Re: Sudden voltage drops from 14.0 to 12.5 - Saab 9000 Bulletin Board - Saabnet.com
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Re: Sudden voltage drops from 14.0 to 12.5
Posted by BobS (more from BobS) on Mon, 14 Jan 2013 11:01:00
In Reply to: Sudden voltage drops from 14.0 to 12.5, Vicsaero95, Fri, 11 Jan 2013 05:09:53
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As others have said, the momentary dimming of the headlights is caused by the A/C clutch, and more so, the starting of the auxiliary front fan. The current through the A/C clutch ramps up as the magnetic field builds up. The resistance of the coil is ~4 ohms so the coil current starts at about 3.5 Amps and goes down from there. The killer is the aux fan DC motor which has a fraction of an Ohm resistance when it is not turning. As a result, the initial momentary current surge into the motor can be 10's of Amps. Once the motor begins to turn, back EMF increases the effective resistance, so at speed there is something less than 10 Amps running through the motor. The high starting current surge causes an appreciable voltage drop in the wiring between the battery - voltage regulator and fan. About the only way avoid the current surge and resulting voltage drop is to start the fan via a resistor like low speed on the main radiator fan.
posted by 97.127.24...
Posts in this Thread:
- Sudden voltage drops from 14.0 to 12.5, Vicsaero95, Fri, 11 Jan 2013 05:09:53
- Low, low, low, how low can you go? 6.1 volts here..., RayF, Wed, 23 Jan 2013 17:53:46
- Re: Sudden voltage drops from 14.0 to 12.5, BobS, Mon, 14 Jan 2013 11:01:00 <-- Viewing This Message
- Just put new battery- seemed to do better, Vicsaero95, Mon, 14 Jan 2013 04:34:05
- Another one here., Dale, Sun, 13 Jan 2013 18:52:23
- Re: Sudden voltage drops from 14.0 to 12.5, Frank , Fri, 11 Jan 2013 10:15:34
- Re: Sudden voltage drops from 14.0 to 12.5, Simon S, Fri, 11 Jan 2013 08:08:34
- Re: Sudden voltage drops from 14.0 to 12.5, Davidk, Fri, 11 Jan 2013 06:08:23
- Re: Sudden voltage drops from 14.0 to 12.5, sam96CS , Fri, 11 Jan 2013 06:04:01
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