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Old 08-30-2012, 02:41 AM   #1
runner19
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Default Alternator Charge voltage dropping when warmed up

2002 Bugeye WRX scoob

Fluke meter used for measurement.
New battery installed. Battery does not die

Original alternator
Startup voltage 14.4V
After the car is driven for approx 8miles(car is wamred up. The voltage drops to 14.0 V.
After a further 8 miles. The voltage drops to approx 13.8 voltage with engine rpm at 2000 rpm.

Voltage at alternator is same as voltage at battery.

I replaced the alternator with a new unit. (not recond unit)

I have the same issue. Startup voltage 14.4v and the voltage drop is the same after driving.

Battery light on instrument cluster comes on when key in ign on position.(engine not running)
Battery light goes off when engine is running.

What could be causing this issue.
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Old 08-30-2012, 07:55 AM   #2
Mulder
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It is normal for the voltage to drop as the engine reaches operating temperature. The regulator is thermally compensated. As long as the voltage remains within the proper range (13.5-14.5V) there is nothing wrong.
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Old 08-30-2012, 11:00 AM   #3
Cougar4
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If the alternator in your car has three wires going to the back side of it then the ECU may be in control of the regulation. Like Mulder stated, the voltage you read when warmed up isn't a real concern but the action does seem a little strange to me also. Perhaps the design helps protect the alternator from heat damage by reducing the current output slightly when things like the output diodes get hot.
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Old 08-31-2012, 09:19 AM   #4
runner19
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Thank you for replies guys. I do appreciate the timer it takes to respond to all these posts.

However, prior to this the car would startup at 14.4 voltage and then hold at 14.2 once warmed up. summer or winter. Also the FSM shows regulator voltage set point 14.2 V

So the new scenario is puzzling. Battery is staying steady at 12.5-12.7 V depending on length of drive.
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Old 08-31-2012, 09:24 AM   #5
runner19
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Update

The alternator has a three pin plug on it.

Regulated voltage as per FSM is 14.1-14.8 Page SC-2
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Old 08-31-2012, 10:19 AM   #6
indytruckboy
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13.8 wont kill you. put a dead battery in there and watch it stay in the 14s. you are wasting time worrying about a non-problem
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Old 08-31-2012, 04:26 PM   #7
Cougar4
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Quote:
Originally Posted by runner19 View Post
Battery is staying steady at 12.5-12.7 V depending on length of drive.
I assume those voltage readings you took across the battery are with the engine OFF. Those readings are about what I would expect to have with the engine off and with a fully charged battery. With the engine running you should have close to the same voltage across the battery as the alternator output post is with no more than .5 volts difference and a good load on the system.

Since you have a 3-wire system it looks like your ECU is controlling the voltage output of the alternator.
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