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TDI Conversions Discussions on converting non TDIs into TDIS. More general items can be answered better in other sections. This is ideal for issues that don't have an overlap and are very special to swaping engines.

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Old June 26th, 2012, 11:48   #1
TDIJ
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Default Alternator wiring?

Essential question at hand... does the alternator in an AHU need to be wired to the ecu or anything else?

I swapped an AHU into my jeep, and have put about 5k on it, at which point the alternator quit. I am about 5 hours drive(and a couple weeks walk without an alternator) from my Bentley manual and can't look it up. Nor am I familiar with how an alternator runs, and regulates.

There are two wires coming off the alternator (aside from the main wire to the fuse box). Do they need to go to anything in particular. I don't have them going anywhere right now, and my assumption is that this oversight when initially working out my wiring has caused my failure, however it may simply be coincidence.

Anyone have that answer for me? I am about to set out to find a replacement alternator, but don't want to ruin another one in quick fashion if i am failing to have it regulating properly.
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Old June 26th, 2012, 14:54   #2
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I think the failure is unrelated ..... Alt will charge fine with just large wire connected ....
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Old June 26th, 2012, 15:02   #3
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One of those small wires (don't remember exactly which pin) is an input: the excitation voltage thru the ALT light on the dash.

No excitation voltage... alternator function will be iffy.
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Old June 26th, 2012, 15:43   #4
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Default Alternator excitation...

So... I am at the point where I am wiring my alternator. I originally was going to use JimBote's advice and run the one large wire to +. Can you expand a little more on the need for the excitation from the alt lamp. I am running M-tdi so no ecu. So this lack of an excitation voltage will cause it to act "iffy". Can you expand on that???

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Old June 26th, 2012, 18:49   #5
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Quote:
Originally Posted by jimbote View Post
I think the failure is unrelated ..... Alt will charge fine with just large wire connected ....
That was my initial thought when I originally hooked everything up, but then when it failed I had the reason to question it. I know you have been very active in the swaps, and not just your own, so I value your confirmation enough to at least go with it until I learn otherwise for sure.

Quote:
Originally Posted by Vince Waldon View Post
One of those small wires (don't remember exactly which pin) is an input: the excitation voltage thru the ALT light on the dash.

No excitation voltage... alternator function will be iffy.
Yes, care to expand on this theory? I'm not comfortable with what the "excitation voltage" is.


Had a friend get back to me on a wiring diagram... looks like one of the wires goes to the ecu, in the same pin as the "220" ground circuit does. The other one goes to the dash.

I'm not running a light... just a gauge off a random power wire in the cab.
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Old June 26th, 2012, 19:16   #6
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No problemo.

Unlike the earlier generators used in vehicle charging systems which had permanent magnets, alternators have no permanent magnets and need to be initially supplied with 12V to reliably start generating electricity on their own.

From the wikipedia article on "alternators":

"The field windings are initially supplied power from the battery via the ignition switch and "charge" warning indicator (which is why the indicator is on when the ignition is on but the engine is not running). Once the engine is running and the alternator is generating power, a diode feeds the field current from the alternator main output equalizing the voltage across the warning indicator which goes off. The wire supplying the field current is often referred to as the "exciter" wire. The drawback of this arrangement is that if the warning lamp burns out or the "exciter" wire is disconnected, no current reaches the field windings and the alternator will not generate power. Some warning indicator circuits are equipped with a resistor in parallel with the lamp that permit excitation current to flow if the warning lamp burns out."

I say "reliably" because there's always a bit of residual magnetism present after an alternator as been used for a while and so it is possible for an alternator to "self-excite" if conditions are right.... for instance... if revved very high under a light load.

So: standard wiring for an automotive alternator (with an internal regulator, like ours) is 2 wires: the heavy-duty ("B+" terminal) output and the excitation input ("D+" or "L" terminal) from the ALT light in the dashboard.

