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I read 100+ Alternator threads

Ok guys sorry to be so needy, but I did my best to research here. I have a feeling my answer is straightforward, but I cant figure out whats going on.

My symptoms:
1. charging light never comes on though the bulb is fine
2. If I start the car and leave it idling, the voltage dips very low and eventually the car will die from running the batt down
3. If I give the car a good throttle blip or 2 the voltage will settle at about 13.4v and will charge.


Is this a non issue, It seems like I might have a voltage reg issue or maybe something else? I will gladly email you a beer for your troubles.

Thanks,
Aaron
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Old 02-04-2014, 09:48 AM
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It's hard to diagnose via the net.

But my guess would be a failing diode in the rectifier plate. (probably)

Also, the brushes and slip rigs wear out. But they can be replaced easily.

Belt tension ok?

Good clean connections on the back of the alternator? Good grounds?
Old 02-04-2014, 10:12 AM
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#1 - you should fix the charging light - maybe its just the bulb.

Older cars had an external voltage regulator and often the alternator is replaced with a newer model with an internal voltage regulator. I can't recall exactly but I seem to remember that when retrofitting the internal VR alternator to the older cars, there is a technical service bulletin that suggests adding a diode to the charge light circuit. I also seem to recall that the charge light circuit may be involved in the process of "exciting" the alternator, which must occur before it produces charging current.

#2 - its normal that the alternator will not charge until you've rev'd the engine enough to achieve the aforementioned "exciting" of the alternator. I'm not a EE guy, so I can't explain the electro physics.

If your charge light were working, you would see that it stays on if you start the car and let it idle, then go out the first time you get the revs up.
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Old 02-04-2014, 10:21 AM
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the light HAS to come on when the key is on and engine off. if it does not, there is something wrong with the wiring for that circuit.
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Old 02-04-2014, 10:21 AM
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somebody explains it better in this thread...I think adding the diode to the charge light circuit helps with the alternator "excitation" process

Just to be sure, alternator question
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Old 02-04-2014, 10:27 AM
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Quote:
Originally Posted by jwasbury View Post
somebody explains it better in this thread...I think adding the diode to the charge light circuit helps with the alternator "excitation" process

Just to be sure, alternator question
Yes it does and if the light doesn't com on at all then either the bulb is bad or the diode. If that circuit isn't completed then the alternator will only put out 12v instead of 14. Took me over a year to find mine, but it was the diode. I just removed it connected the wires and it worked fine until i could find a replacement diode.
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Old 02-04-2014, 11:21 AM
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Nothing excites me more than an excited alternator! How seductive is that?

I think you guys have hit the nail on the head re: the diode. I had never had issues with mine, but did replace the voltage regulator last spring. Not a huge chore, but solved my charging issues. Just another funky German engineering thing....a diode up at the idiot light. How idiotic is that?

If everything is working correctly, even at idle (once the alternator is initially excited by a blip on the throttle) you should be seeing low13 volts at the battery.
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Old 02-04-2014, 12:03 PM
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thanks all you guys are cool as always. Everything seems normal about my car except the light not working I guess. I don't want to take it on a long trip until I am sure though for fear of being stranded. The link you guys produced is for adding a resistor not a diode in parrellel to the bulb.

A few follow ups: What should I double check for the warning light circuit? Like I said the bulb is fine. If the voltage regulator was going out would that cause the issue? Is the rectifier part of the regulator?
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Old 02-04-2014, 03:39 PM
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I pulled the dash gauge out and it looks like the spade connectors going to the plastic diode thing are a bit chared. That must be my problem. I'm going to replace it and see what happens
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Old 02-04-2014, 09:04 PM
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Here is the thing You might have trouble replacing it. They are NLA from Porsche. So i had to keep inquiring in the parts forum whenever people are parting out cars. Several people here tried to Help me out and sell me one for a ridiculous price but I eventually found one for a couple dollars. In the mean time you can just bypass it.
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88 turbo Guards red Targa slant nose, and yes I am a horsepower junkie, 3.4liter,7.5 to 1 JE pistons, Adjustable WUR, Imagine fuel head, 1 bar waste gate headers,allthe cis toys. Now apart to become the next EFI monster. fabbing my own intake, headers Individual throttle bodies, MS-3, pauter rods, Xtreme twin plugged heads, gt-2 evo cams cop's.
05 Cayenne S lapis blue
Old 02-05-2014, 04:03 AM
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Quote:
Originally Posted by aschen View Post
... I don't want to take it on a long trip until I am sure though for fear of being stranded...
Cheap and non intrusive way to monitor. Works very well and its accurate.

