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Alternator idiot light is on

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Old 04-23-2019, 07:20 PM
  #16  
aam993
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Lightbulb

Originally Posted by IXLR8
Very cool, thanks for the inspiration! Straight to the top of the to-do list! Well, after getting rebuilt LSD back into the car.
Old 04-24-2019, 09:08 AM
  #17  
Cameltoe & Heel
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Had the same problem, battery light on, would sometimes go dim at higher revs....

Took alternator out and had it tested, the rectifier was diagnosed as the issue and replaced, re-installed it and now working perfectly.

See below

http://911uk.com/viewtopic.php?t=133...14c731511b5337

Trev
Old 04-24-2019, 01:17 PM
  #18  
fsa
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I have that dash voltmeter and it's terrific.
I added the unit below which allows power to other gadgets- phone charger, Valentine, etc.
A bit cumbersome, but functional, cheap, well made.

https://www.amazon.com/Rocketek-Cigarette-Lighter-Adapter-Splitter/dp/B07JZ7Y3CD/ref=sr_1_4?hvadid=174258156924&hvdev=c&hvlocphy=9032826&hvnetw=g&hvpos=1t1&hvqmt=b&hvrand=11282799766058050804&hvtargid=kwd-92907686000&keywords=dual+12+volt+socket&qid=1556122128&s=gateway&sr=8-4#customerReviews https://www.amazon.com/Rocketek-Cigarette-Lighter-Adapter-Splitter/dp/B07JZ7Y3CD/ref=sr_1_4?hvadid=174258156924&hvdev=c&hvlocphy=9032826&hvnetw=g&hvpos=1t1&hvqmt=b&hvrand=11282799766058050804&hvtargid=kwd-92907686000&keywords=dual+12+volt+socket&qid=1556122128&s=gateway&sr=8-4#customerReviews
Old 04-24-2019, 03:58 PM
  #19  
Kika
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Originally Posted by IXLR8
Install one and connected to where it should be connected, directly to the battery.

This one is "on" all the time as it draws less than 2 mA.

This reading is at idle...
I also have this one, and in the same location. I wired it to switched power, so it isn't directly across the battery, but I figure it is close enough. basically I want to know if it is above or below 13V.
Another Rennlister had a bunch of the little plastic housings, and gave them out a few years ago. got the voltmeter off Amazon. all in all, a pretty simple DIY. maybe a 3 on the 1-0 scale.
Old 04-24-2019, 05:05 PM
  #20  
JMarchetto
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Default Update to idiot light

Well, all the suggested easy fixes have not resolved the issue of the light. So I have removed the alternator again (not so ugh, found short cuts and was efficient) and had the rectifier checked and was all good. Will re-install and see what happens. When I spoke to the alternator rebuilder, he had me test the blue wire for continuity to the block in the engine and it tested fine. Also, had me connect a voltmeter to the blue wire and then turn the key to on (wrapped the battery/power wire) and had only .5-.7V. He felt it should be in the 2-3V range. Also, if I were to ground the blue wire with the key on, the "idiot" light should come on which it did not. So, back to square one if the light stays on after re-installing I'll keep you all posted and thanks for your help.
Old 04-24-2019, 07:20 PM
  #21  
IXLR8
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Originally Posted by Kika
I wired it to switched power, so it isn't directly across the battery, but I figure it is close enough.
You can always do a quick check and see if your voltmeter reads what the battery voltage reads with a DMM. That should be done anytime you connect a voltmeter to a circuit other than to the battery itself. I did the same thing on my 991 since it has a voltmeter as part of the factory instruments...wanted to see how close it was to the battery voltage.


Originally Posted by Kika
Basically I want to know if it is above or below 13V.
I guess you want to know if the alternator has failed to charge. If I was down just below 14.0V under the same conditions, then I have a problem like a bad diode in the rectifier pack.

This voltmeter install started back in 2003 when I installed one on my motorcycle.
Old 05-27-2021, 12:17 AM
  #22  
dave2020
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Originally Posted by pp000830
If your car reads between 13.5 and 14.2 volts with the engine running and between 12.5 and 12.8 a few seconds after you turn it off your alternator is most likely working correctly and is most likely wired correctly.
I looked at the wiring diagrams and the wire from the battery light in the dash clock is a shared circuit that goes to the alternator, alarm, abs & central informer controllers so it is not a simple system.

This leads me to the idea of simply disconnecting and reconnecting the battery as it may reboot/reset some internal software switch in one of the controllers related to this circuit.

An idea I had was that the alternator diodes are part of what is called a rectifier bridge circuit where sets of diodes are set up like one-way valves to redirect the alternating current from the alternator rotations to direct current. For example, something called a full wave bridge design using more diodes produces a smoother level of DC current, a half wave bridge design produces what looks like pulsed DC current. Conceivably if one or two of the diodes in the rectifier circuit internal to the alternator are bad creating a pulsed DC condition this may be causing the warning light controller to turn the light on. From the wiring diagram, the alternator has multiple diodes in it that suggest it is designed to produce very quiet electrical power. The good thing is that this condition can be check without removing the alternator as it presents as a noise in the alternator circuit. Any qualified technician should be able to tell you if this theory applies to your car.

A very very long shot is that the solder joints in the clock are known issues in our cars and to pop out the clock and reheat the internal solder joints on the board is relatively easy, I did it at the kitchen table years ago and seem to recall the solder joints to the clock motor need to be released to get the board out. Then make sure the bulb connections are good.

Some folks replace the some of the dash display lights with LED bulbs. These are not resistive loads like the original bulbs and also act as one-way electrical valves, this may be part of the issue if you have replaced any of your display bulbs.

Finally, do you know why the alternator failed in the first place as this root cause may still be present? Have you had any servicing done recently, just before the alternator issue cropped up?

Hope this all sorts out. The upside is you can still drive your car.
Andy
Great in-depth explanation. Thank you. Based on what you described with the noise, I think I may have an alternator problem. Just posted this...

https://rennlist.com/forums/993-foru...itic-draw.html

I'm noticing a wining noise from the engine bay. Thought maybe I just never noticed it before and that it was normal but now thinking maybe could be a blown alternator diode?

Old 05-27-2021, 10:14 AM
  #23  
IXLR8
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Originally Posted by dave2020
I'm noticing a whining noise from the engine bay. Thought maybe I just never noticed it before and that it was normal but now thinking maybe could be a blown alternator diode?
The whining noise is probably a bearing in the alternator or fan and those are separate from each other. The other whining noise if it comes from an alternator is the one you would hear when the radio is turned on so that is not the case if you are hearing it from the engine compartment.

By the way, a three-phase alternators have 6 power diodes.



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