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Installation of a car reverse alarm system

On Page 1 I surveyed the car thoroughly and worked out a plan for installing the Conrad Parking System. Everything looked straightforward and installation commenced with the sensors on the rear bumper.

A major factor in my case was the curvature of the bumper, as the extreme LH and RH sensors should point rearwards rather than radiating out towards the sides, otherwise the signal's echo is degraded. Each sensor includes a flexible angled seal to help you ‘aim’ them rearwards more accurately. If the bumper also curves upwards or downwards, the spacers will allow you to compensate for this to a limited extent. You really need the best truly vertical and horizontal alignment that you can achieve, to allow the sensors to transmit accurately behind the car and receive an echo back again.

Conrad labels each sensor as L and R (extreme left and right) and L1 and R1 for the inner two. Labels at the connector end ensure that installation errors are avoided.

Early Test

Before going any further, I couldn't resist rigging up the system to test it, as it would be silly after a lot of hard work to find that it was faulty (unlikely, but you never know). Some Blu-Tack fixed the four sensors onto a box, then I connected everything and powered it from a 12V supply: everything was fine. I’d advise storing everything safely in its box somewhere where you won’t tread on it or lose anything in the busy workshop.

An early test to ensure that all's well, just in case The control unit fully connected. The sensors must be connected to their corresponding terminals.

Items Used

Haynes Manual
21.5mm HSS cutter (supplied)
Low speed power drill
Masking tape
Deburring tool (or use a half-round file)
Self-amalgamating tape (car accessory or DIY stores)
Plastic trim disassembly tools (optional)
Tie wraps, adhesive cable bases etc.
Torque wrench for seatbelt rail (see text)
12V tester or multimeter.

So start by marking out the four sensor locations on the rear bumper, making 100% certain that there is clearance behind the bumper after the sensors are inserted.

I find it best to mark a centre line as a reference (see photos below), so I used the centre boot latch as my centre guide, applying masking tape down the bumper and drawing a vertical centre line (CL). The instructions state the sensors can be fitted approximately 30 to 40cm apart, so firstly I found the extreme LH and RH positions consistent with the sensors transmitting rearwards rather than 'outwards'.

Rear Bumper Marking Out and Drilling

As there are four locations, the distance measured between these LH and RH sensors should be divided by three, to locate the two innermost sensors (“there are four telegraph poles, but only three gaps”, as my maths teacher used to say).

After a lot of experimentation I decided mine would be 96 cm apart between the ends, so the four sensors would each be 96/ 3 or 32 cm apart. As every engineer knows, the maxim is ‘measure twice, drill once’ — never is this truer than when drilling big holes in a car’s bodywork! So as an extra double-check, I measured again to confirm that the inner two sensors were 16 cm each side of my centre line, using masking tape to draw lines on.

Some masking tape was run along the bumper to align the sensors horizontally. I took plenty of time to mark the drilling centres, and when I was totally happy that the sensors were spaced at 32cm, the four holes were drilled with the cutter supplied.

A high quality 21.5mm HSS (high speed steel) cutter is included in Conrad’s kit, and this should be used in a slow speed power drill. The masking tape also prevents the drill from slipping (a trick used when drilling ceramic tiles) and a slow startup speed is best. The drill cuts a pilot hole which guides the main cutter through the plastic bumper; ensure the drill is in a horizontal plane parallel with the ground (my indispensable Ryobi One+ 18V drill has a spirit level built in which helps - it's very surprising how badly you can misjudge it when you're so close-up).

Four holes were soon drilled successfully, after which I used a deburring tool to take off the sharp edges. This will also help the sensors to seat properly; you could use a half-round file carefully. I suggest you next slip the sensor's four connecting leads through their flexible spacers (facing the correct way round - the sloping face to the bumper) while you remember!  Although the seals are not mentioned in the instructions they are shown, sort of, in a diagram.

With drilling complete I pierced a hole though the plastic bung in the spare wheel well and the sensors leads were fed through it, pulling reasonably tight. A trick is to bind them together with self-amalgamating tape (an incredibly useful product!) near the point of entry and then pull the wiring back out a little: the tape will jam the loom back into the bung to form a reasonable seal. Some silicone sealant could be used but I didn’t feel the need.

