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Old 08-30-2009, 12:15 AM
Junior Member
Join Date: Aug 2009
Posts: 2
Default 99 miata charging issue

the battery keeps draining and will not stay charged. i have replaced both the battery and the alternator, both have been tested and both are new. i have checked the output on the alternator at the terminal and it is reading thesame as the battery while the car is running i can see the voltage drop. i have also checked the wireing harnesses and they all seem to be plugged in. any ideas woud be greatly appreciated. thnx
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Old 08-30-2009, 07:33 AM
Senior Member
Join Date: Aug 2007
Posts: 1,268

Passenger side fender at the firewall under the hood is a fuse block. The main fuse is bolted in. Remove that fuse and check/clean all bolt on connections to that box. That is the most common cause. Also be sure the main alt connection is good.
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Old 08-30-2009, 08:21 PM
Senior Member
Join Date: Mar 2009
Posts: 466

A couple of things: regardless of the battery age, if its state of charge is such that it can't feed at least 12volts to the car, it alternator won't charge. You must start with a fully charged battery, or else get a boost start. Once running, check the voltage across the battery terminals with the engine at fast idle. You should see around 14 volts if the alternator is doing its job. If you don't, the first thing to check is alternator drive belt and tension.
If you satisfy these conditions but find the battery going slowly flat, you have to suspect sneak current discharge due to faulty components/ wiring problems.
If you have any aftermarket stuff on your car, these are the prime suspects and should be isolated/ unplugged.
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Old 08-31-2009, 09:12 AM
Junior Member
Join Date: Aug 2009
Posts: 2

thnx guys, i believe the battery was not charged fully i will try that this evening. the main fuse is new i just replaced it. the car has a turbo system and most of the harnesses are disconnected, just the vital ones are conneceted. all the gauges are aftermarket, all eight of them.
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Old 08-31-2009, 09:12 AM

15v, 99, alternator, battery, charge, charged, charging, fuse, main, mazda, miata, nb, nitrous, problems, replacement, rx8, stay, wont

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