I flattened the battery on my 2013 Suzuki SX4 S-cross 1.6 petrol SZ4 by leaving the lights on. The emergency call-out man jump-started my car and, directly afterwards, I realised an icon on the dash told me the seatbelt/ airbag circuit had gone. The call-out man connected his kit directly to the battery – should he have connected his negative to an earth point or some other way to save the ECM? The opinion of the garage was that he suspected the ECM damage was caused by the battery recharging or a weak battery, which I have had replaced.
Another point you may be able to clear up for me: with the ignition off and no lights on, the brake lights come on with the brake pedal depressed. Is this right? Peter Horne You mention that the call-out man connected his kit. If you are referring to a stand-alone jump pack, then connecting directly to the battery is perfectly acceptable. The main reason for the advice to connect the negative jump lead to an earth point on the vehicle and not the battery itself is to prevent the possibility of a spark when attaching or disconnecting the lead. Stand-alone jump packs are protected from this type of sparking.
If the vehicle was started with jump leads from a second vehicle, there is the possibility of surge damage occurring if the correct procedure was not followed. The jump leads should be fitted with the donor vehicle engine off, then the donor vehicle started up and left ticking over for a short time while the voltage balances out. After this, the vehicle with the flat battery can be started.
I would agree that the damage to the airbag circuit occurred as a result of the flat battery charging at a high rate when the engine was first started. If this was not via a jump pack, but done with jump leads from a running vehicle, that may have been the reason for the surge.
The brake lights will operate with the ignition off. Many manufacturers wire the brake lights in this way.