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

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?

#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.

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.

somebody explains it better in this thread...I think adding the diode to the charge light circuit helps with the alternator "excitation" process

http://forums.pelicanparts.com/porsche-911-technical-forum/119642-just-sure-alternator-question.html

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.

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.

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?

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

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.

Cheap and non intrusive way to monitor. Works very well and its accurate.

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

http://forums.pelicanparts.com/uploa...1391603443.jpg

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

that thing is pretty cool I might pick one up. Thanks!

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....

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.

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?

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.

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.

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

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.

I have that same problem (promotes quite a bit of staring)! :D

Radio shack diode works. But it sure sounds like you need to get the alt rebuilt.

Hey guys, I'm having an charging issue also, I'm on my 4th alternator and regulator , can someone take a pic of the diode and where it's located , thanks

Pull out your temp/oil pressure gauge. Pull on the wire that goes to the battery light, top middle. Follow wire back to the the plastic connector. Diode is in that connector.

My other question is, can welding on the chassis mess up the alternator ?

http://forums.pelicanparts.com/uploa...1391661143.jpg
I got a little creative with a 3 amp ratshack diode. I haven't tried it yet but when I hooked the 2 leads together the alternator light came right on.

What makes you say that? The chared diode? My car only has 30 k miles so the brushes should be OK. Maybe a new voltage reg would be a worthwhile safety precaution?

What makes me say that is your description of item 2 & 3 in your original post.
An alt rebuild will also include going through and/or replacing your VR.
Plus any quality rebuild guy will test it before tearing it apart and if it's all fine shouldn't cost you more than your time to pull the alt and make sure your don't have a fire hazard of wiring under your shroud.
You still haven't said the year of your car.

OK thanks. My car is an 84. I thought 2 and 3 where caused by the exciter diode. I'm surprised it charged at all from what I read. I'll look into having the alt tested

Aschen,

This is the article I always refer to: http://forums.pelicanparts.com/porsche-911-technical-forum/667753-alternator-light-doesnt-go-out-after-startup.html

I installed my new diode and the light operates as it is supposed to now it turns off the instant the motor fires. It also starts charging with out a throttle blip.

The only issue I see is the voltage hovers at 14.04 v. At idle. Does that seem a bit high to anybody?

Good job ,perseverance paid off!

Does it ever go above 14, like when you rev it up to 3-4 grand? I think the spec on the VR is right around 14. You don't want much higher than that or you will start ruining expensive electronics.

Just a follow up in case any body is searching the site in the future. I used diodes marked 3 amp epoxy rectifier diodes from radio shack. They are not marked with a specific type however. I got one of those cigarette lighter volt meters and it shows around 14v consistently while driving and about 13.5 if the headlights are on. I checked this against my modern commuter car and it was about the same. All signs point to positive for this 59 cent solution.

Thanks for the follow up

Can you be a little more specific. I'm looking on Radio Shack's website, and see quite a few 3A Rectifier Diodes. Which was it? See link: Search Result

Thanks, Shannon

I think your charge voltages are fine. Most 12v lead acid batteries charge at around 13.8. Stone cold and fully charged, a 'full' battery should read around 12.8.

Just for reference, when my regulator gave up the ghost, it was charging at around 17v and my tip-off was the smell of hydrogen sulfide gas as the battery boiled away. Didn't hurt any of my electronics, but did cause my AFR meter to read way low...as in really rich. It was the AFR that first weirded me out, then the smell, then I hooked up a volt meter to see what was going on. It happened only after shutting the car down fully warm and letting her sit for a few minutes. My guess is that it then heat soaked the regulator and cause the over charging to surface.

Ratshack part number 276-1141

Thanks mark I figured the voltage would be erratic and higher if the vr was gone

Pretty well the same story as Mark.

Happen during a Porsche coffee run and since I was following someone with a brand new muffler, I dismiss the smell as that.

Bolled the battery, but no harm to the electronic. The VR would be good for 5 minutes than put out 17 Volt as soon as it got warm. Changing a VR in a Cab is a real PITA.

Its a good reason to either installed a permanent voltmeter or a plug in the cigarette socket type