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Preface: I drive a 2004 Peugeot 206, manual

On the motorway the other night, one of my tires blew out. I managed to pull over and change to the spare wheel in about 30 minutes (having never changed a tire before! Lesson learned!) but because it was dark I decided to leave the dipped headlights on in addition to the hazard warning lights. Additionally, I stupidly left the sat nav plugged in. As a result, when I got back in to start the car, my old battery had given up the ghost and was completely dead (soon enough, even the hazard warning lights stopped working).

After being jump started the mechanic put a meter across the battery and said he suspected I may have some issue with the alternator or an earthing wire however, after driving for a while, the battery charged enough to start the car the next morning so the alternator is clearly working to some efficiency.

Whilst driving after the jump start and while the battery was still very low on power, I noticed that when I applied the foot brake heavily, the engine would stall but I managed to bump start it on the existing forward momentum. I wonder whether this occurred because applying the foot brake caused the power draw on the alternator (from the brake lights) to be too much and there was no longer enough power to spark the engine (I am not much of a mechanic, this is only a suspicion).

Normally, I would think the alternator provides more power than the battery and this should not occur but if the alternator wasn't working at maximum efficiency, is this a plausible scenario and how would I confirm?

I have since swapped in a new battery and the stalling has gone away, additionally, although I tried not to apply the brakes too heavily the morning after, I had no stall incidents with the slightly charged battery.

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2 Answers 2

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I managed to pull over and change to the spare wheel in about 30 minutes (having never changed a tire before! Lesson learned!)

Well done. It's not the world's most challenging job but you can hurt yourself if you're stressed (say after a blowout).

Whilst driving after the jump start and while the battery was still very low on power, I noticed that when I applied the foot brake heavily, the engine would stall ...

Normally, I would think the alternator provides more power than the battery and this should not occur but if the alternator wasn't working at maximum efficiency, is this a plausible scenario and how would I confirm?

I think you called it on the first try. Keep in mind that the alternator has three major jobs (in a very simplified sense): producing spark, running the accessory systems (of which the headlights and power brakes are critical components) and charging the battery. A deeply discharged battery is going to require significant continuous charge vs. a light trickle charge. If you add an additional mechnical load on the system, you remove some of the available kinetic energy that was being converted into electrical energy. If net energy dropped too low, you could below the critical voltage to maintain the idle revs.

In my car, I have a gauge showing the voltage in the system. If you don't have a similar indicator, you can do a casual test by trickle charging your battery for a few hours (using a hardware store battery charger) and seeing if your car still stalls on braking. Running the headlights and AC will also put additional draws on the system.

Now remember, no stall is not the same as no problem. However, a super dead battery is a lot of load for your little alternator to carry. That was likely the cause of your stalling.

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Thanks. I have edited the question to add some additional details regarding symptoms since. Is it possible that a fully working alternator may not provide enough power for the car if the battery is dead? Are you suggesting the extra draw on the power actually came from having to charge the battery heavily? Additionally, is it possible to check the output of the alternator with a fully charged battery? –  WilliamMartin May 13 '13 at 12:47
    
Where exactly is this brake pump you speak of located? The only extra electrical load should be the brake lights. It would be strange if that was enough load to drop the spark. –  Mike Saull May 13 '13 at 14:48
    
Talking fast and loose again - I had a brain fart and couldn't remember the name of the vacuum booster. The connection between the air-pump nature of the engine generating vacuum assistance is admittedly kind of a long mental jump though so I'm going to take it out. I'll replace it with additional mechanical load. –  Bob Cross May 13 '13 at 16:50
    
Would braking also consume some power for ECU and ABS? –  WilliamMartin May 14 '13 at 13:41
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Keep in mind it's not just an increase in load, but also a reduction in engine RPM. When your engine slows down during braking, your alternator output drops. So you're drawing more current from the battery, and putting less back. –  TMN Jun 3 '13 at 15:39

Under hard braking it is possible to lock a wheel. If you lock a wheel without disengaging the clutch you will stall the engine even if you are still moving.

If however the stall occurs while the clutch is disengaged or while you are in neutral then either you are somehow losing voltage and therefore spark, or the brake vacuum is somehow effecting your air/fuel ratio which I guess could be possible if you have a leaking brake booster or check valve. (Might get too lean?)

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I normally depress the clutch when I break. The problem has not returned since the battery was replaced so it's quite suspiciously related! Would an MOT and Service check catch something like a leaking brake booster or valve? The car passed today. –  WilliamMartin May 14 '13 at 13:30
    
I may edit the question to specify that heavy braking is probably too strong a term. It was occurring when I was braking coming up to traffic lights at around 30 mph, just a normal reasonable brake (not a light tap) –  WilliamMartin May 14 '13 at 13:49
    
It was probably electrically related then. –  Mike Saull May 14 '13 at 14:32

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