MK4 TDI alternator upgrade question.

Tdi<3er

Member
Joined
Oct 8, 2024
Location
Manitoba, Canada
TDI
MK4 BEW wagon, MK6 CJAA Golf manual, MK6 CJAA Golf Sportwagen, E70 X5 35d
From what I’ve gathered looking around, the mk4 TDI’s came with 2 types of alternators, 120amp for the manuals and 90amp for automatics. I have an automatic 05 BEW and I need to know if the 120amp alternator is a direct fit instead of the 90? I’m slowly going over it and replacing things here and there as it’s a 20 year old car now, and I don’t really like how the lights dim when I open/close the windows… I also want to install a light bar as the light output on the mk4’s is pathetic so the extra juice wouldn’t hurt.
Thank you.
 

Mozambiquer

Vendor , w/Business number
Joined
Mar 21, 2015
Location
Versailles Missouri
TDI
2004 VW Touareg V10 TDI, 2012 Audi Q7 V6 TDI, 1998 VW Jetta TDI. 1982 VW Rabbit pickup, 2001 VW Jetta TDI, 2005 VW Passat wagon TDI X3, 2001 VW golf TDI, 1980 VW rabbit pickup,
From what I’ve gathered looking around, the mk4 TDI’s came with 2 types of alternators, 120amp for the manuals and 90amp for automatics. I have an automatic 05 BEW and I need to know if the 120amp alternator is a direct fit instead of the 90? I’m slowly going over it and replacing things here and there as it’s a 20 year old car now, and I don’t really like how the lights dim when I open/close the windows… I also want to install a light bar as the light output on the mk4’s is pathetic so the extra juice wouldn’t hurt.
Thank you.
Yes, it's a bolt on, though I would make sure the battery fuse block has the 150 amp fuse, not the 100 amp that the cars with the 90 amp alternator had. And if it's not been mentioned, I've got alternators for those, new OEM or reman, and also build 180 and 220 amp bolt on alternators for the BEW.
 

JB05

Top Post Dawg
Joined
Oct 20, 2005
Location
Il.USA
TDI
Golf,2005,anthracite blue
Yes. I swapped my OEM 90 amp to a Bosch 120 amp reman with an X suffix in the part number back in Jan. 2011. I've since added coolant GP's
manually switch on or off. 2005 BEW Golf with o9A auto transmission.
 

wonneber

Top Post Dawg
Joined
Oct 12, 2011
Location
Monroe, NY, USA
TDI
2014 Jetta Sportwagon,2003 Jetta 261K Sold but not forgotten
From what I’ve gathered looking around, the mk4 TDI’s came with 2 types of alternators, 120amp for the manuals and 90amp for automatics. I have an automatic 05 BEW and I need to know if the 120amp alternator is a direct fit instead of the 90? I’m slowly going over it and replacing things here and there as it’s a 20 year old car now, and I don’t really like how the lights dim when I open/close the windows… I also want to install a light bar as the light output on the mk4’s is pathetic so the extra juice wouldn’t hurt.
Thank you.
If you have the 90 amp alternator you should replace the positive wire from the alternator to the battery positive post.
I wouldn't trust the lighter gauge wire.
Is there a 90 amp fuse in the fuse block?
Been to long since I had a MK4 JSW. :unsure:
 

BobnOH

not-a-mechanic
Joined
May 29, 2004
Location
central Ohio
TDI
New Beetle 2003 manual
Suspect the actual issue is a weak or undersized battery or flakey big wire connections.
 

oilhammer

Certified Volkswagen Nut & Vendor
Joined
Dec 11, 2001
Location
outside St Louis, MO
TDI
There are just too many to list....
I never put the 90A units back on, because the 120A units cost less. Probably more of those made I guess.

But yeah, the main fuse is bigger, although if you don't have the triple heaters then you likely won't ever call for the full output anyway.
 

Vince Waldon

Top Post Dawg
Joined
Apr 25, 2009
Location
Edmonton AB Canada
TDI
2001 ALH Jetta, 2003 ALH Wagon, 2005 BEW Wagon
My one additional thought would be: if your lights are dimming appreciably when you raise/lower the window that particular symptom is probably not an alternator capacity issue... since the normal draw is way less than 90 amps... and likely won't go away just by swapping out alternators.

More likely is a voltage drop issue... generally a corroded big ground or big positive... that's shedding volts when asked to deliver high current to the window motors.

You may spot it when you're swapping the alternator... or may need to dig a bit more as you do the upgrade. Or, your old alternator may in fact have been on it's last legs, and the upgrade fixes it!
 

xrdan

Member
Joined
Oct 22, 2024
Location
Ga
TDI
2003 Jetta
With the engine running check the alternator terminal voltage against the battery positive terminal voltage. I had over a one volt drop. I replaced my cables on another thread- now they read the same. It turns over and starts way faster now. Cheap and easy fix.
 

Tdi<3er

Member
Joined
Oct 8, 2024
Location
Manitoba, Canada
TDI
MK4 BEW wagon, MK6 CJAA Golf manual, MK6 CJAA Golf Sportwagen, E70 X5 35d
I never put the 90A units back on, because the 120A units cost less. Probably more of those made I guess.

But yeah, the main fuse is bigger, although if you don't have the triple heaters then you likely won't ever call for the full output anyway.
Sorry could you elaborate on the "triple heaters?"
 

aztecducky

Veteran Member
Joined
Jan 31, 2011
Location
San Antonio, Texas
TDI
2003 Jetta Wagon
I always like to mention on these types of posts that the cable from the battery negative to chassis ground is a common issue on these cars. It mounts under the battery tray and tends to get loose and/or corroded over time. Always check that ground if you're having any kind of electrical/low power issues.

I have a 370A alternator on my Jetta, but going that big has consequences. The main one being that it doesn't charge at all under about 950 RPM, which isn't a big deal if your battery isn't dead lol
 
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