The LH and RH extreme points were eventually decided, then divide the distance by 3 to locate the inner sensors. Note the centre line for an extra reference. Mark the locations on the bumper using masking tape. A good quality 21.5mm HSS cutter is provided in the Conrad kit. A slow-speed drill should be used so that it doesn't slip off and scratch the paintwork...
Starting at a very slow speed, drill the pilot hole. The masking tape will help prevent the drill from slipping. Ensure the drill is in parallel with the ground: ask someone to help if necessary. Continue to drill right through the bumper. One mounting hole shown just after drilling. The rough edges (burrs) should be removed next.
You can use a special deburring tool if you have one (Conrad, Westfalia), to remove the burrs around the hole and provide a better seating for the sensor......otherwise try using a half-round file or a cone-shaped stone in a slow drill. The wedge-shaped rubber seals should be slid over each sensor next (the sloping side facing towards the bumper) then push the correct sensor into the correct hole.
The transducers push snugly into the bumper. Use the wedge-shaped rubber seal to align it on the bumper's profile, facing rearwards. The rubber seal compensates for the curvature of the bumper to get a better directional alignment, facing rearwards as best as possible.
Each lead can then be routed into the car boot. They are clearly labelled to avoid mistakes. Use self-amalgamating rubber tape to bind the wires, then pull back into the bung slightly to form a seal.
End-on view showing the locations of the fitted ultrasonic transducers. A 3/4 view of the sensors - the curvature of the wraparound bumper determined the left and right locations. Later they would be spray-painted (see follow-up below).

Control Box Installation

Onto the next stage: siting the control box. It was very easy to carve a generous space out of the nearby polystyrene block using a knife and Dremel cutting disc: make room for the excess cabling too, which can be coiled up. You might have to hunt around in your car for a suitable location for the box, which might involve removing some plastic trim around the boot or inside the rear wing.  Check that the fused power cable will reach your car’s reversing light feed,too.

At this stage you could make another interim test, by hooking the sensors into the control box along with the VFD and power cable, and hook it to a 12V supply. This will confirm that you have successfully installed the sensors and their wiring.

The control unit prior to mounting in the boot Cutting the moulding to shape to carry the control box. Consider mounting in the rear light cluster compartment or similar, out of harm's way.
You might need to make some space too for the coiled-up excess wiring, located near the control box. The sill and driver's kick panel were removed to allow the cable to run into the boot. Beware side airbag sensors (arrowed), and never interfere with inertia reel seatbelt mountings (explosives).


Car Interior Installation

I turned attention next to the dashboard area and the driver’s footwell. A Hayne’s manual was invaluable for revealing the position of trim panels and those pesky fragile plastic trim clips, so I knew how to dismantle everything. I started at the dashboard end of things. The driver’s footwell trim came out perfectly easily. A removable end-panel on the dash allowed the signal cable to be threaded up onto the dashboard. I coiled up 6"-9” under the dash to allow some slack if needed in the future, taping everything down to avoid vibration noise against the dash.

The cable was routed down through the footwell, along the sill and towards the rear seat/ hatch area, though the driver's seatbelt rail had to be loosened before the side kick trim could be prised out (which was nothing like as easy).

My car also has side airbags in the front seats and the airbag sensor unit is bolted to the side sill. Under no circumstances clout it or interfere with it in any way. Full marks to Conrad for remembering this and highlighting it in the instruction manual.

Never tamper with seatbelt inertia reels themselves either, as they may contain a tensioner which is an explosive that acts in a collision. The driver’s seatbelt rail, which had to be loosened to allow the trim to come partly away, must be re-tightened with a torque wrench to the setting specified in the manual. Do not overtighten or it may be severely weakened.

Rear boot/ hatch

With completion within sight, the next stage is to route the signal cable into the rear hatch or boot area. Rather than taking the car interior to pieces, I coaxed the cable underneath the edges of the plastic panels of the inner wing and boot lining.  The carpet mostly allowed some ‘slack’ and then the cable was routed under the hinge of the tilting seat and into the rear boot, near to the petrol tank sender unit wiring loom.

There were no problems routing the wire underneath the plastic boot-liner panels around the edge of the boot’s floor though I did have to remove or loosen a panel or two. The main thing is not to trap the wire where it might be crushed or damaged by the trim or (in my case) when tilting the rear seat up or down. Allow some slack for movement as necessary, and re-inforce the cable(s) with some self-amalgamating tape at any pressure points to protect against chafing or wear.

Dashboard and Power

The lightweight (75g) display unit can be fixed to the top of the dash with a variety of methods, eg Velcro (not supplied) on the bracket or I used a double-sided adhesive strip to stick the bracket down – clean the dash with e.g. IPA solvent to remove polish etc first. However I didn’t want to screw the display down and I might have to think of another method sometime (a windscreen mount?). Importantly, the display needs positioning to avoid reflection problems, as the display’s glass window is highly reflective. The mounting bracket can easily be bent or adjusted for height.