INNOVA 3721 Battery and Charging System Monitor : Amazon.com : Automotive

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Old 02-05-2014, 04:31 AM
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Quote:
Originally Posted by Ken911 View Post
Here is the thing You might have trouble replacing it. They are NLA from Porsche. So i had to keep inquiring in the parts forum whenever people are parting out cars. Several people here tried to Help me out and sell me one for a ridiculous price but I eventually found one for a couple dollars. In the mean time you can just bypass it.
Thanks ken. I saw your posts in many of the threads when searching for diodes.

The good news is I have several EEs that report directly to me so I should be able to figure out a suitable replacement. My intuition is that nearly any standard diode with enough current capacity should work but I need to confirm with an EE
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Old 02-05-2014, 08:30 AM
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Quote:
Originally Posted by JJ 911SC View Post
Cheap and non intrusive way to monitor. Works very well and its accurate.

INNOVA 3721 Battery and Charging System Monitor : Amazon.com : Automotive

that thing is pretty cool I might pick one up. Thanks!
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Old 02-05-2014, 08:31 AM
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Has anyone attempted a cross-reference to say a Radio Shack diode or Digi-Key maybe? A diode is a diode, provided it has the right power rating to handle the amps going through it. Just a thought....
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Old 02-05-2014, 10:40 AM
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sombody reccomended a part# 1N4002, but a EE just confirmed my suspiscion that nearly any with a sufficent current rating will do. I think I will try to buy something around 1 amp.
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Old 02-05-2014, 10:49 AM
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Because I grew up with 60's British cars and bikes, I get uncomfortable driving without a voltmeter. In my 930 I use a small digital one that plugs into the cigarette lighter. About $4 on eBay.

Car LCD Cigarette Lighter Voltage Digital Panel Meter Volt Voltmeter Monitor | eBay

And yes, a properly rated diode from DigiKey or Radio Shack will be just fine, but does the shack still sell stuff like that?
Old 02-05-2014, 11:02 AM
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no all you get from radioshack these days is cell phones and cheap electronic gadgets. not many components unless it's for wiring your stereo with crappy connectors. That was the first place i checked for a diode to make my own. All I got was no we dont cary that type of stuff anymore. And that particular store is now a nail salon lol.
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88 turbo Guards red Targa slant nose, and yes I am a horsepower junkie, 3.4liter,7.5 to 1 JE pistons, Adjustable WUR, Imagine fuel head, 1 bar waste gate headers,allthe cis toys. Now apart to become the next EFI monster. fabbing my own intake, headers Individual throttle bodies, MS-3, pauter rods, Xtreme twin plugged heads, gt-2 evo cams cop's.
05 Cayenne S lapis blue

Last edited by Ken911; 02-05-2014 at 11:19 AM..
Old 02-05-2014, 11:17 AM
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I'm not sure about your 84, but the 86 and later US cars with the factory alarm run the excite wire through the alarm module. If the alarm module is removed you have to jump the two wires to get the alternator charging again.
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Old 02-05-2014, 11:54 AM
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Im gonna try radio shack on my way home. The one by my place still carries some components. We only had surface mount diodes in the lab downstairs.

Ironically I was in radio shack for the first time in years on sunday. It amazes me that it stays in buisness, but there were still a few items that caught my interest. I made a 100$ impulse purchase of an arduino begginers kit. If you don't know what an arduino is, it is an open source microprocessor development architecture for education (and big kid projects too). It is a neat movment of people building gagets. I was starting to think kids just played vilolent video games and looked at naked girls on the internet. It is refeshing to see some technical hobbies remain
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Old 02-05-2014, 12:59 PM
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Because I grew up with 60's British cars and bikes, I get uncomfortable driving without a voltmeter. In my 930 I use a small digital one that plugs into the cigarette lighter. About $4 on eBay.
Amen on the Prince of Darkness experience 8-)

Nice - thanks for the tip! Just ordered the exact same thing off fleabay, $2.89 shipped!

The other one looks fancier with the battery graph and LED lights, and I like the tilt-down feature, but the socket plug is quite long and seems to protrude a lot.
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Old 02-05-2014, 01:04 PM
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