will lead and there are no corner sensors. However, the two sensors are useful for final adjustment when you are parallel to the curb.

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