Hi all,

This is not a "question" post, this is just a summary of what I did to my car, and thought it might help others as well. Due to large bulbs in my Euro lights and a hefty stereo planned, I needed more output. I picked up a 143A alternator from a 1998 S420 and experimented with it. I swapped my original pulley onto the 143A unit because the S420 pulley didn't stick out far enough. Otherwise it bolted right up - same mounting brackets, bolt spacing, everything. The new unit is larger and comes pretty close to the lower radiator hose but doesn't touch. So how does it perform? Read on:

I got the car fired up and tested the alternator output. Remember, my stock unit was 70A and had what appears to be two 10-gauge wires to the junction block near the battery. The new one is a 143A with a zero-gauge cable to a new distribution block and to the battery. I tested full load by turning on every electrical item I could, measured voltage with a Fluke 12 VOM, and current with a Fluke 36 DC clamp meter. All tests were done with a "hot" alternator, after driving the car at least 15 minutes with a decent load.

The stock unit could only cough up about 45 amps at idle, but did put out around 75amps maximum (revved up to ~2000 rpm). Voltage was poor... Never got higher than 13.6-13.7 volts even under light load or no load. Under moderate load it drifted down to lower 13's, and at peak load it went below 12 volts (!) as the alternator simply could not put out the power.

The 143A unit could put out roughly 100 amps at my ~600rpm idle speed, and appeared able to put out max current at barely above idle, not even 1000rpm! The highest load I could generate, including some current to charge the battery, was 125-130 amps. At 1000rpm, it put out the 130 amps and did so at 14.1 volts! And at idle, same thing, 14.1 volts (with a smaller load, under 100 amps.) The only time voltage dropped was with VERY high load at idle and I don't think I ever saw it go under 13 volts, and that was with an "artificial" load that is unlikely to occur in normal use.

My "normal" load is low beams (90Wx2) and ACC fan, plus whatever the chassis electrical stuff pulls (EDS, gauges, etc.) This appears to be roughly 40 amps, plus some current to charge the battery. The good news is, with my "normal" load I get a solid 14.1-14.2 volts, even at idle! VERY nice. I'd like the voltage a bit higher (14.4-14.6) but there is not an adjustable regulator available for the 143A unit (AFAIK). No big deal, I'm happy with the setup as-is. The stock unit badly needs an adjustable regulator, and I'd highly recommend one for those of you not interested in the full big-alt swap. Set it when stone cold to 14.6v or so, it will drop a bit as it gets hot, and you want to set the HIGHEST point.

The only down sides are extra weight and horsepower loss. The 70A unit weighs about 11.5 pounds, the 143A unit is 16.8 pounds. Not a big deal. But when you spin the shaft of the 70A unit, it feels light and spins easily. The 143A is MUCH heavier (massive) and definitely takes some extra crankshaft power to spin, even with no electrical load. I'd guess it could easily sap 3-5 HP more compared to the original. Still worth it, IMO.

Verdict: Would I do it again? Yes. I already bought another 143A unit & new pulley, and plan to install that on my other 1987 300D in a few months! For the record, a used 143A unit can be bought used for ~$75-125 or so. The donor car must be a 1996-1999 S or SL class with a V8 (SL500, S420, etc.) Others may work but I don't know what years & models, and most others are smaller (100-120A).


Photos of everything are here (nothing new from last time though):
http://www.W124performance.com/images/W124_stereo/

I'll post a few photos below this, if I can find one small enough to squeak by the 65kB limit...



Regards,

__________________
Check out my website photos, documents, and movies!