Neep help with my p5

About a year ago I bought a 03 p5 and I'm now experiencing voltage problems I just had it checked at my local mazda dealership today and the service department said that it was either an alternator or my ecm any thought on what cause my battery to die while the car is off and what would cause a consistent battery light on and off?

Most likely the alternator, belt, or battery connections.

Agreed. Clean the battery terminals and connectors. That's free. Charge the battery and see if it takes a full charge. Then take that to an auto parts store like autozone. They will check the battery and alternator. Might have to remove them first but they do it for free.


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I've replaced the alternator three times and I have a new battery it's not that I have some other issue with the car. Is there a voltage regulator outside the alternator if so where is it? Also does the ECM have anything to do with all this?

Dizfunction said:

I've replaced the alternator three times and I have a new battery it's not that I have some other issue with the car. Is there a voltage regulator outside the alternator if so where is it? Also does the ECM have anything to do with all this?

Click to expand...

the ecm controls the alternator
i have an extra P5 , 5 speed ecm. PM me i see youre in maryland maybe i can help you out.

ap123 said:

the ecm controls the alternator

Click to expand...

Check to make sure that control wire is making good contact and isn't corroded/partially broken.

If the wire disconnects, the alternator effectively shuts off.

The alternator isn't shutting off its running over 15 volts.

Dizfunction said:

... my battery to die while the car is off and what would cause a consistent battery light on and off?

Click to expand...

My car did the same thing. It was the alternator.

Something fried inside the alt. and killed my battery in one night.

After jump starting the car, the voltage shot up to 17 volts. I turned on all electrical loads to bring down the voltage and drove it to the mechanic. The alternator was replaced under warranty but still cost me $180 for labor.

I'm thinking it's your alternator because the ECU shouldn't be able to drain your battery overnight regardless of what it's doing.

Your ECU may possibly be instructing you're alternator improperly which cooked it but I'm pretty sure your alt. is toast now.

In my case a new alternator fixed the problem.

Some guys here on the forum have had to replace their alternator 3-4 times before they got one that worked. Remanufactured alternators can be a real problem but the OEM ones are a lot more money.

Our car's battery light comes on if the voltage is too high.