Mazda Mazda3: BK alternator voltage regulator failure — charging drops or overcharges
BK alternator voltage regulator failure — charging drops or overcharges is a common electrical fault on the Mazda Mazda3 on the 2003–2009 cars, and we rate it major. Putting it right costs $450 to $1,200 at New Zealand independent workshop rates. Below is what goes wrong, the symptoms to notice on a test drive, where to look, and what to ask the seller before you hand over any money. It is one of 37 documented faults on the Mazda Mazda3.
- Vehicle
- Mazda Mazda3
- Years
- 2003–2009
- System
- Electrical
What you are looking at
No photograph available
What goes wrong
The voltage regulator inside the alternator is a recognised weak point on the BK. It can fail either way: undercharging, which flattens the battery and eventually leaves the car dead, or overcharging, which cooks the battery and can take other electronics with it. The symptoms are easy to blame on a tired battery — slow cranking, dash lights dimming at idle, a battery that keeps going flat — so plenty of these cars get a new battery when they needed an alternator. If a BK has had more than one battery in recent years, that is the finding, not the batteries.
What you would notice
- Battery warning light, especially at idle or under electrical load
- Headlights dimming or pulsing at idle
- Battery repeatedly going flat despite being new
- A smell of hot battery, or a swollen battery case, from overcharging
How to check it
Where to look. Alternator at the front of the engine, and the battery it charges
With the engine idling, switch on the headlights, the heater fan and the rear demister together and watch the dash lights and headlights for dimming or pulsing.
Look for the battery warning lamp: it should light with the ignition and go out promptly once running, and any flicker at idle is a note.
Open the bonnet and look at the battery itself for a swollen case or corrosion, and check whether it looks newer than the car's history would suggest.
Ask how many batteries it has had — more than one in a few years points at the alternator, not the battery.
If a workshop inspection is possible: ask for charging voltage to be measured at idle and at speed, with load applied.
What it costs to put right
$450 to $1,200 at New Zealand independent workshop rates. That is for this job on its own, not for the car.
Ask the seller
- “How many batteries has it had, and how recently?”
- “Has the alternator ever been replaced?”
- “Does the battery light ever flicker at idle?”
Now check the Mazda Mazda3 you are actually looking at
Everything above is true of the model, and it is free. What it does not tell you is whether THIS car has been looked after: its odometer history and whether the reading has ever gone backwards, whether there is still money owing on it, whether it has been reported stolen, how many owners it has had and what it is really worth today. That comes from the official records for that plate. Enter the plate for the free details, or get the full history and a market valuation from $13.49.
Where this comes from
- Samarins — 'Mazda 3 2004-2009: pros and cons, problems'
- CarParts.com — 'Mazda 3 reliability and common problems'
Mazda Mazda3 buyer questions
Is bk alternator voltage regulator failure — charging drops or overcharges serious on a Mazda Mazda3?
We rate it major and common on the Mazda Mazda3. The voltage regulator inside the alternator is a recognised weak point on the BK. It can fail either way: undercharging, which flattens the battery and eventually leaves the car dead, or overcharging, which cooks the battery and can take other electronics with it. The symptoms are easy to blame on a tired battery — slow cranking, dash lights dimming at idle, a battery that keeps going flat — so plenty of these cars get a new battery when they needed an alternator. If a BK has had more than one battery in recent years, that is the finding, not the batteries.
How much does it cost to fix on a Mazda Mazda3?
$450 to $1,200 at New Zealand independent workshop rates. That figure is for this job on its own. A car showing the symptoms on this page is usually worth either walking away from or negotiating down by at least the repair figure.
Which Mazda Mazda3 years are affected?
Our research records this fault on the 2003–2009 Mazda Mazda3. Build dates move around, especially on imported cars, so treat the range as the years to look hardest at rather than a guarantee either way. The official records tell you the exact build and first registration date for the car in front of you.
How do I check for it before I buy?
With the engine idling, switch on the headlights, the heater fan and the rear demister together and watch the dash lights and headlights for dimming or pulsing. Look for the battery warning lamp: it should light with the ignition and go out promptly once running, and any flicker at idle is a note. Open the bonnet and look at the battery itself for a swollen case or corrosion, and check whether it looks newer than the car's history would suggest. Ask how many batteries it has had — more than one in a few years points at the alternator, not the battery. If a workshop inspection is possible: ask for charging voltage to be measured at idle and at speed, with load applied. Look here: Alternator at the front of the engine, and the battery it charges