Hi everybody,

I am having a problem with the electrical system of my Mazda that cannot be determined even by the dealership.

The problem started on October 2018. Battery light came on, along with check engine light, dsc and asc lights. I have a tablet installed in my dash and an OBD scanner, so using Torque app I was able to check voltage at PCM...it started dropping out of nowhere, momentarily, under 12V (11.5-11.7) and triggered all the warning lights. At the same time I got fault codes regarding intake temp sensor and O2 sensor. I checked the obd reading for intake temp, and whenever I had the undervoltage situation the temp reading fell from the 22 degrees celcius (actual temp at the moment), to -40!!!!!
I went to a local specialist shop (ex Mazda dealer), and decided we should change the alternator. Due to cost of a new alt here (about 750 euros), we went ahead and installed a used one from a low milleage donor. Everyhting was ok for about 9 month (until following summer). Then, the dreaded battery light lit again.
We tried again another 2 used alternators but nothing improved...intermittent undercharging again...I then decided to remanufacture my old alt after I was reassured by the mechanic that this particular alt part no. was ok for remanufacturing (I even got a 1 year warranty) but again battery came on after installation.
I decided I had no option but to bite the bullet and take the car to the dealership...they kept the car for almost a week, and they couldn't find anything wrong ...they checked all the cables, the wiring loom fuse boxes etc...so they suggested they should install a new alt and if that didn't work, then a new PCM (750 and 1000 euros cost respectively!!)..I obviously took the car and left, as I could not accept them proposing to start changing parts and wait to see the outcome, without any reassurance...

I then, started monitoring what was going on with voltage for a long time, using torque and I found out the following:
The problem is happening only on very hot summer days (more than 35 celcius) and very cold winter days (less than 3-4 Celcius)....when in normal outside temps it just magically stops...
I also noticed that when I am pressing the gas pedal and there is load at the engine, the alternator charges fine and the battery light goes off...when I leave the pedal and let the car roll without engine load, the light comes on almost immediatelly, and it goes off again when rpm reach idle!!!
The belt is about 2 years old (original part) with no slack at all, and the battery is a year old (the most expensive of the Bosch line of batteries).. I checked the wires from MAF and they look ok without cracks etc...I also ordered a new aftermarket MAF and tried it, but the light came on again...

I also took some measurements with a voltmeter at my garage and they were the following:

Alt (+) --> bat(-) 14.13V
bat(+) --> bat(-) 13.96V
Alt(+) --> ground point near alt 14.08V
bat(+) --> ground point near bat 13.99V

They all look normal.....I couldn't take the same measurements with battery light on, beacuase, as I mentioned earlier, when at idle the car charges fine no matter what!!!!!

So as you probably understand I am having a very strange problem (5 alternators cannot be all bad!!!!) and any help will be greatly appreciated.

My Mazda, is a european 2008 Mazda 6 , 4 cylinder, with ~80K miles, maintained by the book. Besides the undervoltage it is running fine with no other symptoms....

Thanx everybody in advance

PS: I have a used PCM in hand and was thinking of getting a used BCM too, but would like to have some opininions before I start changing parts....

 

Alternator is controlled by the PCM.

I've replaced a few PCM's over the years for charging issues.

 

Wow this is the first time I have seen or heard of anything like this as a professional mechanic. Maybe you have a short in the charging system they can be hard to find sometimes and sometimes it will come and go as you described.