MG-Rover Club Nederland

Rover/MG hard to track down battery drain --- how to fix.

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.

The thing that I find intriguing is your comment about the immobiliser sound? On the ZRs predecessor, the 200, the immobiliser engaged itself if the engine was not started within 30 seconds of opening the door or after 30 seconds of stopping the engine. If an attempt was made to start the car, nothing would happen but you got a warbling sound from the immobilser to warn you of the problem. Pressing on or other of the key fob buttons would then disable the immobilser and allow you to start the engine.

The 25 and ZR used a slightly different system where there was an induction coil around the ignition switch that interrogated the key fob on start up. If all was well the immobilser would be disengaged and the engine could be started. If the fob was missing, or was the wrong one, was out of sync or the battery was flat in the fob then the same symptoms would appear, no start and the warning warble.

The fact that you couldn't jump stat it either (or did you mean bump start?) seems to point to either a faulty immobilser/fob for any of the above reasons.

With (what I assume was a battery booster) it then started probably means that the immobilser was able to 'sync' with the fob and the engine started.

If you had an alternator fault I would expect the battery warning light to be on assuming that the bulb has not blown (does it illuminate when you have the ignition on with the engine off?).

It sounds to me as if the battery in your fob may be on its way out?

If it was just a flat bettery due to leaving lights on/radio etc then you would just get a clicking from the relay, starter, the lights dimming etc but you should not get the immobiliser warning, plus it would have been possible to bump/jump start the car.

Before splashing out on a new battery, change the one in your fob. Might be just as simple as that. If in doubt about your battery get an auto electrician to do a drop test on it. Even though some batteries will show 12V, they do not have enough current left in them to start a car due to damaged plates etc. A drop test should confirm or deny whether your battery is knackered before splashing out unnecessarily for a new one.