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914World.com _ 914World Garage _ Alternator light on?
Posted by: 914Toy May 30 2020, 01:19 PM
My alternator light comes on with ignition on, and stays on with engine running, and voltage stays at 12V regardless of rpm's. Was charging regularly at nearly 14V. The fan belt is in good order at spec tension. The alternator was reconditioned a couple of years ago and now has about 5,000 miles on it. The diode behind the gauge was also replaced, but I have had the gauge in and out a few times working on other issues - possibly damaging it(?)
Before removing the alternator for testing, could the diode behind the gauge be faulty preventing charging? If so, what is the spec of these diodes?
Your suggestions will be welcome.
Posted by: dr914@autoatlanta.com May 30 2020, 02:12 PM
pull the regulator and make sure that the light goes out. If it does plug in a new voltage regulator (probably not that) so if the belt is tight the alternator has a problem
My alternator light comes on with ignition on, and stays on with engine running, and voltage stays at 12V regardless of rpm's. Was charging regularly at nearly 14V. The fan belt is in good order at spec tension. The alternator was reconditioned a couple of years ago and now has about 5,000 miles on it. The diode behind the gauge was also replaced, but I have had the gauge in and out a few times working on other issues - possibly damaging it(?)
Before removing the alternator for testing, could the diode behind the gauge be faulty preventing charging? If so, what is the spec of these diodes?
Your suggestions will be welcome.
Posted by: 914Toy May 30 2020, 03:38 PM
pull the regulator and make sure that the light goes out. If it does plug in a new voltage regulator (probably not that) so if the belt is tight the alternator has a problem
My alternator light comes on with ignition on, and stays on with engine running, and voltage stays at 12V regardless of rpm's. Was charging regularly at nearly 14V. The fan belt is in good order at spec tension. The alternator was reconditioned a couple of years ago and now has about 5,000 miles on it. The diode behind the gauge was also replaced, but I have had the gauge in and out a few times working on other issues - possibly damaging it(?)
Before removing the alternator for testing, could the diode behind the gauge be faulty preventing charging? If so, what is the spec of these diodes?
Your suggestions will be welcome.
Thanks for your comments. I have a hatch in the firewall, so will be removing the alternator for replacing the built in voltage regulator.
Posted by: Mikey914 May 30 2020, 03:51 PM
If you replaced your alternator light with a LED. The problem is the LED.
Posted by: 73-914 May 30 2020, 05:11 PM
pull the regulator and make sure that the light goes out. If it does plug in a new voltage regulator (probably not that) so if the belt is tight the alternator has a problem
My alternator light comes on with ignition on, and stays on with engine running, and voltage stays at 12V regardless of rpm's. Was charging regularly at nearly 14V. The fan belt is in good order at spec tension. The alternator was reconditioned a couple of years ago and now has about 5,000 miles on it. The diode behind the gauge was also replaced, but I have had the gauge in and out a few times working on other issues - possibly damaging it(?)
Before removing the alternator for testing, could the diode behind the gauge be faulty preventing charging? If so, what is the spec of these diodes?
Your suggestions will be welcome.
Thanks for your comments. I have a hatch in the firewall, so will be removing the alternator for replacing the built in voltage regulator.
the voltage regulator is on the relay board not on the alternator
Posted by: ndfrigi May 30 2020, 05:22 PM
pull the regulator and make sure that the light goes out. If it does plug in a new voltage regulator (probably not that) so if the belt is tight the alternator has a problem
My alternator light comes on with ignition on, and stays on with engine running, and voltage stays at 12V regardless of rpm's. Was charging regularly at nearly 14V. The fan belt is in good order at spec tension. The alternator was reconditioned a couple of years ago and now has about 5,000 miles on it. The diode behind the gauge was also replaced, but I have had the gauge in and out a few times working on other issues - possibly damaging it(?)
Before removing the alternator for testing, could the diode behind the gauge be faulty preventing charging? If so, what is the spec of these diodes?
Your suggestions will be welcome.
Thanks for your comments. I have a hatch in the firewall, so will be removing the alternator for replacing the built in voltage regulator.
the voltage regulator is on the relay board not on the alternator
Keith’s car is a 6 conversion
Posted by: Steve May 30 2020, 06:09 PM
It's a 2.7 six. The alternator has a built in regulator. He is using a standard alternator light, not an LED.
Reading the manual... One side of the light goes to D+ on the regulator (blue wire) and the other side of the light goes to ignition + (RD/BK wire).
If the light is on, it's getting power. Slight chance the blue wire is grounding, which will also enable the light. Can you get to the blue wire directly on the alternator and put a light on it and see if it makes a difference? Otherwise your going to have to pull the alternator again and get it tested.
Posted by: porschetub May 30 2020, 06:39 PM
If you replaced your alternator light with a LED. The problem is the LED.
With an LED lamp it won't work from the get go,only the stock lamp will work for excitation of the altenator.
Bugger when you have a built in VR as changing them is a bit of a chore with the six, not so bad with a firewall opening.
Posted by: Mark Henry May 30 2020, 07:07 PM
When I was building my engine there was enough cases of internal regulator failure that I went with a 70amp external regulator type.
But I also put in a huge hatch, so if I have to I can pull the fan in car.
Posted by: 914Toy May 30 2020, 07:49 PM
After checking the wiring behind the gauge, which looked OK, I spent the afternoon removing the alternator from the car via the firewall hatch. Will be ordering a new voltage regular tomorrow from PP. Expect to have it back together by Wednesday in time for the rally., 
Posted by: 914Toy May 31 2020, 08:06 AM
What is the purpose of the diode attached to the alternator light terminal. The Haynes manual shows it connected in parallel with the blue alternator wire, connected to the brown with white stripe wires, but detail is difficult to be sure?
