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  #1  
Old 10-02-2011, 05:03 PM
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W114 Charge Indicator Light?

Hey Guys,

I just got my new to me 1973 280C up & running and the generator warning light is on in the dash. Is there any easy way to test if the voltage regulator is bad or if it's actually the alternator?

Thanks!
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Old 10-02-2011, 07:27 PM
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From my own experience..

Apply a volt meter across the battery, it should read 12V +/_ .5 volts Start the car. While cranking, it should drop no lower than 11.5 volts. if it does, charge battery and re-test.
When running, with a good alternator and voltage regulator, voltage should be 13.5 -14.4 volts at roughly 1000 RPM. A Bad voltage regulator won't increase voltage when RPM is increased. If at idle your voltage is 12.0 and dropping, try a higher rated regulator (if one will interchange)

A burnt bulb won't allow the alternator to function, and an LED bulb in place of a regular bulb won't allow it either, because it is a Diode.

Ensure that the battery is fully charged. an alternator will not charge a dead battery..
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Old 10-03-2011, 10:21 AM
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Battery is brand new.

I do need to check it with the car running & see if it's getting ANY kind of a charge or not.
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Old 10-03-2011, 12:18 PM
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You might also make sure that the belt is tight--those 55-amp alternators draw a couple of horsepower--the belt in the pulley should be tight enough that you can't really turn it by the vanes


Tim Kraakevik
kraakevik@voyager.net
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  #5  
Old 10-03-2011, 04:07 PM
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Quote:
Originally Posted by PanzerSD View Post
Apply a volt meter across the battery, it should read 12V +/_ .5 volts Start the car. While cranking, it should drop no lower than 11.5 volts. if it does, charge battery and re-test.
When running, with a good alternator and voltage regulator, voltage should be 13.5 -14.4 volts at roughly 1000 RPM. A Bad voltage regulator won't increase voltage when RPM is increased. If at idle your voltage is 12.0 and dropping, try a higher rated regulator (if one will interchange)

A burnt bulb won't allow the alternator to function, and an LED bulb in place of a regular bulb won't allow it either, because it is a Diode.

Ensure that the battery is fully charged. an alternator will not charge a dead battery..

X2, very good advice.

The reason you need the ALT bulb to function is to provide excitation for the alternator. If you don't provide this initial current, the alternator will not put produce output once it's turning. Generators retain their magnetism and do not require this initial voltage.

Once the alternator is spinning and showing a rising voltage the light gos off because the path through the alternator was to ground, and now the voltage on alternator side matches the voltage supplied to the bulb through the key switch and the instrument cluster.

Thus, an alternator that stops alternating will trigger a voltage differential, and light up the ALT light, alerting the driver.

Jim
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Old 10-04-2011, 02:37 PM
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Also, check whether the light changes brightness at all when it is "on". If it changes, that means the alternator/generator is not providing enough juice to counteract (just as said before) at certain (low) rpms. Typically you should see a little dimming or flickering with changes in rpm.

BUT, if the light stays on solidly lit uniformly, you may want to check the cable connections. I have almost always seen variation in luminicity of the ALT bulb when the alternator was going bad in my cars. The few times I had it on solidly, i actually had forgotten to reconnect a wire...

Good luck,

Bert
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Old 10-05-2011, 12:04 PM
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Drove a half mile down the road to the store & back to see if maybe the brushes were just corroded from sitting. Lost all electrics (headlights/wipers/etc) very quickly but the car stayed running & got me home.

Alt on order form Phil.
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