Pulsating lights - why?

[WolfeMacleod]

I always meant to post but kept forgetting.

When my lights are on, they pulsate. Both dash lights and headlights.
At regular idle, they don't do this. Only when I get on the gas.

It doesn't seem to get faster with higher revs, nor does the pulsating get more...intense...

Is this a normal thing in 911's? If not, what causes it?

[carrera turbo]

hello
not normal

i would check the charge voltage with lights on and lights off both at idle and higher revs. sounds like a regulator problem or diode depending if you are internal regulated or external. i have seen the ground on the back of the alt. cause this type of problem as well as the one up front on the battery. check the voltages check the grounds and get back to us.

thanks ed

[Flieger]

This is just a crazy thought without reviewing my electrical textbooks,

The diodes in the alternator might be going. The 4 diodes with 2 in series in parallel branches I believe is called a bridge rectifier. This functions to cut the peaks off of the AC sine wave and help convert AC to DC.

[T77911S]

an alt is a 3 phase generator. it has 3 pairs of diodes. one diode on each pair allows the positive portion of the sine wave of that phase to go to the battery and electrrics, the other diode sends the negitive portion to ground. if you look at the output of a bridge rect. with an O scope, it would look like a ramped DC, or a saw tooth. IE, the voltage may start at 13 volts and ramp up to 15 volts, then drop down and do it again. if you add a capacitor to the output, it smooths out the ramps and make a flat DC voltage. yes i am at work and putting off doing work right now, its early. if you just had one pair of diodes, the ramps would be steeper, or there would be more of a swing, say, 11 to 15 volts. (i dont know the exact volts, just numbers to give you an idea of what it looks like). plus, the 3 phase is more efficient.

[rick-l]

Only when you get on it like raise the rpms? It could be a diode breaks down with the higher peak inverse voltage but ???? It could be interacting with the regulator in some strange way. Does it do it without the car accelerating, like in your driveway in neutral?

I think Porsche has 6 phase alternator but could be wrong.

[RWebb]

I don't see how it could be a cap. at the speed things are happening during the rectification. My guess would be the voltage regulator but not sure re an '85...

IIRC, this sounds like what ancient cars did when they used to have DC generators in them - think I recall seeing this when I was a young kid...

Bottom-line you need to drop the unit off at an alternator shop.

[Superman]

Lord knows why those brilliant German engineers chose to build these cars without voltmeters.

[darrin]

Does your seatbelt warning light randomly come on? Do you smell a sulpher odor in your cabin? Check your battery to be sure it isn't cooking (and spilling acid). Sounds quite similar to voltage regulator problem I had when I first purchased my carrera years ago. My failure to timely address/diagnose the problems led to a failed tach, a fried DME, and an acid-damaged battery box area . . .

[john walker's workshop]

typical overcharging issue with 90A carrera alternators w/internal regulators. new alt time.

[VFR750]

I had the same problem. New voltage regulator solved the problem.

[john walker's workshop]

the problem with just replacing the regulator is that the regulator's brushes usually have worn deep grooves in the alternator's thin commutator, sometimes most of the way through it. so the life of the commutator is very short and the brushes often don't seat well and/or get chewed by the rough commutator grooves. the commutator is too thin to machine smooth again, so it needs to be replaced. quicker to replace the entire unit.

[KTL]

Had the same problem on my car. New alt. fixed it.

Definitely a wise idea to get a digital handheld voltmeter (or a nice sensitive analog) in the system so you can see the charge characteristics of the alt. as revs change. An extreme overcharge situation is bad news.

I experienced this first hand with a faulty reman alt. I happened to be installing some gauges at the same time I did my alt. When done, my VDO slow-sweeping voltmeter would sway back and forth, and then go to max. Well that blew out my headlights (expensive 80W/100W H4 bulbs of course) and made some other things go wacky- same seatbelt light came on like darrin mentioned. So get that alt. replaced sooner than later!

[VFR750]

Well, I understand about the wear risk. I guess the best way is to first look.

But I have to say, I replaced the voltage regulator 24,000 miles ago. Still working fine, and a lot cheaper than a new alternator.

[dshepp806]

I'd watch both components (as one is failing presently),..I (personally) change both out as a system...I know,..more $$ should it be only one of two......don't really care about the $$$$..they work SO closely together, I make sure H/W is new.

Alternator (as JW suggested) can be tested.

Matter of age and use of the hardware........

Best of luck to you..

Doyle

[WolfeMacleod]

Originally Posted by darrin

Does your seatbelt warning light randomly come on? Do you smell a sulpher odor in your cabin? Check your battery to be sure it isn't cooking (and spilling acid). .

Nope, not at all. In fact, I just replaced the battery because the car had sat over winter and would not hold a charge.
I can remove the battery and it will run just fine, so I don't think we're under-charning at all either. I've exploded a batery before years ago, so I'm VERY familliar with that Sulfur smell.

Originally Posted by rick-l

Only when you get on it like raise the rpms? Does it do it without the car accelerating, like in your driveway in neutral?

Yes, even sitting in the driveway. Any raising of the RPMs will cause it to happen.


Come to think of it, this has happened since the day I bought the car. Four years ago.

John, wanna test my Alt?

[rick-l]

my 88 had deep grooves in the slip rings at 65,000 miles. I got an alternator online for $125. Before I installed it I took it apart to see what they did for that price and it did have new slip rings on it.

Put a voltmeter on the battery and Rev it up.

[john walker's workshop]

Originally Posted by WolfeMacleod

I can remove the battery and it will run just fine,


.

John, wanna test my Alt?

that's the best way to kill an alternator.

you already did the test, the lights pulsate, so it's overcharging. don't boil the battery dry.

[Traveller]

Originally Posted by Superman

Lord knows why those brilliant German engineers chose to build these cars without voltmeters.

Very few cars have them, and if they do, who knows if those brilliant engineers inserted them into the circuit where they should go...in a non load bearing circuit or preferably directly across the battery terminals.

I'm installing one that I made for one of my motorcycle that has gone to heaven.

It draws a mere 2 mA so I can connect it permanenetly across the battery without worrying about any significant drain.

It'll be placed at the end of the console above the buttons and attached to the carpet with Velcro.

Here are some pics...
LCD Voltmeter

[Traveller]

Originally Posted by WolfeMacleod

When my lights are on, they pulsate.

Are you sure you're not trying to clear traffic ahead of you.

I haven't read all posts, but have you taken some voltage readings across the battery? Or try setting the meter to AC and turning the range down till you get a reading.

I've read a few of the alternator explanations and started to .

A three phase alternator puts out three sine waves, each 120° apart, producing three positive half cycles and three negative half cycles.

The diode pack (rectifier) flips the three negative half cycles to the top. The end result are six positive half cycles which overlap. The ripple follows the resultant until a battery (a capacitor) smooths it out to near DC.

Have a look...
Understanding Alternators

[Traveller]

A very useful site for auto electronics such as alternators, starters (and their parts) as well as other components...

Transpo.de

They have offices throughout.