WonkoTheSane New Reader
Aug. 26, 2013 10:29 a.m.

Hey guys,

I had an odd problem pop up on my 2007 Rx-8.. I noticed it once before, but it really became prevalent at the track on Friday...

When I get into the upper RPMS (call it 7-9k), and stay there for a second or two, the battery light on the dash will come on. I've got a newish Optima Yellowtop that checked out okay (measured 820 CCA for the 550 it was rated for) when I took it to Advance yesterday to check it out on their load tester.

I logged it with Torque when it's doing this, and it says that I've got ~14.1V at the ECU when this is going on, so it doesn't seem to be the alternator. I've tightened the belt a bit which appears to be in good shape, and I reclamped the connections at the terminals..

Any ideas?


beans HalfDork
Aug. 26, 2013 11:08 a.m.

Shift light?

davidjs Reader
Aug. 26, 2013 11:16 a.m.

Maybe the voltage regulator is going out, and you're getting "bad" power at the high RPMs?

Swank Force One MegaDork
Aug. 26, 2013 11:17 a.m.

Does it even have a voltage regulator? Or is this more silly Mazda fare where the alternator output is controlled by the ECU?

Chas_H New Reader
Aug. 26, 2013 11:28 a.m.

Probably a case of the slip rings no longer round and the brushes are being thrown off at high speed. A rebuilding shop-not a parts store- can test your alternator and repair it. Avoid exchange rebuilt alternators. Controlling the alternator through/by the ECU works well. It's people that don't understand the system that are the problem.

codrus HalfDork
Aug. 26, 2013 11:32 a.m.

Pretty much everyone controls the alternator with the ECU these days. There's a bunch of advantages from an OEM perspective, such as better idle control.

The battery light on the dash is a pretty simple device, IIRC it's just a diode. If it's on, then current is flowing from the battery to the alternator through the light, meaning that the voltage is lower at the alternator.

When the alternator failed on my Miata, it manifested as a dropoff in available current at higher RPMs. I noticed this because getting on the freeway at night the lights would dim and the boost would fall back to the mechanical level (as the electronic boost controller lost the voltage it needed). At idle and low RPMs the car was fine. To diagnose it, we used a jumper to lock the field on max, stuck an oscilloscope with a current probe on it and ran it up to 5K RPMs, the output fell to around 30 amps vs the 65 that it's supposed to be.

WonkoTheSane New Reader
Aug. 26, 2013 11:42 a.m.

Yeah, I'm not getting any noticeable power drop off at high RPMs, which is good, the first time I noticed it was during an apex where I was being passed by a Ferrari... That's not a good place for loss-of-power oversteer to kick in! :)

Anyone know of any rebuilding shops in eastern CT area? It sounds like the alternator is going to need to go under the knife, eh? :(

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