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Old 01-28-2004, 09:59 AM   #1
Scooby Specialist
Member#: 23547
Join Date: Aug 2002
Chapter/Region: Tri-State
Location: Trenton, NJ
13 Focus ST

Default Loose belt or alternator on last leg?

Last night while driving home in an ice storm I started to notice that my headlights were dimming every once in a while. I checked the voltage and the needle was jumping around from about 11 to 12.5. My TT also has a meter in it and I was able to confirm that swing in voltage there too. This continued all the way home and at various RPMs, including idling at a traffic light.

This morning I poked around under the hood and everything seemed fine. I couldn't check the tension of the alternator belt though. I started the car and immediately got a normal voltage reading. The voltage continued to read fine for the rest of my 40 minute drive.

WRX, 46k miles, original battery.

What do y'all think?

Edit: After a little bit of 'alternator' searches, I think its worth adding the following: During the ice storm I had the defroster on full blast, the rear defroster on, both sets of wipers running, and the radio on. Oh, and I have not done the grounding mod YET.
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Last edited by grindMARC; 01-28-2004 at 11:51 AM.
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Old 01-28-2004, 12:46 PM   #2
Scooby Specialist
Member#: 36576
Join Date: May 2003
Chapter/Region: Tri-State
Location: New Fairfield, CT
2k2 WRB wrx WAGON
16G'd & FMIC'd


doesn't sound non-normal. When you get off the accelerator (decreasing the rpm) the alernator belt is not spinning as quickly so the voltage the alternator puts out decreases (quick voltage drop that normally causes the headlights to dim, especially when other "high draw" components [front/rear defroster] are turned on). When the voltage gets to a certain point (low), the car draws voltage from the battery. When you drive the car on any given day, most of the voltage supplied to the car comes from the alternator.
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Old 01-29-2004, 12:05 AM   #3
Scooby Guru
Member#: 7327
Join Date: Jun 2001
Chapter/Region: NWIC
Location: Seattle, WA
2008 Mazdaspeed3
2006 Wrangler Sport


at idle, the voltage definitely drops, especially if you're using an underdrive pulley. A solution could be to increase the idle speed 75-100rpm with Deltadash or similar electronic control.
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