Shining Alternator Light- root cause?

[wullie]

Help please!

A couple of weeks ago I noticed that the alternator light turned on during driving.
After checking the BBS and the 101 Project book I am still confused.
Starting the car the red light is really bright, also at higher revs the light does not turn off but is less shiny.
The cars voltage at the battery is 17.6 V during idling and off.
I assumed that the voltage regulator is not working OK. Therefore I ordered a new voltage regulator, but I still have the same problem!

Please help!

Thanks, wullie

[dweymer]

The voltage is measuring 17.6v with car turned off?

[wullie]

Yes

[philip guiral]

I had the same problem with a 73 911 shortly after purchased. I found that the voltage regulator was wired wrong. The ground should be the brown wire not the black one. Hope your problem is that easy to fix.

[wullie]

Hi Philip,

thanks for your reply. But no, the brown is the ground.. Blue is ign and Black is field..

[seventyeight911]

Originally posted by wullie
Help please!

The cars voltage at the battery is 17.6 V during idling and off.

ya, thats wierd,

look at the wire, it is probably rubbed through somewhere and touching something and grounding and the dimming is it just making less contact. 17.6 volts seems a little high, usually a 3 volt increase is necessary to the 14-15 volt range is the norm on most cars and should only be 12 volts or slightly more when the car is shut off unless you meant off idle and even then, it should be about 12.7 - 13 at idle and no more than 14-15. probably should not be more than 14.7 volts at 2000rpms or more. also, if it is cold, it will read a higher voltage output than if it is hot. from 0 degrees F 14.6-15.4V to 160 degrees F 13.3-14.1V; these values are on the slightly high side too. i would say mid 13 volts is good.

[Zeke]

How do you get 17v out of a 12v battery? I'd use another meter before going any further.

Wullie, have you checked out the tech articles? There's a lot on alts.

[wullie]

Hello Zeke,

you were right. I measured 12.6V when the engine is off, 13.3 when the cold engine was around 2500 rev. The alternator light was still (not bright) shining.

[bigchillcar]

17+ volts? even when turned off? that doesn't make sense...turned off, a 12 volt battery shouldn't produce 17 volts...if the engine is running, alternator online, i suppose a faulty vr could improperly regulate to 17 volts...but it shouldn't read 17 with the car turned off!
ryan

[bigchillcar]

sounds like what mine has been doing of late though...light on until the alternator diodes excite...normally, my light goes out at start as soon as the rpm's pass about 1200, now it doesn't extinguish until nearly 2200...i think it may just be brushes starting to wear in my case...think my vr is ok.
ryan

[Early_S_Man]

wullie,

A battery being cooked by a bad alternator in the 16 - 20 Volt range can take 2 - 3 minutes to settle/return to the 'sane' 13 - 12 Volt range after the engine is turned off ...

I am suspicious of a replacement regulator behaving just like its' predecessor, but it is entirely possible! You probably should check the ground strap at the alternator ... both ends. It needs to be shiny-clean, and very secure!

I had bad luck with a replacement alternator on my wife's Nissan just two weeks ago ... original lasted 10 years, two months and 138K miles, whereas the rebuilt unit lasted 8 days and 220 miles! The regulator cooked the stator until the Karo Syrup colored windings turned black, and had the same phenolic smell as burned clutch or brake linings!

[seventyeight911]

17 volts will fry something. the VR should regulate about 14.7 volts max. I dont think that diodes have to "excite" they are a one way electrical valve, thats all. if you have to wait for them to "excite" to work then when you turn your car off your battery will drain because that is what they not allowing that current to run back into the altinator as it is an electrical motor when it is not being turned. also, they are for converting the ac to dc to charge the battery.

[wullie]

Hello all,

sorry for the confusion! I checked my volt meter and replaced the battery.
Now I measure the following volts:
12.1 when off
11.7 Volts when ideling around 1000 revs !
12.7 around 2500 revs

According to the previous posts I guess that is the reason why the alternatorlight is on all the time.

Now my question would be what the root cause is.

Now, I have one of the old alternators, see picture, where I can change the brushes. Would this be the root cause for the low voltage?

Regards,

Thomas