Our alternators have a third terminal labeled "DFM" which evidently stands for "Digital Field Monitoring"... the Bentley says specialized equipment is needed to utilize this terminal for testing purposes...my guess is that it heads to the ECU for monitoring purposes.

Clear as mud?!
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Last edited by Vince Waldon; June 26th, 2012 at 19:27.
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Old June 26th, 2012, 20:04   #7
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Originally Posted by Vince Waldon View Post
Clear as mud?!
Pretty much... can you water that mud down for me a little?

Sooo.... this would go to say that the alternator could potentially not supply power at all if not receiving power from an "exciter" wire?

Correct me if I'm wrong but to mimic this in an swap situation like mine, you would simply ignition power a light bulb, running the other end to the alternator wire. this would then operate the bulb when ignition is on, and grounding out through the alternator when not running. The bulb would then go off when started as it would be getting power from both ends?

Once the engine is running and the alternator is powering, the exciter wire wouldn't be doing anything? However it could be needed for it to start powering upon initial engine startup?

Am I seeing through the mud at all?
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Old June 26th, 2012, 21:22   #8
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Yup you've got it. Stated slightly differently, in case SEARCH sends someone here some day:

Track down a small 12V lightbulb (dome light, running light, something like that). One terminal goes to a source of switched power. The other terminal goes to the exciter terminal on the alternator (labeled D+ or L, depending on your alternator).

With the key in the ON position power is supplied to the exciter terminal of the alternator thru the bulb, which will light up.

Once the engine starts and the alternator starts generating electricity on its own the alternator's output will equal the ignition switch voltage and the bulb will go out.

For bonus points... mount the light bulb on your dash. Now you have a real ALT warning light that lets you know if the serp belt dies... probably a good thing to have.
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Last edited by Vince Waldon; June 26th, 2012 at 21:26.
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Old June 26th, 2012, 23:22   #9
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Cool... in conclusion... I am comfortable stating that my alternator just went out... not as a result of not being wired properly. I'll probably hook up an idiot light because I can though... when I get to that point. i still haven't hooked up my CEL or Glow Plug light yet.
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Old June 27th, 2012, 04:36   #10
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Quote:
Originally Posted by TDIJ View Post
Cool... in conclusion... I am comfortable stating that my alternator just went out... not as a result of not being wired properly. I'll probably hook up an idiot light because I can though... when I get to that point. i still haven't hooked up my CEL or Glow Plug light yet.
you are correct leaving the exciter wire off most likely did not cause the failure..... and your jeep already has an alt light that will suffice....I used the stock toyota lamp and all is well....
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Old June 27th, 2012, 09:30   #11
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Yup I'd agree.
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Old October 1st, 2015, 00:53   #12
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I got a new alternator for my 98 Beetle TDi and got the new style "D" connector for it. However, the new alternator doesn't have any markings on it as to which terminal tab is for the ECU "exciter" or which is for the the light? The new "D" style connector has one tab towards the flat side of the "D" and the other towards the round side of the "D". Does anyone have any idea which tab is for which, or IF it matters?
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Old October 1st, 2015, 17:04   #13
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If the bulb is too big or the field terminal is hard wired to the ignition switch, it will back feed from the alternator and prevent shut off in the key off position.
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Old October 5th, 2015, 14:53   #14
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Quote:
Originally Posted by mtushmoo View Post
If the bulb is too big or the field terminal is hard wired to the ignition switch, it will back feed from the alternator and prevent shut off in the key off position.
So of all the mechanics and re-builders I know. They all recommended to put the right wire on the right and the left on the left. So..... I did that. The alternator works. However, the battery light does not come on when starting, or if I turn off the car and let it coast down.
So, now my question is do I have the two wires backward??
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Old October 5th, 2015, 16:23   #15
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gotta have a idiot light to charge... and you did not hook it

a real test light with a real light bulb, not led, can be used to see which of the 2 terminals it is... the other wire is for the computer to know when there is a load on the alt.. not a w terminl unless 90a...

Last edited by CRSMP5; October 5th, 2015 at 16:26.
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