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Discussion Starter · #1 ·
Just putting the final touches to my subbox design and final plans for the sound system. How large is the stock alternator on the B5 (assuming it's a fairly standard 120amp unit). I'm not doing a huge system this time around, but will the stock electrical system be fine for a max 40amp draw (that will be the absolute worse case scenario for the amp I picked up).
 

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V6 = 120A
1.8T = 90A
This isn't necessarily true from what I've read. When my alternator started failing this winter I looked into replacing it with a 120 amp unit (my car is an early AWM 1.8t with a 90 amp)

I found many conflicting "rules" about this. 90 vs 120 amp. Valeo vs Bosch. Some said only V6s had 120s. Others said some 1.8s did as well. Maybe late AWM only? In the end I just had my own unit rebuilt by a local shop because it was cheaper and easier than a swap.

That said a 120 amp swap is certainly possible and many have done it.
 

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I just finished a 150A swap today. Got a 150A from a 03 Passat with the V6. Bolted right up to the stock mounts. The B+ and plug move with the other alt, but I was able to plug in the harness without issues.

The only thing is the 150A comes with a 6 groove pulley. The 5 groove from the old 90A has about 12mm from the mount to the first belt valley (from the alt foward), where the 6 groove is about a 2mm or so. However, when you measure everything out, the 5 groove belt will ride in the front of the 6 groove pulley just fine.

I also got rid of the wire that runs from the alt to the starter, and replaced it with a wire from the alt directly to the battery. I was seeing a 1.2-1.5 volt drop in the original setup. Now, I am seeing no drop in voltage.

Now to see how long the belt lasts riding the way it is...
 

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Discussion Starter · #7 ·
Well, the system I'm planing isn't overly fancy... But the current requirements are high enough that I felt that I should look into it - make sure I've got all the details worked out.

So 1.8T - that will be a 90 amp alt. I'll assume it has 40% overhead (seems to be a general guideline) - meaning the alt will safely support approx 36 amp draw from additional accessories (like amplifiers). Approx 25 amps less then I ideally want to see. Now yea, that's a very big rough guess, makes several assumptions about the electrical loading, including assuming that the amp will actually draw it's fused 60 amp's (bet it's less). Reality is I could be fine with the stock alt. But a 120 amp alt would be a safer bet. I'll have to add it to the list of stereo to do's.
 
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