Flashing Lights With Heated Front Screen

[wolf]

Hi all first post!

Wondered if anyone has any ideas on this?
With the head lights on they flash at a regular frequency when the heated front screen is
switched on but do not do it any other time.

Not had a chance to investigate yet just hoping for some ideas?

Thanks

[Mirez]

When you say flash are they physically turning on/off or just dimming significantly? Assuming its the latter (as the first would be freaky lol) then the key is the amount of energy required to power the heated screen.

I would suggest your alternator's regulator, or specifically the rectifier is on the way out. Basically the alternator produces an AC current from the motor and its the rectifiers job to convert that to DC current that the car runs on. There are 4 diodes that make up a rectifier and if ones failing what you end up with is an output that would look like 14v, 14v, 3v, 14v in a loop as the alt cycles, its that drop that you are seeing manifesting as headlight flicker. Normally when the alternators not heavily loaded the battery voltage will compensate but when a large demand (such as that screen) is placed on the electrical system then you'll start to notice it.

Unfortunately testing will be tricky unless you have access to an oscilloscope as a normal volt meter won't refresh anywhere near quick enough for you to see the drop. A less scientific way is to put the car on a battery charger when running, the charger's output should compensate for the drop and the lights wont flicker but its by no means proven using this method as it depends on the output and sophistication of the charger.

[galaxynnik]

would the flashing rate change with the engine alternator speed - what sort of frequency ac does the alternator poduce ?

[Mirez]

Yes, as the engine revs are increased the frequency of flickering would also increase until the eye can no longer perceive a fault. In reality that would probably only take a little increase to achieve because of the high coupling ratio between the engine and alternator. Imagine a fan spinning slowly, you can clearly see each blade but as the fan speed increases the eye perceives the blades to merge - its the same principle only with volts

The frequency generated depends on a number of factors including the pulley size, engine rev to alternator rev ratio and the number of poles the alternator uses. I've not examined a Galaxy alternator in that much detail but I believe they are 6 pole so using some guessed figures for the ratios I would expect that to equate to around 40hz at an idle of 800rpm, and around 200Hz at 4000rpm.

[galaxynnik]

well even at 40hz I reckon you would see a flicker rather than a flash in the headlights even if 2/6 poles on the alternator werent working or the rectifier was on the way out i.e. half wave rectified

to the o/p what sort of frequency are the lights flashing t and is it engine speed dependent?

[Mirez]

Absolutely, also bear in mind that its a 3 phase rectification so the alternators output is generally quite dirty and not true DC. You'll normally see it as a rippling line with quite a degree of variance if you look at it on a scope:

[attachimg=1]

Thats a GOOD graph by the way, if you look at one that's failing you'll see large downward spikes where there is essentially no output

[galaxynnik]

Thats a very smooth output is there any inductance/capacitative smoothing involved?

[Mirez]

It wasn't one that I personally tested the graph just happened to be on the shared drive looking at Matts notes its from a Phaeton and I believe they do have a cap installed in the fuse box to address ripple but I wouldn't say it was THAT smooth.

Then again not sure where he measured this as I was only using it as an example

[galaxynnik]

well its pretty good for a real world engineering example given vibration belt slip etc 

[attachimg=1]

[Chrispb]

I think I would be right in saying a small amount of flickering doesn't have any affect on charging my old Mk2 has done that for years without problem, on saying that I never noticed it in the headights only with the interior light on

[Mirez]

See THATS a smooth graph, but yes it's not bad I suppose and having confirmed it was from a Phaeton and they do have a smoothing capacitor in the fusebox. Interestingly its a cap with two blades and is designed to fit in a maxi-fuse holder so could be an interesting retrofit.

Chris, no I would be surprised if the battery cant charge off the alternator - it will just take longer.

[wolf]

Thanks for the replies.

The lights flicker(dimming) at the same rate no matter what the engine speed.

Not sure on the frequency but it is noticeable. I have no problems with anything else electrical and charging is ok, just started last week.

I know that screen takes a significant current draw so was suspecting that the alternator/rectifier was probably the cause.

Will have to have keep an eye on the alternator then and see what happens?