Strange Alternator light issue

[keynsham1]

It went to get my 1978 911SC out this afternoon. I haven't driven it for about four weeks. When I put it into the garage all was fine. Today it started easily, but the alternator light stayed on. The UK cars do not have a seatbelt warning light but instead have a handbrake warning light and a brake fluid level light. Funny thing was that the brake fluid level light also stayed on when the engine started. This is a simple circuit which just connects live to earth when the fluid is low and lights the bulb. I disconnected the switch for the fluid level but the light remained on. This is odd as there should be no power!

I have checked all plugs and sockets and even pulled the alternator forwards to check the wiring. All looks fine. The alternator brushes and slip rings are good and there is no sign of any overheating.

I wonder if the two lights being on is related? It seems possible that I am not getting 12v back from the alternator on the blue wire from the D+ connection. Baybe a bad connection?

Any ideas would be appreciated!

[afterburn 549]

unplug all your fuses , that will tell B4 or after the fuses.
If after, hook one at a time back up till you find the circuit.

[keynsham1]

On further inspection all wiring seems to be fine. The brake fluid warning light is lit and extinguished in the same way and with the same circuit as the alternator charge light so it will come on if the alternator light is on. So no issue with this any more.

I believe it must be the alternator asn the external voltage regulator would most likely give a high reading across the battery when the engine is running. I have removed the alternator and there was nothing obvious. I decided to put it back in case it was an undiscovered wiring bad connection (me being hopeful) but it is not any better.

Is there a way of testing the alternator when it is fitted to the car other than measuring battery voltage? I think I have no other choice but to remove it again and take it to be checked out.

[afterburn 549]

If you unplugged it would that not help?

[keynsham1]

Quick update. My alternator was tested and had zero output! It is presently being rebuilt.

[keynsham1]

I have had my alternator completely rebuilt and tested and a very good job has been done.








So you can imagine my disappointment when I refitted it, started the engine, and my alternator light stayed on!

Further investigation has shown that the blue wire from the bulb to the engine loom connector is showing zero volts with the ignition on and engine not running, and no different when the engine is running. This explains why the alternator is not charging as it needs this feed to energise the coils. Nothing in equals nothing out!

As the light is on and stays bright, I have to assume that somewhere between the bulb and the engine connector, this wire is earthing out so the bulb stays lit, but no volts gets to the alternator feed.

I suspect the fault is in the dashboard wiring somewhere, but I was wondering of anyone knows where the loom runs from the dashboard to the engine bay? I believe it is inside the car but I am not sure where, and I haven't had a chance to look yet.

[timmy2]

The Blue wire runs directly from the back of the gauge lamp through the main through body harness to the rear fuse panel male 14 pin connector on pin 11.
From there via the female engine harness pin 11 it has 2wires soldered in the female pin and one goes directly to the back of the alternator and the other goes to the remote Voltage Regulator plug.

Have you tested or replaced the remote Voltage Regulator? Also there is a radio noise suppressor that may be plugged into the remote VR (between the VR and the plug to the VR from the engine harness) that can fail.

So, test continuity from the Alt blue wire to the female 14 pin, the remote VR connector and the female 14 pin 11. Then test continuity from the male 14 pin 11 to the blue wire at the gauge lamp. (Ignition off)
Check each of the above to Ground as well.
This will tell you if the wire is open or shorted to ground.

[rgmrgm]

I had a similar problem and it was the radio noise suppressor

[keynsham1]

I have checked all the wiring. I have bypassed the suppressor, tried another voltage regulator, and even run a replacement wire from the bulb to the blue wire on the alternator, all with no effect. I seem to have battery voltage at the alternator brushes so the feed is good. I have removed the alternator now and it is back for testing again as all tests lead to it still being faulty even though it has been rebuilt. Watch this space!

[timmy2]

Did they test the windings and test/replace the diodes?
Rebuild or replace the cummutator rings, deglaze them?
Brushes seated /bedded properly?
Ah, the old days working on 100 megawatt generators in a dam!
I don't miss it...

