# mk4 charging system help



## j0hnnybmx (Oct 18, 2004)

brand new alternator, brand new battery, alternator wont charge the battery while running... i'm assuming it's the exciter system that's not functioning correctly, so can anyone help me understand the two wire harness that comes off of the alternator a little better?, i have a bentley but the wiring diagram isn't a lot of help


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## BassNotes (Mar 16, 2005)

*Re: mk4 charging system help (j0hnnybmx)*

What voltage do you measure across the battery terminals when the engine's running?


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## j0hnnybmx (Oct 18, 2004)

with a full charge it starts at ~12.3, works its way down as it runs because of no help from the alternator.


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## BassNotes (Mar 16, 2005)

*Re: (j0hnnybmx)*

Open the fuse block on top of the battery. Check the metal fuse furthest to the left, as well as the thick black wire from it to the alternator. Also check the thick red wire from the fuse block to the positive battery terminal.
When the engine's running, the wire from the alternator should be around +13.5 to 14 volts with respect to the chassis. If you find that voltage at one point and then ~12.something volts at a point closer to the battery, then you'll know there's a break somewhere in between.


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## BassNotes (Mar 16, 2005)

*Re: (BassNotes)*

There are a pair of wires from the alternator that go to a four-pin connector near the starter. The Bentley manual wiring diagram for the AWP engine (I have no idea if that's what you have) says that the T4 connector is only present on cars with A/C, so I would expect that the alternator should work even without it connected (I'd have to check the A/C circuit diagrams to verify; perhaps you should pull the fuse for the A/C before you disconnect T4 for alternator testing).
The more important alternator connections are B+ (thick black wire) and ground. Ground appears to be through the alternator body and its mechanical mounting to the engine. It's hard but not impossible to make a bad connection through this.
The other side of the ground connection is how it connects to the negative terminal of the battery. The negative battery cable has two ground connections: one in the middle, to the unibody, located under the battery tray, and the other at the end, connected to the engine block (at the upper side of the bellhousing). If this end connection is loose, then you almost certainly will not get good charging. (I found this ground slightly loose on my car some months ago, and it was a little corroded as well, thanks to a small pinhole leak in the coolant hose to the oil cooler.)
Checking the B+ line is covered in my previous post.


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## j0hnnybmx (Oct 18, 2004)

thanks man. still trying to figure this out.. haven't had much time lately..
i'll let you know what i find out.


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## BrianAnderson (May 13, 2008)




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## fast4dr (Nov 25, 2010)

im having the same problem. and now to find out if the battery light is burnt out it will not work. it completes the circit for the exciter wire to give it its 12v+. ive checked grounds fuses battery alt and still my charging system dosent work. i dont kno why my battery light on my dash does not work.


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## sardo_67 (Dec 26, 2009)

not sure if this requires a new thread or not but i just put a JL Audio W7 in my car and it dims the lights, is there a larger alternator i can put on the car or any other wires that i can upgrade? my battery is in the trunk with 0 gauge wire running up front and i have a good ground on it. the sub isn't completely installed yet but i'm wondering if there are any larger alternators that i can get for my car. it's an 03 MK4 TDI 5speed with the monsoon system


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## Wolfschnee (Jun 14, 2009)

consider a capacitor. You can probably get on that $50 and will do the trick.


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## sardo_67 (Dec 26, 2009)

I plan to get the cap but that is only designed for big hits, my lights dim with most music when turned over half way up. Even with the cap I still would need a better charging system to keep power in it.


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## BassNotes (Mar 16, 2005)

A cap is a band-aid fix. Snake oil.


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## sardo_67 (Dec 26, 2009)

Yes I realize that the cap isn't a good fix or even really needed but what is the correct way to better my system in the car?


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## BassNotes (Mar 16, 2005)

sardo_67 said:


> Yes I realize that the cap isn't a good fix or even really needed but what is the correct way to better my system in the car?


 Did you read the upper part of this thread?


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## sardo_67 (Dec 26, 2009)

Ya I read threw from te first post and it's about trouble shooting the charging system. I'm not sure if I need a new thread but I wanted to know if I can upgrad my charging system with a bigger or better alternator, maybe certain wires too. My system works fine but it just draws a lot on the battery and dims lights.


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## Non_Affiliated (Feb 12, 2002)

If you put a large amp in the car and now the lights dim, I suggest upgrading your Grounds, and power wire to lage Gauge wire. If it's a really large system them your looking at needing a large alternator, i.e. higher current output. 

How many amps is the stock alt?


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## BassNotes (Mar 16, 2005)

sardo_67 said:


> Ya I read threw from te first post and it's about trouble shooting the charging system. I'm not sure if I need a new thread but I wanted to know if I can upgrad my charging system with a bigger or better alternator, maybe certain wires too. My system works fine but it just draws a lot on the battery and dims lights.


 That's right. It's about troubleshooting the charging system. You don't get it? 

You want to "upgrade" your alternator or wires but you don't want to get into the causes of your lights dimming and battery draw? :screwy:


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## sardo_67 (Dec 26, 2009)

Oh ok I see what you mean now. I understand what I need to check and go over, I already identified the areas that need improvement. What i don't know is what can be done after that, just want to know if there is a step after grounds and wires. I ordered the correct amp for my sub and have been slowly working on the car but I'm living in a hotel and not at home so my resources are limited to a parking lot which has been cold. I'm trying to wait till I can get home in a heated garage before I get reall into it.


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## BassNotes (Mar 16, 2005)

You need to check your connections. There's a very good chance that your wires are fine and your alternator is fine. Common trouble areas are where the alternator lead attaches to the fuse box on top of the battery (does the connection get hot while the engine is running?) and the ground connections on the bellhousing and chassis (under the battery). Battery post clamps often aren't as well installed on the posts as they should be. Use a voltmeter to check all of them.


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