99 miata "battery" light: Alternator? Battery?
 

My "battery" light came on yesterday. I noticed that as I was driving that If I hit the gas the light would go of for about 10~15 seconds. Having the a/c on made it come on quicker. I made it to fiances apartment and the car would not start.

Took the battery out today and took it to o'reillys. The battery was to low to test. They charged it and said it checked out ok. So I am guessing it is my alternator.

Can I check my voltage through my OBDII scanner to see if it is working. If not how do I check it with a voltmeter/multimeter. I'd really like to make sure since a "new" unit is around 400 bucks (but planet miata has used ones for around 50.). I'm tired of this as with my wedding coming up and work I'm really pressed for time and I've already getting pissed that i've had to change what feels like a billion little things over the past month.

I was looking through my 99 shop manual and it seems there is one more step than on my 92 as I would need to remove the manifold brace.

Replace your IAT sensor.

To check voltage with a voltmeter:

Start car
Open trunk
Check voltage at battery

The Miata alternator has got to be the easiest alternator I've ever replaced. $400 seems a bit ridiculous - where did you get that price?

If it's the battery, I've got one in my garage that's yours for $25 shipped if you're interested. It works fine, I just swapped it out for an internally regulated NA alternator when I installed my megasquirt.

Battery check out well yesterday. It held a charge.

the 425 is from Mazda for the alt.

Just ordered one from planet miata for 50bucks. I'll try intall it this weekend

did you test your alternator?

unfortunatly I do not have time to till this weekend. I can always sell or keep the unit.

I have a bunch of wedding comitments weeknights this week and a few workshops for work, as well as I have to be a driver for someones oral surgery later this week.

I can test it this weekend. If it is I will have the part of not then I can exchange or sell it.

Seriously, try the IAT sensor, 95% that is your problem. Any time I've seen a miata with a battery light on, its always the IAT sensor.

When it is the IAT the battery still charges doesn't it? My battery was at 7% charge when I went to the store. They had to recharge it before they could test it.

It charges lower than normal. The stock ECU uses the info from the IAT sensor (air temp) to charge the battery at an optimal voltage. Worth a shot before replacing an entire alternator.

Since the use of an OBD-II scanner was mentioned earlier, I'm assuming that the OP has the ability to look at the ECU's opinion of what the IAT sensor is putting out in real-time.

Is the IAT number plausible?

You could also unplug the IAT sensor and measure the resistance directly across it with an ohmmeter. At 70°F, a good IAT sensor should read somewhere in the general neighborhood of 2.1 - 2.8 Kohms. Resistance will go down as temperature goes up.

I did not have to scan it. On the way to work on it the water temp gauge started going down and the engine starte to sputter. It left me stranded on the side of the road. Took the battery out and it was at 0% charge. Had an Oreillys charge it up for me. Made it to where I was going and installed the new alt. Just need to tweek the tension now. Squeaks for about 5 seconds on warm up.

99 nb
 

[QUOTE=Reverant;875542]It charges lower than normal. Hey man i have a 99 nb just built it like a broken box of legos still figuring stuff out but its boosted and i was told to run a iat from a gm i calibrated it with the ms but if something was off in the tube on that setting would it make my alt not charge

No, the MS doesn't care about the air temp as far as charging goes.