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is this voltage drop or bad alternator

1Gina2G

Proven Member
811
2
May 6, 2011
Beaufort, South_Carolina
so after a good drive, with the engine warmed up, when the headlights are switched off, the idle changes by sounding a little bit faster then it did, even though the idle was never sounding bad at all.

the battery reads 14.3-14.4V upon initial start up with everything off, goes to 14-14.1V with everything on. strange enough after todays drive i turned everything off, checked voltage and it was at 13.7V, but basically no charging time between shutting off (electrical) and checking voltage, is this why?

battery is brand new, just a few days old, and is holding a charge just fine, no power drains in the car (fixed them)

also all fuses are good

thanks everyone :thumb:
 

dsmornothing

Probationary Member
605
5
Jul 23, 2009
Ishpeming, Michigan
The alternator charges around 14v but the battery will not maintain much more than 12.6 after the engine is off. 6 cells in an automotive battery for a voltage of 2.1 per cell.
 

1Gina2G

Proven Member
811
2
May 6, 2011
Beaufort, South_Carolina
Not sure if you misread anything but when I say turn off I just mean electrical, I'm aware of what the battery voltages drops too when off as I mentioned it also holds a charge.

Honestly I feel like rewiring my alternator with bigger wires (factory wires currently) at the same time, the alternator is used and came with the car, not sure how old it is but it appears to be working and charging the battery. :hmm:
 

5150DSM

Supporting VIP
856
3
Jan 5, 2004
Sacramento, California
Sounds normal. The alternator has a sensing wire to the voltage regulator that tells the alternator how much recovery voltage (based on load) is needed. After a good drive the battery should be near full charge and with all accessories off the load demand is minimal, therefore, the battery requires less overall charging voltage. This is simplified but should answer you well enough for your purposes.
 

PaqueDSM

Probationary Member
14
0
Jan 18, 2013
Costa Rica, Central_America
It's totally normal, the battery is charged, and you are turning off all the electrical accessories, the voltage has to drop... the regulator and the ECU are working fine...
 

BowDownRacing

Proven Member
65
1
Jan 12, 2013
Torrance, California
I know how I test my alt is to disconnect the battery while the car is running and if it turns off it means the alt is not working IDK if this helps but it always has worked for me.
 

luv2rallye

DSM Wiseman
7,883
756
Jun 7, 2003
Minneapolis, Minnesota

DSM1G90

Proven Member
5,105
61
Apr 6, 2008
Nampa, Idaho
As by the quote from STEVE:

"Some people think that it is ok to test your alternator by starting the car and then pulling the battery cable from the positive terminal, saying that if the car remains running that the alternator is good.

Like he said, this is a bad idea because the alternator can load dump and spike the whole electrical system with >100v and even if it doesn't, the battery filters the alternator output and the AC ripple on the output will again flow through the whole electrical system. Both of these can damage the ECU and other electronics in the car."

Worst trick in the world to do to any vehicle.

-DSM
 

1Gina2G

Proven Member
811
2
May 6, 2011
Beaufort, South_Carolina
I know how I test my alt is to disconnect the battery while the car is running and if it turns off it means the alt is not working IDK if this helps but it always has worked for me.

LOL I know you can not do this with any vehicle running an ECU. Old school trick thats now abandonded practically. Glad to hear my issue sounds normal, first DSM so I wouldn't know any better. thanks again everyone! :thumb:
 
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