Exciter wire? Or diode in battery light?

My battery light came on. I noticed many threads saying its a diode in my cluster. My alternator does work but only when you make a connection from the battery to the blue exciter wire on the alternator ..
I need a quick temporary fix for this. I was told I can hook a wire to my battery and another to the alternator with a switch and this will be a temporary fix..
Any ideas? I just need it to work until I have time to fix it right..

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Well, I know this does work.. Does the exciter need constant power? Also, is 12v from the battery too much?

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Hi probably best to have a resistor in series. The exciter should only need maybe 20mA max. R=V/I=12/20mA = 600 ohms.

I am having a similar trouble and hopefully I can find a resistor.

After a bit more research, it looks like the exciter may need more current than 20mA, maybe as much as 0.25 Amps. Therefore, the resistor would have to be about 12/0.25 = 48 ohms. Power = I2R = 0.25 squared * 48 = 3 Watts.

Sorry about my first answer. And here again, I may not know what I'm talking about!

You can run B+ directly to the excitation field, but you get what's known as 'full field' which is great for testing but can burn out an alt pretty quickly.

Hi,

WORDS OF WARNING...DON'T BLOW YOUR ALT!

After suggesting a resistor in one of my posts above, I tried various values. With about 100 ohms, the current was pretty high, maybe 150mA as I remember. I then tried 1Kohm, and it appeared to cause the alternator to charge properly with only 11mA flowing. I then rigged up a wire to leave in place as a Band-Aid solution. However, I found the alternator started "cycling" from charging to not charging with the output voltage bouncing around. I then tried about 500 ohms and 200 ohms with the same result. Frustrated, I eventually hooked an un-fused lead directly from the batter. There was a spark on the battery terminal and then the alternator puts out 18V steady, indicating a blown voltage regulator. Brand new alternator, so that's too bad. I ordered a new regulator module.

So the question is, does anyone know of a safe way to Band-Aid the alternator in this case? I have seen various recommendations (http://uk-mkivs.net/topic/20843-battery-warning-light-wont-come-on-alternator/) to drive the alternator pin through a lamp or from the battery, but it seems scary after my experience.

(I think that my issue is a problem with the blue exciter wire. It's strange because I can measure 12V at the wire, but my dash light never comes on. If I ground the wire through a meter, only 6mA flows. I am thinking that there is some problem with this wire not having the current to light the warning lamp and do the excitation. My plan is to trace the wire and figure out why there is no warning lamp. Does that make sense?)

Cheers

Why are you guys being cheap and not replacing the proper components on an electrical device that can produce 120 Amps? It takes only 5 ma to stop your heart. If you don't know what is the proper "fix" take it to someone who does and get it done right. You rish a fiery death of at least your TDI.

5mA directly across the heart, which takes quite a bit more than 12v thru skin, luckily.

OP...a small 12v lightbulb (turnsignal, for example) between +12 of the battery and the alternator excitation lead is a safe way to test things out...all you are really doing is mimicking the factory wiring of every car made before LEDs

Why are you guys being cheap and not replacing the proper components on an electrical device that can produce 120 Amps? It takes only 5 ma to stop your heart. If you don't know what is the proper "fix" take it to someone who does and get it done right. You rish a fiery death of at least your TDI.

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You are right that I messed up by connecting 12V directly to the alt pin. To answer your question about "why" it's a long story but has to do with a weekend alternator replacement that ended with a broken car Monday morning, including a power steering leak caused by the "repair" and a blown up voltage regulator, a mad wife, and desperation to get my car on the road working late at night using a band-aid solution. You may know the feeling. Ultimately I needed the car on the road if possible, I didn`t have the time to trace the exciter wire problem (which I have to do now) and a band-aid solution was described on the UK web forum. My other problems are a life-long do-it-yourself streak that allows me to fix anything eventually, often after breaking some part of the car in the learning curve. Overall I save big money for my incompetent efforts, and I get to buy tools instead of a car payment. The random confusion of web forums and half-baked information is also to blame, as I found a Ferrari site discussing resistor values to use on the exciter wire, and I was skeptical of using a filament bulb to replace a car circuit involving an LED. Finally I am a professional electronics engineer and an expert at electrical wiring repair. PS 12V won`t hurt you unless you get literally burned or blinded by the very high current capability. Using care is a good idea for sure. On second thought, maybe I should give up my own car repair Cheers.

Thanks a lot Vince.

Underlook said:

I was skeptical of us a filament bulb to replace a car circuit involving an LED.

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Ah... but as an electonics engineer: using a non-linear resistor (aka the filament in the bulb) to safely limit current over a range of input voltages should make perfect sense!

That's the beauty of rockin' it ole skool with a light bulb, and why it's been the case for decades...up to when they started using LEDs in clusters... LEDs that usually have to be augmented with both series and parallel resisters to mimic the beauty and simplicity inherit in the lowly tungsten light bulb.

True, but LEDs can produce much cleaner monochromatic light with greater intensity and less current. There are advantages to either though

Thanks for the info Vince. Always more to learn! Cheers