90 amp alternator on manual mk3 TDI

Currently i have a 120 amp but after driving with the battery light on the past few days(lucky i had an AMG battery drove over 6 or 7 hours on it) i started checking voltages and got it tested and it failed.

Reason for the question is i do not have the three coolant glow plugs plugged in or the fuse or relay block for it so im wondering since that's why the manual has the 120 i should be fine with the 90 amp. I did not have them last winter either and my heat seems to be coming on fine and the cars temp seemed to get up there at good rate.

On a side note will i need them this coming winter due to deleting the egr cooler it might cause it not to heat up as fast. hasnt had any of the vacuum lines hooked up not sure if that means it would not be benefiting the heat at all at that point.

It'll be fine but the alternator will work harder. I know lots of people with TDI's running the 90A.

Ditto what Abacus said, and it's good as a substitute for a while but be aware that it lacks the clutched pulley that the 120 amp model the Mk3 cars got from the factory, so if you plan to run it for an extended period of time I'd invest in a clutched pulley for it.

Steve

^^^^^
What they said.
However, its best to rebuild these alts vs replacing them. its not that costly to repair and if you know how to solder and use basic tools, its not a difficult job. Whats the voltage from the alt? is it high or low or not at all? high means failed regulator, low means the same, not at all could be as easy as a failed connection. In any case bigger is always better, especially if you have a stereo system or things like HID lights or other things.

Maybe slightly premature (brushes still had some life on them), but at 180K miles, I polished the slip rings and replaced the regulator/brush assembly ($142.30 shipped). I'll replaced the alternator clutched pulley ($39 ish) when I do the T/B and should be good for another 100K miles. Except for required maintenance of course.

The parts house re-mans are usually garbage and by the time you are all said and done will cost more in money and frustration than if you had done maintenance with quality parts the first time.

If you drove for a long time with the light on, most likely your brushes have worn out and have damaged the slip rings from arcing. AGM batteries do not work well in these cars. They are overpriced and usually don't have an adequate AH (Ampere Hour) rating.

AGM batteries dont work well for a few reasons, the most being that the DC curve is a strait curve like most cheep chargers. vs a proper charger for AGM being a real curve sin wave. Alternators only put out strait wave format and this ultimately kills them. The other big deal on them is that they need to be discharged down to about 60% and then recharged to 100% slowly. A normal lead acid battery can take the abuse of constant discharge and charge at the same time. I use AGM's for boats ect but they do handle car audio well because the cost vs LI-Ion

ALso im still running the stock 120A Alt on my jetta with 360K on the clock. I can tell its not as good as it was, it runs about 13.8 amps on average vs when i got the car at 200K and it would run about 14.2 on average.

Mongler98 said:

AGM batteries dont work well for a few reasons, the most being that the DC curve is a strait curve like most cheep chargers. vs a proper charger for AGM being a real curve sin wave. Alternators only put out strait wave format and this ultimately kills them. The other big deal on them is that they need to be discharged down to about 60% and then recharged to 100% slowly. A normal lead acid battery can take the abuse of constant discharge and charge at the same time. I use AGM's for boats ect but they do handle car audio well because the cost vs LI-Ion
ALso im still running the stock 120A Alt on my jetta with 360K on the clock. I can tell its not as good as it was, it runs about 13.8 amps on average vs when i got the car at 200K and it would run about 14.2 on average.

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It's a good time for you to pull the regulator out and inspect the brushes and slip rings. My brushes were down to about 3/8" and the springs were not putting much tension on them. I'm pretty sure they weren't arcing yet however the slip rings did have grooves in them that I was able to polish out. Charging voltage had not started to deteriorate yet.

VW is kinda vague on the AGM battery thing. Bentley's for my car says there are two types of batteries on these cars and always replace your battery with the same type. No explanation as to why. My assumption is that if you could find an AGM spec-ed regulator to fit your alternator, you could go with an AGM battery providing it had the correct AH rating.

To convert over would probably cost about the same as 3 flooded cell batteries. Certainly not worth the extra expense or hassle.