It is a repair case on repair Audi A8L J518 module,the vehicle keyless function can not work,and can not start the engine.For Audi repair,please check more cases here:Audi Trouble Repair Projects.
Use ODIS diagnostic to get trouble code,it show below fault code:
00183 – Interior Access/Start Authorization Antenna 1 (R138)
Measure the antenna resistance value is 11Ω and the connection between J518 to antenna work well.So it maybe the J518 module get trouble.
R13 R139 and R154 are parallel in the control unit,mechanic disconnect it and measure the resistance value of R154 IS 2Ω,it mean the trouble maybe in the diagram or R154.After checking the wire,mechanic found the wire under the armrest box was damaged,after fixing trouble symptom disappeared.
Without keylessGO function:
WIth KeylessGo Function
Replace triode and driver chip