Jump-start harm

Car Mechanics (UK) - - Diagnostics Doctor -

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.

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