Problem :battery going completely discharged on 03 plate Rover 25 if car parked for more than 30 hrs without starting. After eliminating the usual suspects (I.E. the alternator diodes and the glove box light) The cause was traced to continuous current drain of between 0.6 and 0.9 Amps through the fuse in the engine compartment fuse box that supplies the imobilser and central locking.

Fuse 5 feeds the Lucas/TRW 5AS combined security immobiliser & central locking controller and the battery backed alarm sounder. By of elimination of other possibilities it was discovered the 5AS unit was not going into sleep mode and was passing current through the ignition key transponder antenna fitted round the ignition switch barrel.

At this stage it appeared the only cure would to be replace the 5AS and probably the antenna, this would require the cars MEMS ECU be programmed to recognise the replacement 5AS and the existing key fob zappers introduced into the 5AS unit memory.

However a much simpler and ***** free ***** fix was used. The 5AS was used for over ten years by Rover and although extra functions are used on some models and all 5AS units are basically are the same and plug compatible. It turns out one of the optional functions is the key transponder. The key transponder’s function is to keep the immobiliser disarmed while the driver restarts the engine in the event of it being stalled. Transponder system was not fitted to early models and the system will function perfectly well without it.as long if the engine has been stalled the driver the unlock button on the fob before operating the starter.
Here is the really good bit to disable the transponder function all that is required is to unplug the antenna --- remove the steering column cowl a 2 minute job and unplug the 2 pin connector --- job done.

These Lucas 5AS units were fitted to most Rover and MG models except the 600, 800 and 75 models between 1992 and late 2003, although the transponder was only fitted from late 1996 onwards. One point to be aware of when checking for this fault is that this fuse carries all the central locking solenoid activation current of 16 amps this is high enough to blow the 10 amp fuse in most digital multimeters.



[Edited on 17/7/10 by britishtrident]

Interesting, my mates zs 120+ does this.

Next door's 216 did the alternator trick, he found out when he touched the alternator and burnt his fingers. We disconnected the battery and had to wait 8 hours before it was cool enough to remove.

Thanks for posting Britishtrident.

Not a lot of threads have such a detailed follow up.
Regards.
AA