Alternator pulley size 1998 S70T - how precise does it need to be?

timetobakethedonuts · Post by **timetobakethedonuts** » 31 Dec 2023, 15:30

I am replacing the alternator in my 1998 S70T. I bought one from NAPA, when I got my old one out (which was an interesting task), I found that the pulley is 56 mm at the outer edges of the splines. The new one's similar diameter is 57.5 mm - 1.5 mm larger. This is a difference in circumference of 4.8%.

I have seen comments about swapping pulleys when new alternator pulley sizes do not match, But how close does the diameter need to be to OEM? The alternator's performance curve shows about a 5% decrease in current output to 85 rpm from 89 rpm when the rpm decreases from 2000 rpm to 1900 alternator pulley rpm (5%) - which to me doesn't seem like a deal breaker. A rough estimate is 900 rpm engine idle will result in 2000 alternator rpm.

Any comments or advice? Replacing the pulley requires a special tool, and currently I have no vehicle as my daily was totaled.

Thank you in advance,
Don

1998 S70t 71,000 mi
2005 V70 2.5T 140,000 mi and killed on I-81 in Binghamton by exploding truck tire

454cid · Post by **454cid** » 31 Dec 2023, 17:32

I'm not sure that circumference would matter. Pulley diameter is what ratios are based on. 1.5mm doesn't sound like a big deal to me, but it would spin slightly slower. I've only ever swapped one pulley, and that was an OEM part.

Is this one of the over run pulleys that wear out after awhile?

timetobakethedonuts · Post by **timetobakethedonuts** » 31 Dec 2023, 18:08

Hi -

Not overrun pulleys, these are both solid. The NAPA specs for the new one say 56mm which a lot of other replacements stated, but maybe this shipped with an SAE pulley of 2-1/4" which is 57.2 mm. I think I will be OK. The bodies of the units are identical, and both say "Bosch) on the back covers. Seems to me the main considerations are if the grooves match (they do), will the current belt fit (I believe it will), and will it produce enough power at idle (I think it will).

I believe you are correct, that I should have used the radii (which relate proportionally to the ratio), not the circumference. So the ratio changes by 2.7%, less than I thought.

Thanks, Don

Post by **BEJinFbk** » 31 Dec 2023, 19:26

There are way too many poor
rebuilds out there in the world...

If the old alternator is the original,
and the bearings are good, you might
consider pulling the regulator/brush
module out, and inspecting the rings.
If they look good, just drop a new
regulator in it and keep original.
An OEM Bosch regulator may be
on the expensive side, but you
know what you’re getting.

Post by **abscate** » 01 Jan 2024, 04:58

That pulley will be fine. Those spline bolts are really tight and might be left hand thread? They are lefties in 1999, but I think 1998 were righties

timetobakethedonuts · Post by **timetobakethedonuts** » 01 Jan 2024, 14:34

BEJinFbk wrote: ↑31 Dec 2023, 19:26 If the old alternator is the original, and the bearings are good, you might consider pulling the regulator/brush
module out, and inspecting the rings. If they look good, just drop a new regulator in it and keep original.
An OEM Bosch regulator may be on the expensive side, but you know what you’re getting.

This sounds like excellent advice. Sadly, I am under a ton of stress with having to get this backup vehicle road worthy ASAP. Getting the old alternator out was a huge project due to the manner I attempted it. Then I fell on my stairs because I was distractedly thinking about this project to add pain to misery. So I am tossing $$$ at the part hoping I get quality. It is a BOSCH unit, as far as that goes.

timetobakethedonuts · Post by **timetobakethedonuts** » 01 Jan 2024, 14:46

abscate wrote: ↑01 Jan 2024, 04:58 That pulley will be fine. Those spline bolts are really tight and might be left hand thread? They are lefties in 1999, but I think 1998 were righties

Thank you for the heads up that thread direction changed around this time - good to know. The car is charging at 13.77 V, which is at the bottom end of the acceptable range. I had noticed before replacement that the new pulley is slightly inward of the old one. Due to pressure to get the car going (which I should not have buckled to), I reassembled everything. I estimate the offset is about 1/16". Naturally I regretted the decision to press forward, when I started worrying about belt wear and noise. It does run smoothly, though I plan to try to assess possibility of in-car pulley removal tomorrow. Also pull the pulley off the old one, and see if the dimensions differ and/or if either has shims/washers. The nut is a 24mm or 15/16" (box wrench), and the shaft is a 12 spline M10. NAPA has a 3/8 drive M10 socket which should clear with a 3/8 bar. I'll see if I can break them loose.