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Registered
Join Date: Jul 2000
Location: Bend, OR USA
Posts: 372
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Alternator??
I have a 1972 2.4T.
I noticed on Friday evening that the Ground light was very slightly lit when driving at night. It would not glow at idle and would then slightly glow when I reved up the engine. It is not bright enough to see during the day. Thoughts here? Is this my alternator going bad? If my battery was very low would this light come on if the alternator was working super hard to charge my low battery? My car has had not been driven for around 2 months. I'm driving like mad now though.... |
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08-20-2001, 08:58 AM
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Registered
Join Date: Jul 2001
Location: Wilmington, NC USA
Posts: 635
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A quickie check for your alternater. With the engine off, check your battery voltage with a multimeter. It should be above 12 volts. Start car, turn on headlights and other electrical equipment and recheck voltage at the battery. If it drops below the reading with the car off(especially less than 12 volts) then either your voltage regulater or alternator or both is bad. I've had good luck in the past with alternator rebuild shops. Good ones can save you lots of money. Good luck.
------------------ 69 911 2.3EZ |
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08-20-2001, 09:40 AM
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I got a rebuilt alternator and found my alternator light glows as my RPMS rise as well. As I recall this is idicative of not enough amperage, or perhaps a bad diode on the regulator.
Reading Bruce Anderson, I know for my year, 85 (maybe its 90) watts is just enough to power the stock electrical system. Of course with my headlights (80/100w H4), fog lights (100w each), and now my stereo amp, it may not be up to it. So I checked voltage, and all seems well. I don't drive TOO much at night, but it has lasted a four hour night drive already. So I am not worried about mine. Check all your grounds, and check voltage as mentioned above. Check battery too. Good Luck! ------------------ Nick Hromyak '85 Carrera 7 & 9 Fuchs Havin' Fun in Sacramento |
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08-20-2001, 10:56 AM
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Registered
Join Date: Dec 1999
Location: TX USA
Posts: 9,804
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Ian,
If your battery measures 12.6 to 12.9 Volts with engine off, and 13.5 to 14.2 Volts with engine on above idle, then all is well with the alternator. It is very typical for the alternator warning lamp on older cars to glow at night when there is nothing wrong at all! If you install a 91 Ohm 5 Watt resistor across the terminals of the warning lamp socket, your problem will disappear and the pre-exitation current available at idle will be improved. The resistor is recommended by Porshe in a service bulletin for '82 SCs that is applicable to earlier cars as well! ------------------ Warren Hall 1973 911S Targa 1992 Dodge Dakota 5.2 4X4 parts hauler |
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08-20-2001, 11:01 AM
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Senior Member
Join Date: Feb 2000
Location: Lacey, WA. USA
Posts: 25,310
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Warren, I have been told that faintly glowing alternator lights suggest a bad diode. Does this make sense?
Ian and Nick, a bad diode is not much of a problem, or so they say. ------------------ '83 SC |
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08-20-2001, 03:30 PM
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Senior Member
Join Date: Apr 2001
Location: Finland
Posts: 1,214
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Hey IAN, could you tell me which colour wire goes to the alternator idiot light?
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08-22-2001, 03:53 AM
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Registered
Join Date: Dec 1999
Location: TX USA
Posts: 9,804
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Super Jim,
There are three sets (called arrays) of three diodes ... two of the arrays of three are for main power generation, the positive array and the negative array, and the third array is for exitation current for powering the rotating magnetic field inside the rotor. Diodes can fail and burn 'open' or short out ... Alternators are really varable-frequency, three-phase AC generators, but the magic of the diodes allow DC current to be taken from the AC generator! Diodes in either positive or negative arrays that fail 'open' reduce the current capacity to be diminished by 1/3 ... but, Voltage could still be 'regulated' to the normal level! This often results in a car that will start and operate normally during many days or weeks of daylight-only operation, but the headlamps get dimmer and dimmer as the battery runs down during one extended night drive! The odd circumstance of a shorted diode in the positive array AND a shorted diode in the negative array will result in a car that runs down a brand-new, charged battery in a matter of minutes to a few hours! A diode in the exitation array that fails open would cause a dim glow of the warning lamp, because 1/3 of the current corresponding to the 'engine off' exitation current (which causes the full-brightness warning lamp) would be drawn from the battery though the warning lamp! ------------------ Warren Hall 1973 911S Targa 1992 Dodge Dakota 5.2 4X4 parts hauler [This message has been edited by Early_S_Man (edited 08-22-2001).] |
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08-22-2001, 07:45 PM
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Right before my alternator went out (73K) the light began to glow at idle, and then stopped glowing as soon as I achieved higher revs...ie over 1500. I thought that meant it was charging while I was driving and I could get away with it for a few days while I researched the problem.....wrong. Car died second day with a completely dead battery and bad alternator. I should have tested the current as recommended here...so beware. I was lucky to break down on a residential street two blocks from home with a vacant parking space to coast into...the Porsche gods were smiling....
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08-22-2001, 08:05 PM
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Registered
Join Date: Jul 2001
Location: Danbury,ct .usa
Posts: 79
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Well said s man
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08-22-2001, 08:05 PM
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Porsche
1985 Porsche 911 3.2
