Alternator and battery question - Miata Turbo Forum - Boost cars, acquire cats.

General Miata Chat A place to talk about anything Miata

Alternator and battery question

Old 10-01-2012, 02:28 AM
  #1  
Junior Member
Thread Starter
 
Join Date: Jun 2008
Location: Delhi CA
Posts: 97
Total Cats: 0
Default Alternator and battery question

My car now has trouble starting. It has just clocked 203k miles. I thought I may have a parasitic drain(I have an amp and capacitor installed). But after unplugging the battery overnight, it still won't start.

When the car is off, the battery reads 12.3V.
When the car is on, the battery reads 15.8V.
When I attempt to start the car, the battery drops to 6V.

Do these readings indicate the battery or alternator is bad? I replaced the battery a month ago, when I first started to have this problem. After installing the new battery(pro rated warranty replacement) the car worked fine. The battery brand is "Super Start" from Oreillys.

Thanks in advanced.
Gregor is offline  
Old 10-01-2012, 04:52 AM
  #2  
Supporting Vendor
iTrader: (10)
 
Reverant's Avatar
 
Join Date: Jun 2006
Location: Athens, Greece
Posts: 5,542
Total Cats: 222
Default

The voltage with th engine running is too high.
__________________
Originally Posted by ninerwfo View Post
The “into to megasquirt thread” mentions that the MS3 basic has 5 inputs with built in pull up resistors - and I was just wondering why it would have so many, while the MS3Pro PnP has just 2 (with no pull ups?)
MS Labs: Plug and Play Megasquirt ECUs for ANY 90-05 Miata (NA, NB1, NB2 w/VVT, MSM), including basemaps to get you started ASAP. PM me or email or details.
For support on MS Labs ECUs: Please email, be sure to include the unit's serial number and include a datalog and your current map.
Reverant is offline  
Old 10-01-2012, 09:16 AM
  #3  
Elite Member
iTrader: (1)
 
Leafy's Avatar
 
Join Date: Jun 2012
Location: NH
Posts: 9,309
Total Cats: 80
Default

Originally Posted by Reverant View Post
The voltage with th engine running is too high.
By about 1 volt. It would have to do that for a while to kill a liquid cell battery but would kill a stock AGM quickly. From those reading it sounds like the voltage control in the alternator went bad and killed the battery. I bet if you put the battery on a cheap tester it would check out as ok, but the expensive load type tester would show it as junk.

My persona course of action would be replace the alternator and replace the battery.
Leafy is offline  
Old 10-01-2012, 09:19 AM
  #4  
Boost Czar
iTrader: (61)
 
Braineack's Avatar
 
Join Date: May 2005
Location: Chantilly, VA
Posts: 75,273
Total Cats: 2,152
Default

and voltage when off is .3v too low!

dropping to 6v during cranking is a bit too low too. I think i see around 9-10v
Braineack is offline  
Old 10-01-2012, 09:23 AM
  #5  
Elite Member
iTrader: (1)
 
Leafy's Avatar
 
Join Date: Jun 2012
Location: NH
Posts: 9,309
Total Cats: 80
Default

Originally Posted by Braineack View Post
and voltage when off is .3v too low!

dropping to 6v during cranking is a bit too low too. I think i see around 9-10v


Dropping to 6v during cranking but having normal voltage with no load is indicative of a battery with a high internal resistance.
Leafy is offline  
Old 10-01-2012, 11:22 AM
  #6  
Junior Member
Thread Starter
 
Join Date: Jun 2008
Location: Delhi CA
Posts: 97
Total Cats: 0
Default

I just noticed the harness that plugs in the back of the alternator has a broken tab and is loose when I push it in the alternator. Could this be the source of the problem?

I will replace the battery this morning, but I'm concerned that if I replace the alternator and the harness is the problem. Is it possible to replace the connector?
Gregor is offline  
Old 10-01-2012, 11:36 AM
  #7  
Boost Czar
iTrader: (61)
 
Braineack's Avatar
 
Join Date: May 2005
Location: Chantilly, VA
Posts: 75,273
Total Cats: 2,152
Default

Originally Posted by Gregor View Post
I will replace the battery this morning, but I'm concerned that if I replace the alternator and the harness is the problem. Is it possible to replace the connector?
is it possible to fly by pushing down on air?
Braineack is offline  
Old 10-01-2012, 11:37 AM
  #8  
Senior Member
iTrader: (1)
 
Join Date: Jun 2006
Location: Sweden
Posts: 1,378
Total Cats: 59
Default

Originally Posted by Leafy View Post


Dropping to 6v during cranking but having normal voltage with no load is indicative of a battery with a high internal resistance.
It can also indicate high resistance anywhere between the battery and the load (e.g. corrosion, badly tightened leads etc).

Dropping to 6V would mean no ECU action at all.
NiklasFalk is offline  
Old 10-01-2012, 12:21 PM
  #9  
Junior Member
Thread Starter
 
Join Date: Jun 2008
Location: Delhi CA
Posts: 97
Total Cats: 0
Default

Well I pushed in the harness, push started the car, and the battery/alternator light is on. I'll be replacing both tonight after work.
Gregor is offline  
Old 10-02-2012, 03:12 PM
  #10  
Junior Member
Thread Starter
 
Join Date: Jun 2008
Location: Delhi CA
Posts: 97
Total Cats: 0
Default

Replaced both battery and alternator. Car now shows 14.7V when turned on.

Thank you.

Does anyone know where I could buy a replacement connector for the alternator harness?
Gregor is offline  
Old 10-02-2012, 03:16 PM
  #11  
Elite Member
iTrader: (1)
 
Leafy's Avatar
 
Join Date: Jun 2012
Location: NH
Posts: 9,309
Total Cats: 80
Default

Junkyard, panic motorsports, ballenger motorsports, eastern beaver.
Leafy is offline  
Related Topics
Thread
Thread Starter
Forum
Replies
Last Post
StratoBlue1109
Miata parts for sale/trade
20
09-30-2018 02:09 PM
bigmackloud
Miata parts for sale/trade
13
09-22-2017 01:34 PM
JesseTheNoob
DIY Turbo Discussion
15
09-30-2015 03:44 PM
btabor
ECUs and Tuning
10
09-28-2015 06:33 PM
ihiryu
General Miata Chat
9
09-28-2015 11:22 AM


Thread Tools
Search this Thread
Quick Reply: Alternator and battery question


Archive Advertising Cookie Policy Privacy Statement Terms of Service

© 2018 MH Sub I, LLC dba Internet Brands

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.