Battery and Check Engine Lights.

[Walsted]

On the way home to Fort Worth from Marshall, the Battery and Check Engine lights came on as I was on the 20 west to 35W south overpass soon after I turned on the headlights. I figured the alternator finally gave up the ghost after only 397,000 miles. I turned off all electrical that wasn't essential, and made it home. I pulled codes, it came up clean but incomplete as I had pulled codes earlier. When I started the car to check alternator voltage, no warning lights.

Has anyone experienced anything similar, and if so, what was the final outcome? Is my alternator on its way out, or maybe a stuck relay or gauge cluster issue? Until today, I haven't seen the check engine light on except for start up, even when the code scanner showed the MIL to be on.

Thanks for any help.

[OZMDD]

I'd go with the alternator.

[motorollow]

Definitely check the alternator. Fellow forum member had an electrical issue on his NB and it turned out to be a short in the fuse box under the hood. Only found it by moving the fuse box while car was running.

[Walsted]

Thanks for the advice. Alternator is showing around 15 volts output. I don't know if it is coincidence, but I did jiggle the under-hood fuses to ensure good contact while checking alternator voltage. If the battery light comes back, I'll check voltage without shutting off the car to see if it is an alternator or indicator issue.

[Walsted]

Battery and check engine lights are back. They seem to come on after the car has been running for about ten or twenty seconds. I'm showing about 15 volts at idle both at the battery and alternator. I'm still thinking it is an indicator issue.

[hornetball]

It may be a high voltage problem. Alternator should be regulated to around 14VDC. Still no code I guess? I "think" that on your year, the ECU does the alternator voltage regulation function -- in which case replacing the alternator won't help.

[POS Racing]

Isn't 15 Volts is on the high side for a working alternator?



What is the CEL??

[OZMDD]

Seems high. I'll run mine later and see what it outputs with a dead battery hooked up to it.

[POS Racing]

As a reference point.

My 2002 sitting not running 12.3 volts.

Running 14.4 volts tapped back to 13.5 volts after it ran for a couple of minutes.

[Walsted]

It may be a high voltage problem. Alternator should be regulated to around 14VDC. Still no code I guess? I "think" that on your year, the ECU does the alternator voltage regulation function -- in which case replacing the alternator won't help.

Thanks.
What happens if the alternator is run at 15V for a while?
Is there a way to determine if it is the ECU or alternator?
Thanks, again

[POS Racing]

Thanks.
What happens if the alternator is run at 15V for a while?

Google search found this :

Manufacturer warranties are typically pretty clear about overcharging as well as undercharging your battery.
Make sure you have a properly working charging system before you lay out big cash on an expensive battery only to cook it worse than my Aunt Lucille cooks meatloaf.
Plus, overcharging can create a very serious explosion potential causing damage to you, your battery, and your system.

[Walsted]

Update: Tried to make it to Marshall, figuring 15 volts isn't that much above 14 volts, and I could grab a new alternator in Shreveport during lunch, tomorrow. On the trip, the Battery and Check Engine warning lights turned off around Terrell, and then were intermittent to about mile-marker 565 or so. Then they stayed on, and I noticed I was losing electrical (coolant temp gauge started to drop below its usual level) about the time I was approaching Longview. I shut down all non-essential electrical and looked for an exit that didn't just lead to a different highway. Car shut down due to no electrical power at mile marker 590 or so, in Longview, and I coasted to a stop where I was easily seen.

Luckily, I pay AAA a lot of money each January for just this contingency, so the power-less Miata and I are at the Marshall apartment. I'm guessing that getting 225,000 miles out of an alternator isn't that bad, and that is if it was new when I bought the car used. The car is at 397,521 miles at the moment, so if it is the original alternator, I should have no complaints, except it could have picked a better week.

If replacing the alternator doesn't fix the problem, I'll likely have to find an ECU. Hopefully we don't have to go that route.

[gabkwong]

It can be a PIA to replace the alternator if you still have the intake manifold brace on. Plus the fact that the bolt that hold it on is very tight from the factory. Good luck!

[Walsted]

It can be a PIA to replace the alternator if you still have the intake manifold brace on. Plus the fact that the bolt that hold it on is very tight from the factory. Good luck!

I have already had the alternator off and on due to a previous part failure. Once I have the alternator and car in the same place, it should be quick and dirty.

[hornetball]

Found wiring diagrams for your car here:

http://madracki.com/miata/images/wiring/99sys.pdf

Looks like I was right about the ECU doing the regulation on your year. Hopefully you can pull your alternator and have the parts store test it before laying down cash for another one. That should tell you whether you need an alternator or an ECU. Good luck. Impressive mileage.

[Walsted]

Put in a new alternator, and so far everything looks good. Scared myself at first, as I forgot to connect the voltage regulator connector before starting the car, and of couse, the warning lights came back, the car died, and I am wondering how much artur wants for an ECU. Then I remembered not connecting the connector, and fixed that issue.

The real test will be the drive back to Fort Worth Friday afternoon.

Thanks for everyone's help and advice, and I hope this thread is useful to someone in the future.