I have a 2007 with the 2.0 and an electrical issue. First hint of a problem was loss of both Low Beams. Lamps were blown, replaced and problem repeated with 24 hours. Checked charging voltage at battery, fluctuating between 15.2 to 16.4vdc. Brought to Autozone, "not regulating", alternator need replacement. Pulled old (not OEM), put in a remanufactured. Had them put new one on test machine first, all good. Exact same issue, started looking at ground connections. Some corrosion, so replaced any suspect ones. Problem dropped somewhat, generally below 15.8, but still very unstable (about 1 vdc max bounce) . Used OBDII tester, watching live data. Not running, there is a difference of 1 vdc between battery/alt output and reported voltage on OBD (lower at OBD). This rises to almost two volts when running. If the voltage climbs above 16.2 for any length of time, low beams will flash and some dash indicator as well, and what sounds like a relay clicking near the center of the dash. No battery warning lights, no code thrown on OBDII.
Talked to a mechanic "with 40 years working on Mazdas" and dealer techs from two different dealerships and also searching forums/Haynes manual. Since none seemed to be in agreement on practically any point, and I have been unable to find any detailed info online so far, I have the following burning questions:.

1. Does the PCM control the output of the alternator? If so by what kind of signal (two wires from alternator, one blue one gray)? I verified voltage at input to PCM same as output from alternator (measured at both alt and batt).
2. How many cables on negative terminal (two on this one, to left fender and bracket on block)?
3. Is the battery temp sensor used on the 2007 model? If so, where in the PCM is it fed?
4. One Dealer stated an aftermarket alternator will not work, has to be OEM. Truth?
5. Is there a document showing all the ground points for the model? level?
6. Has anyone seen the relatively large difference between voltage at terminals and the reported OBD? Since there is a difference when the engine is running or not, just larger when running, but not consistent.

Note, I work on marine control systems and I have never spent this much time or effort chasing a single fault. Normally I would suspect a poor connection somewhere given the large fluctuations, but the "experts so far just repeat "change the alternator" or "bring it in, we'll be happy" to charge an arm/leg. Anyone who has had this particular issue, I'd love to hear. Trying to correct for new owner (young lady, not ready for large repair cost, I'll deal with the lot that sold it to her after it's corrected). Many thanks.

 

Yes the PCM controls the alternator.

Usually two main earths

Not sure on which alternators work with the PCM, I can say my dealer has fitted well known brand non genuine alternators before without issues.

Factory Workshop manual has a wiring section that details all the earth points.

different voltages are offered via live data, eg Alternator, Battery, individual modules.

 

Thanks for the response. So, in your experience, which is more likely. A poor earth connection somewhere or a failing PCM channel? Hate to pay for a new PCM and still have same issue. I'll order the factory book for wiring points. No opinion on the battery temp sensor?

 

I would be leaning towards the PCM.

However, make yourself some some temporary earth wires and see if that helps.

 

I found the Mitchell schematics, so I have the list of earth points now. Leaning towards PCM replacement, because of the difference between input and displayed voltage. The degree of fluctuation is what makes me think it's a bad connection somewhere. Thanks for the ideas.

 

Got a replacement PCM, coded to VIIN. Immobilizer circuit is active, no start. Way forward?

 

Hello, I'm having exactly the same issue. Weird voltage spikes, relay switching, lights randomly turning on, all the circus you had. Have you managed to solve this?