Car Mechanics (UK)

Auto Electrics

Part nine: Key­less en­try sys­tems.

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There was a time when key­less en­try sys­tems (KES) were only fit­ted to high-end ve­hi­cles. As tends to hap­pen with tech­nol­ogy, KES have now be­come common on a much wider range of cars. There are many dif­fer­ent brand names and acronyms for the sys­tems but, by and large, they func­tion in a sim­i­lar way. The key can re­main in your pocket or hand­bag and there is no need to press the un­lock but­ton on the key-fob to en­ter the car. Once in­side the ve­hi­cle, the key is de­tected and the en­gine can be started with the push of a but­ton. For driv­ers, the fea­ture is a sim­ple con­ve­nience, but the tech­nol­ogy be­hind it is quite so­phis­ti­cated.

Key­less en­try sys­tems utilise some of the fol­low­ing tech­nol­ogy:

Transpon­ders De­vices for re­ceiv­ing and trans­mit­ting ra­dio sig­nals.

Mul­ti­ple an­ten­nae For low fre­quency, ul­tra-high fre­quency and ul­tra-wide band com­mu­ni­ca­tion.

Ca­pac­i­tive sen­sors Door han­dles use these to de­tect the driv­ers’ hands.

Fixed and rolling codes For au­then­ti­ca­tion of ve­hi­cle keys.

Se­rial, LIN- and Can-bus com­mu­ni­ca­tion be­tween mod­ules.

The fol­low­ing break­down of the steps that take place be­fore a door can be opened is a generic view of KES op­er­a­tion and is not spe­cific to any brand or model.

Let’s start with a driver en­ter­ing a ve­hi­cle that is locked and armed. The driver’s door’s ex­te­rior han­dle de­tects a change in ca­pac­i­tance as the driver’s hand en­ters the airspace around it. The ca­pac­i­tive sen­sor sends a sig­nal to the KES mo­d­ule. The KES mo­d­ule sends a com­mand to the low fre­quency (LF) an­tenna in the driver’s door.

The an­tenna sends an LF 125KHZ sig­nal to the key. The key re­sponds with the au­tho­rised UHF sig­nal. The ra­dio re­ceiver de­tects the sig­nal. The ra­dio re­ceiver con­verts the sig­nal to se­rial com­mu­ni­ca­tion and re­lays it to the KES mo­d­ule. The KES mo­d­ule val­i­dates the sig­nal. The KES sends a ‘key valid – open doors’ mes­sage via Can-bus to the cen­tral gate­way mo­d­ule. The gate­way mo­d­ule re­lays the mes­sage via Can-bus to the door con­trol mod­ules. The door con­trol mod­ules open the locks and the driver opens the door.

All of the steps out­lined op­po­site take place seam­lessly in fewer than 500 mil­lisec­onds and the ma­jor­ity of driv­ers will be un­aware of the com­plex­ity at work as they reach to open the door. The most common prob­lems with smart keys re­late to bat­ter­ies. When trou­bleshoot­ing, this is a good place to start. Some keys will need re­pro­gram­ming after re­plac­ing the bat­tery, but this process will vary from model to model and in­struc­tions can some­times be found in the own­ers’ man­ual.

 ??  ?? LF an­tenna sig­nal.
LF an­tenna sig­nal.
 ??  ?? Can-bus sig­nal.
Can-bus sig­nal.
 ??  ?? LF door an­tenna.
LF door an­tenna.
 ??  ?? Smart keys with transpon­ders.
Smart keys with transpon­ders.
 ??  ?? Pi­co­scope in­duc­tive wand can be used to mea­sure LF sig­nals.
Pi­co­scope in­duc­tive wand can be used to mea­sure LF sig­nals.
 ??  ??
 ??  ??
 ??  ?? Key­less en­try in­te­rior mo­d­ule.
Key­less en­try in­te­rior mo­d­ule.
 ??  ?? Satel­lite an­tenna.
Satel­lite an­tenna.
 ??  ?? LF in­te­rior an­tenna.
LF in­te­rior an­tenna.

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