Alternator wiring?

TDIJ

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Oct 27, 2011
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Durango, CO
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AHU Wrangler
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.
 

Vince Waldon

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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. :)
 

JayTee013

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Jan 22, 2011
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2000 jetta TDI, 1992 toyota Pickup TDI project
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???

-Jesse
 

TDIJ

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Oct 27, 2011
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Durango, CO
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AHU Wrangler
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.

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.
 

Vince Waldon

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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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TDIJ

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AHU Wrangler
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?
 

Vince Waldon

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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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TDIJ

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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.
 

jimbote

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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.
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....
 

tvandermeuse

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Sep 28, 2015
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98 Beetle
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?
 

mtushmoo

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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.
 

tvandermeuse

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98 Beetle
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??
 

CRSMP5

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May 18, 2009
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NE OHIO
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idi
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...
 
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Joined
Jan 23, 2018
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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....

My Toyota has a 3 wire harness and my TDI (alh) alternator has 2 wires. How’d you wire yours up?

Thanks!
 
Joined
Mar 12, 2019
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02jetta
5 speed swap wiring

Hello
I’ve finished my swap an wiring. I have two problems . #1 my reverse lights stay on , is the switch on the transmission normally open ? # 2 my alternator just took a crap. When I key on I get power to both side of the two prong plug on the alternator , should I only have power to the “curved side of the plug prong “ or both sides ? If I have key on an remove the large plug to the computer then I just have power to the curves side of the plug to the alternator. Any help would be appreciated.
 

[486]

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Mar 1, 2014
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MN
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02 golf ALH
Hello
I’ve finished my swap an wiring. I have two problems . #1 my reverse lights stay on , is the switch on the transmission normally open ? # 2 my alternator just took a crap. When I key on I get power to both side of the two prong plug on the alternator , should I only have power to the “curved side of the plug prong “ or both sides ? If I have key on an remove the large plug to the computer then I just have power to the curves side of the plug to the alternator. Any help would be appreciated.
on an 02 motor the two small wires to the alt are field and lamp

the light on the dash (also used to excite the alt) and a wire from the field coil to tell the ecu what percent load the alternator is loaded down (to switch on or off the coolant glow plugs, nothing more)

field ends up with some power when keyed on, but nowhere near full 12v as the lamp wire will not supply that much current to the regulator, talking maybe a couple tenths of a volt
lamp should be essentially grounded, key on not running, and the light in the dash illuminated
 

CasaEd

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Can you see the color of the wires in the plug ? 1 is normally blue, that is the warning light wire, the other wire is usually brown/red, that wire go's to the ECU.
 

jimbote

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Jul 10, 2006
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spiral arm, milky way (aka central NC)
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iirc, there's a yellow wire from the toyota's cluster (charge lamp) that goes to the blue wire of the vw alternator, that's all i have to charge aside from the large wire... my alternator light works as required
 
Joined
Jan 23, 2018
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Toyota TDI
on an 02 motor the two small wires to the alt are field and lamp

the light on the dash (also used to excite the alt) and a wire from the field coil to tell the ecu what percent load the alternator is loaded down (to switch on or off the coolant glow plugs, nothing more)

field ends up with some power when keyed on, but nowhere near full 12v as the lamp wire will not supply that much current to the regulator, talking maybe a couple tenths of a volt
lamp should be essentially grounded, key on not running, and the light in the dash illuminated
So, in a toyota swap, the field wire will be left alone?

iirc, there's a yellow wire from the toyota's cluster (charge lamp) that goes to the blue wire of the vw alternator, that's all i have to charge aside from the large wire... my alternator light works as required
I assume you just "dead-head" or unpin the field wire? What are the other two wires in the toyota harness used for?
 
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