Park­ing sen­sors

Car Mechanics (UK) - - Help! -

I have a 57-plate Ford Fo­cus 1.6 Ghia five-door with non­func­tion­ing rear park­ing sen­sors. When I se­lect re­verse, there is no beep­ing, al­though the re­verse light il­lu­mi­nates. I’m not sure if they are OEM or aftermarket sen­sors, but they are black in colour while the car is sil­ver. I have care­fully cleaned them and tried lis­ten­ing for any click­ing noise, but I can hear noth­ing. Mike Boswell

The first point to es­tab­lish is whether the sen­sors are fac­to­ry­fit­ted or aftermarket items. The nor­mal de­fault func­tion for a failed fac­tory-fit­ted sys­tem is a con­tin­u­ous beep when se­lect­ing re­verse. It can also be read for faults by a ded­i­cated code reader.

The con­trol box or park­ing aid mod­ule should be lo­cated in a space be­hind the rear off­side whee­larch. Trac­ing back the wiring from the sen­sors to the con­trol box, you should then be able to de­fine if the wiring is a fac­tory-fit­ted loom or an ac­ces­sory con­nected into the rev­ers­ing light cir­cuit. An­other giveaway is that aftermarket units have a built-in speaker, whereas Ford units have a sep­a­rate speaker mounted near the rear seat­belt mech­a­nism.

If the sys­tem is an aftermarket unit, the most cost-ef­fec­tive so­lu­tion would be to re­place the com­plete sys­tem; these are read­ily avail­able start­ing from around £15 per set on ebay. If the sys­tem is an OEM unit and as it is not mak­ing any sound, the feed to the unit should be checked – this is pin num­ber 1. The earth con­nec­tion should also be in­spected – this is pin 16. If the feed and earth con­nec­tions are fine then the prob­lem may be the speaker; this can be tested us­ing an ohm­me­ter and the re­sis­tance should be 50 ohm + or – 7.5 ohm.

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