Ok, so I spent the greater part of today troubleshooting my car's electrical system to see why I have constantly low charge voltage... So here are my observations.

This car has an AL129x 115amp alternator upgrade, BTW.

With the Headlights, Driving Lights, Rear Defogger, and heater on HI setting the car will have a dropping voltage that finally settles around 11.9volts at idle.. Engine has to be revved up to about 1200 rpms, at which the voltage will settle at 14.5v and show about 80amps of consumption by the car. (I had an ammeter that monitored current traveling to the fuse box.) Headlights draw 10amps, as do the driving lights.... Just about right I figure for 60w bulbs... The rear defogger draws 19amps, and the CCU set on ECO with blower latched on hi draws 24amps.

Anyhow, It appears that at 800rpm idle the alternator can only put out about 35amps or so...

So, I decided to look into this... I measured the diameter of the alternator pulley and it is 3.25 inches in diameter. The drive pulley on the engine is about 7 inches. This equates to about a 2:1 ratio, meaning at 800 rpm engine speed, the alt will be turning about 1600 rpm.. Now, the alt bearings can withstand 15,000 rpm easily I'm told, so at a 2:1 ratio, they will never max out as 7,500 rpm is alot higher than our engine's 5,500 rpm limit... So, here is my hypothesis: The 55amp alt that was originally on the car appears to not be original (no MB stickers or symbols) but actually a replacement. I'm wondering if the pulley I have is two large? I would think that downsizing to a 2 inch pulley would give me 50% more power at idle, as now the alt shaft will be turning 2400rpm at 800rpm engine speed, which, right now 2400rpm is attained at about 1100 rpm engine speed, which is exactly where the alternator can keep up with my loads........ Plus, the 15,000 rpm alt shaft speed won't be achieved until 5,000rpm engine speed, which I don't see very often.

So, what do folks think of this? Does anyone know what the alternator pulley diameter is on their car? Anyone feel giving and want to measure their's to compare to mine?

__________________
-Matt

EPA Section 609 Certified MVAC Technician
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Oil Burner Kartel Member #10

Ahh the smell of Diesel Fuel, it's like coffee in the morning!

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1982 300SD Turbo Diesel (231,500 miles!) RIP

1984 300SD Turbo Diesel Custom (235,500 mi on driveline.) - On Road!!

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