[keynsham1]

I have a new concern now. I took the alternator back to the repair shop and they retested it. Apparently the rectifier diodes were fried again! They are rebuilding it for free and when done I am going to see it on their test bed so I know it is working properly. Thing is, I am now worried that it is the wiring in the car causing the failure. I cannot tell if it was repaired correctly so it may have come back faulty after the first repair but I cannot prove that.

I have replaced the voltage regulator with a new one. I have checked continuity of all wiring and also resistance of the lead from the alternator to the battery to make sure it is not giving a high resistance. From what I can understand provided the alternator is good, all that will blow the diodes is excessive current and this can only be caused by high resistance in the line to the battery so as this is very low, it should be OK. I am still worried about causing $300 of damage at the first turn of the key though!

[JFairman]

Fantastic alternator when using extra AC fans, headlights, stereo amp, etc.
Always shows a good charge while motor's running.
Porsche 911 Alternator 1984 1989 High Amp 160 Amp High Output Generator | eBay

[timmy2]

Make sure your battery is fully charged.
Bad diodes from the start I would imagine.
If you traced all the wiring as described you should be ok.

[keynsham1]

I have just been to see my alternator guy. He wasn't looking very happy. He replaced the rectifier pack in my alternator and put it on his test rig. As soon as he turned it on, it blew again! This is a result for me as it means my car is fine and the alternator is faulty. He was determined to sort it out but he said that after working in the trade for 20 years, this was the first time he was beaten. All looks fine, it just doesn't work! So it looks like I will be buying a new one after all.

[keynsham1]

Well my alternator did get repaired, and I refitted it to my car and guess what? No charging and light still ! Yesterday I spent a good few hours with an auto electrician and we gave the entire circuit the once over. It seems that the new voltage regulator I bought doesn't work. It does nothing at all so the alternator doesn't get excited and acts like it is dead. The old voltage regulator did make the alternator come to life, but it is faulty as it doesn't regulate the battery charge voltage and the light stays on so somewhere within its workings there must be a short of some type. The conclusion is that a new working voltage regulator will fix all my problems.....hopefully. One odd thing though. On the blue wire from the alternator, we were picking up the tachometer signal (square wave on the oscilloscope). Is this normal?

[keynsham1]

A new voltage regulator has not fixed my problems! Actually the replacement one I have just bought doesn't even allow the alternator to excite and so nothing happens at all.

Having read some advice from this website, I will try today to connect the D+ an DF wires together directly without the voltage regulator plugged in. If the D+ side is working correctly, 12v should initially be supplied to the coil via the bulb before the engine is started to excite the coil. When the engine is started this 12v is replaced by the feed from D+ which will also feed back towards the bulb, making it go out. If this doesn't happen it would seem that the alternator is still bad and the D+ diodes are not working properly. The check done by the shop was just to supply 12v to DF and demonstrate that the alternator would be excited and produce output. This is fine but it doesn't check the D+ feedback circuit which I now suspect is faulty.

Update later!

[keynsham1]

So I have returned my alternator to the shop. The guy there has decided he does not know why my alternator seems to be faulty. Apparently it had been repaired previously and he had tried to copy the wiring for the three diodes feeding D+, I suspect incorrectly. He has offered me a full refund or an offset of that amount against a replacement alternator. This is the way I will go. It seems however that the replacement will be the later type with an internal voltage rectifier. This is good and apparently what Porsche recommend as an upgrade but I am not sure what wiring is required.

Can anyone tell me what wiring changes are needed? I assume that there will be a B+ for the battery lead, a D_ which will be the earth, and a connection for the blue wire from the warning lamp. The current black and brown wires going to the voltage regulator will be redundant.

[timmy2]

Just fold back and tape off the black wire at the alternator. Remove and tape up the 3 prong connector at the Remote VR location. Good to go.
You may have to install a diode or resistor at the gauge lamp. There is a service bulletin out there about it. Search the regular Pelican 911 tech forums for it.
I have it at home somewhere, if you can't find it I will post it up later tonight or tomorrow.

[timmy2]

Technical Service bulletins I promised.
Page 4 talks about the resistor installation that may be required. Enjoy.

[keynsham1]

Thanks timmy2, massively helpful for me!