With all wiring successfully routed to the boot and the various cables connected to the control box, the final step is to connect the power. A reversing light circuit is needed, so with the ignition on and reverse gear selected, use a tester to check which reversing lightbulb wire goes ‘live’ (+12V) and which one is earth (chassis). Use the two Scotchlok-type connectors supplied to hook the red and black wires to the electrical supply. The power lead has a 2A bladed fuse near the control box.

A trim panel was removed to allow the cable to enter the boot through the floor, avoiding problems from the hinge of the rear seat. Splice the power supply leads into a reversing light circuit. Use a tester or multimeter to see which one is "live".
Control unit connected up with all wiring, excess bundled up using tiewraps. I simply covered it with some duct tape to keep it all in place. The VFD fixed on top of the dash. The end panel of the dash was removable to allow the wire to be routed up.

Completion and Testing

The installation was completed by re-assembling any panels etc. in the boot, and in my case I simply used some extra-wide duct tape to secure the lightweight control box and surplus wires (coiled up and tie-wrapped) underneath the polystyrene block in the boot floor. The result was a totally invisible installation, and 'nothing' is what I like to see!

As mentioned, a tiny 3-way dip switch on the VFD rear selects Off, Visual or Visual & Sound. One way of testing the system's response is to turn the ignition on and select reverse gear (the unit beeps once), then ask someone to approach the car from several metres away. Initially the animated display reads ‘---‘ for distance but from about 1-2 metres (depending on conditions) a distance reading will display (in metres only). The audio beep increases in pitch until eventually at minimum distance a flashing STOP sign shows on the display. Meaningful blue markers strobe on the display show detections on the left or rightside rear, though due to the way the VFD is made numerous non-illuminated electrodes are also visible that clutter the display and makes it look 'busy'.

Obviously, like all such car reversing alarms, the system relies on detecting the echo from an obstacle directly behind, so it may not detect very low-level or slender obstructions, items on slopes, bollards or posts, or absorbent items like e.g. shrubs. Used sensibly it will give you a useful guide at bumper level to help you maneouvre your car more confidently. It's a perfect example of a sophisticated microcontroller application condensed into a compact unit.


In use, I found that the Conrad Parking System detected more prominent obstructions effectively, and the distance measurement was helpful. The animated display was appealing and informative, though it's much less legible in broad daylight or direct sun. I would suggest keeping the audio alert permanently switched on; it's clear but unintrusive.

When reversing into a garage the system was at its best, displaying the clear distance from 2 metres behind. Sometimes it may fail to alert depending on the environment or the nature of any obstacles, so it's best used as an additional guide to maneouvering your vehicle.

The instructions for the Conrad Parking System were clear and comprehensive. Installation wasn't difficult for the confident DIY'er but allow the best part of a day to fit the system. The hole cutter (supplied) ensured accurate drilling of the rear bumper.

Overall I was impressed by the quality and appeal of the Conrad Parking System which is proving a very useful driving and parking aid. If it helps to avoid just one minor scrape the device could pay for itself ten times over. Alan Winstanley.

  • The Conrad model no. 852475-89 reviewed here costs £34.95. Other models in the Conrad range include an eight-sensor front/ rear system, an invisible antenna wireless unit that scans the whole width behind the car, and pro-quality Valeo systems.
  • See the update below for replacement sensors and spray-painting them.

Follow-up 2013

Several years later, replacement sensors were painted to match the bodywork. Spray paint to match your car can be sourced online.

After several years, one of the sensor pairs became faulty and the alarm stuck fully on when reversing.

I bought a simple reversing kit (ebay) for about £10 and used the four sensors from that. The two-pin connectors are the same but the plastic sensors had a 'visor' style and work just fine.

As the new sensors were 22mm dia. not 21.5mm, they did not quite fit so the bumper hole was carefully enlarged by 0.25mm radius using a Dremel sanding drum to get a very snug interference fit.

The new sensors were sprayed to match the body colour. Two coats plus one coat of lacquer can be applied without damage. A custom spray paint was delivered by Paints4U.com and was a perfect match for the car's paint code.

More Resources

Conrad Electronic GmbH http://shop.conrad-uk.com/

Amazon (Hayne's Manuals for your car) www.amazon.co.uk

"Honest John" Daily Telegraph motoring column, wise advice on Car Hire Insurance Industry Scams

Ebay (search for disassembly tools and spare sensors) www.ebay.co.uk

Aerosol paint mixed to match your car www.paints4u.com

Prices and availability correct at the time of writing. August 2013.

All content and photos Copyright © EPEMag.Net 2010 and may not be copied or re-used in any other publication without prior permission.

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