Posted by: Spoke May 31 2020, 08:47 AM
What is the purpose of the diode attached to the alternator light terminal. The Haynes manual shows it connected in parallel with the blue alternator wire, connected to the brown with white stripe wires, but detail is difficult to be sure?
Keith,
Can you show a picture of the schematic you refer to? I have an '86 930 and it doesn't show the diode you're referring to.
There shouldn't be a diode in series with the GEN light as the current in the GEN light needs to be able to go both directions.
Please post the schematic and/or my email: spokester@yahoo.com.
BTW, while your alternator is out it might be worthwhile to have it tested if possible.
Posted by: Spoke May 31 2020, 09:00 AM
...could the diode behind the gauge be faulty preventing charging?
Below I'm showing the simplified schematic for the 914/4 charging system. The 911 charging system is very similar except the VR is located with the alternator. Notice there is no diode to speak of. Not sure what the diode on your gauge is for. Not sure even what the gauge has to do with the charging system except to hold the GEN light.
Posted by: Mikey914 May 31 2020, 09:28 AM
That's why I thought he had put a diode in the indicator light position.
Posted by: 914Toy May 31 2020, 10:40 AM
...could the diode behind the gauge be faulty preventing charging?
Below I'm showing the simplified schematic for the 914/4 charging system. The 911 charging system is very similar except the VR is located with the alternator. Notice there is no diode to speak of. Not sure what the diode on your gauge is for. Not sure even what the gauge has to do with the charging system except to hold the GEN light.
This diagram is very clear thanks. I am removing this diode from behind the gauge, and ordering a new voltage regulator which I expect will solve the problem.
Thanks Jerry.
Posted by: Mikey914 May 31 2020, 11:25 AM
removing the diode alone should make it work
Posted by: Mark Henry May 31 2020, 12:10 PM
Also some alternators have two B+, two D+ and two D- terminals, don't try to pair these extra terminals up, just pick one of each.
Posted by: 914Toy Jun 2 2020, 07:15 PM
I have replaced the voltage regulator with a new one from PP on my 2.7 L 911 engine. After a light sanding clean of the two slip rings to mate with he voltage regulator carbon brushes, I installed the new regulator back onto the alternator. I started the engine and reved it up to about 2000 rpm - unfortunately - still no charge, with the warning light remaining on red and the voltmeter showing slightly over 12V - the battery voltage. The regulator is the $100 Valeo 5 star rated unit on PP which they say is the same as Porsche uses, but it is made in China (?).
After looking at a few "how to trouble shoot Alternators" videos, and checking all connections and correct attachment of the three wires to the alternator, I have to conclude that the alternator has failed. Subject to any further advice from this brain trust I plan to remove the alternator again tomorrow and take it for a professional checking.
Posted by: 914Toy Jun 3 2020, 03:10 PM
had the Alternator checked this morning. Not charging because on or more of the diodes had failed. Expect to pick up the alternator tomorrow morning and have it back in the car by the end of the day. So I expect to join all of you on Saturday morning early
Posted by: porschetub Jun 3 2020, 05:57 PM
had the Alternator checked this morning. Not charging because on or more of the diodes had failed. Expect to pick up the alternator tomorrow morning and have it back in the car by the end of the day. So I expect to join all of you on Saturday morning early
Good result Keith,can be frustrating sorting issues with charging systems
,my Bosch 55a unit has kinda has been a PITA after being full rebuilt,NOS Bosch regulator and rebuilt relayboard added later have helped somewhat but don't think these old units play well with AGM battteries at its max charge under load is around 13.5V but it still appears to be working when it needs to 
.
Posted by: 76-914 Jun 4 2020, 09:44 AM
had the Alternator checked this morning. Not charging because on or more of the diodes had failed. Expect to pick up the alternator tomorrow morning and have it back in the car by the end of the day. So I expect to join all of you on Saturday morning early
Keep your old alt. R&R the defective diode then you'll have a spare.
Posted by: worn Jun 4 2020, 01:25 PM
What is the purpose of the diode attached to the alternator light terminal. The Haynes manual shows it connected in parallel with the blue alternator wire, connected to the brown with white stripe wires, but detail is difficult to be sure?
I worked through this a few years ago. Here is a link to the post:
http://www.914world.com/bbs2/lofiversion/index.php?t262302.html
I even made a spiffy schematic. OK, maybe not actually spiffy
Posted by: 914Toy Jun 4 2020, 03:13 PM
Alternator had broken diodes. Service replaced the diode pack with a checkout. Installed the alternator this morning - all systems go with steady 14.6 volts.
Lessons learned: 1) our 911 engines from the early/mid 70's through the 80's do not have a diode behind the alternator light gauge, 2) the three wires attached to the alternator need to be insulated well to avoid grounding (in my case the small blue wire may have ground out at the alternator where it is difficult to see in the final stages of remounting the alternator to the engine, and 3) agree with Kent, these 911 alternators can generally be repaired for much less than buying a new or even refurbished unit.
Now ready for Saturday's Tour
Posted by: porschetub Jun 4 2020, 04:06 PM
Some more tips I picked up when doing mine ;
1- make sure the ground strap engine case to altenator is in good condition,mine was pretty crusty so I just replaced it,this advice came from Perry when he made my loom.
2- make sure you mark the top of the f/glass air deflector before removing as it can only work one way.
3- I found the nuts holding the air deflector were all loose,I replaced them with nylock nuts and larger washers to help prevent cracking the thin f/glass flange.
4- more sure you don't overtighten the fan housing strap otherwise the fan will contact in places...ask me how I know.
In summary its not an easy job so best to do it right the first time.
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