300SDL AL129x Alternator Swap Dilemma
 

Is it a simple fix of finding a larger serpentine belt, or am I going to have to buy a 143 or 150 amp alternator that will fit a 300SDL W126 OM603 engine?

Unless I'm mistaken, I just learned the hard way that an AL129x 115 Amp alternator will not fit a 300SDL W126 OM603 engine simply because the serpentine belt is too snug.

I purchased a '92 Saab 9000 AL129x alternator from eBay and began wiring it into my 300SDL. Everything bolted in normally, and I was ready to go after upgrading the wires, but when I tried installing my serpentine belt, it seemed to be slightly too tight. I couldn't get it back on even after releasing some slack with the spring tensioner (I unbolted and released the spring tensioner when I pulled off the old alternator.)

The serpentine pulley from the stock alternator (AL65x) seemed to match up with the replacement (AL129x) when I eyeballed them together. The only thing I can think of without dismantling everything again is that the distance from the pulley to the mounting bracket is slightly larger on the AL129x.

I've read several posts on this forum that said it had worked and others that said it wouldn't work.

Am I going to have to buy another 143 (AL0162X) or 150 Amp (AL0766X) alternator from a V8 W140 now and be stuck with an AL129x that I can't use? If so, are these the cars that I can pull one from since they all have a V8 engine?

1991–1993 400SEL
1993–1998 S420
1991–1993 500SEL
1993–1999 S500
1994–1998 S420/CL420
1992–1993 500SEC
1993–1999 S500/CL500

Last Question: If I buy a 143 or 150 Amp, do I need to modify anything, change any spacers so the serpentine belt will line up properly, or change the pulley from the stock AL65x? Do I need any special tools to change this pulley out?

Thanks for any suggestions or help. I don't want to make another mistake in getting the wrong alternator or making an improper modification.

If it's the same as the '87 300D I have installed one from a LR Discovery. I needed new wires with ring terminals and my original pulley.

Thanks for the reply. Do you recall what year the Land Rover was that you pulled it from or how many amps it's rated for? I noticed that the LR Discovery uses a Bosch alternator, but the model and amperage changes depending on the year.

I also noticed that your '87 300D has the same engine as mine. I may try to find this alternator if it is less expensive and has a decent power output.

Do you need any special tools to change the pulley, or can you just grip the pulley and unbolt it with a socket wrench?

It was a '98, 130a.
Yes a socket-wrench and a strap-wrench to hold the pulley and you should be fine.
I've also used a 90a from a '93 190E 2.3 which was a direct plug and play, and it seems that the alts in my E320s were the same fit.
Are you certain that you don't have the belt around something wrong? It seems like the mounts are pretty standard.

400E also had around a 110-115A alt that bolts up.

The one on my SDL is a 143A and same as my 300SEL.

If you don't already have a strap wrench, I'd not buy one, I'd put the money towards an impact wrench. even the 12v $40.00 one from walmart or harbor freight is better than not having one. and makes changing the pulley a simple matter.

Autozone or Kragen
 

FYI...Bought the perfect fit for my '86 SDL at Autozone or Kragen a few years ago while not able to locate a Bosch. Cost me about $80. Simple to replace. Did not come with the pulley so I pulled the one off my old Alternator. 60,000 miles later it is still running strong.

Thank you everyone for your advice. I broke down and bought a 150 Amp AL0766X from a salvage yard since I heard that many have used this model successfully with the 300SDL. The donor vehicle was a 2000 Mercedes E430 with 62k miles, so I think this one should last for a while so long as it fits. I checked the following thread and it seemed to match the list of compatible alternators for an OM603 engine.

http://www.peachparts.com/shopforum/showthread.php?t=53236&highlight=150+amp+alternator&page=2

The only disconcerting thing is that there is a lot more resistance when I spin this one by hand. I read that it may sap an additional 3-5HP. 'Hit Man X', have you noticed any difference in acceleration, idle speed, or fuel economy with your 143 amp on your 300SDL?

'VSTech' I may get the 12V impact wrench from Harbor Freight as you suggested. The pulley I'm going to take off my stock alternator is kinda rusty. I'll have to find another weekend to finish my swap.

Last question: I've prepared my two replacement cables, and I'm going to wire about 6 feet of 0/1 AWG wire from the alternator post DIRECTLY to the battery, rather than to the stock distribution block first (as shown in the images at the bottom of the above link)? I traced the path of the old wiring in order to avoid exposure to excessive heat from the exhaust manifold. I'm using Monster cable, so I think it's robust enough, but I want to make sure the insulation won't melt off over time.

I figure the straight shot to the battery will be a better connection with the least path of resistance since I have a 500-Watt amplifier and 1-Farad capacitor in the trunk. I'll leave the old wire in place that connects from the battery to the stock distribution block in order to complete the circuit. I'm assuming this won't hurt anything since it's all part of the same circuit anyway. Am I correct?

The larger alternator will indeed sap a few horsepower. But it will be negligible overall. Will not lower MPG, acceleration, or affect the running of the engine.

The larger alternator will only sap more horse power when it's producing more current. The fact that it can produce 150A doesn't mean it's always producing high current.

yeah.
it should NOT be difficult to spin in your hand... only under load. if it's tight, take it to a auto-electric shop and have it spin tested... likely it needs bearings or armature cleaning...

The armature doesn't resist rotation until it's energized by the regulator, the resistance to rotation is proportional to the current draw.
If the replacement alternator is hard to spin; that suggests the bearings.

I'm no expert. But I was under the assumption that the horsepower loss was half because of added weight, and half due to the higher current draw. Aren't the "insides" of the 150A much heavier than a stock alternator? Meaning that just spinning the pulley on the 150A requires more energy than the stock alt, whether its producing full current or not.

From reading other threads the 150A is actually lighter due to more cooling vents cut into the casing, but there are no free rides it will use more energy to spin when putting out max amps.

Everyone, thanks again for your insight. I mainly wanted to ensure that I'm not going to reduce my fuel economy with this upgrade considering the price of diesel.

'JohnM', I agree that the mass of the components could be causing this resistance due to inertia even without a current draw. The 150 Amp AL0766X is much heavier when you pick it up out of the box compared to the AL69x stock alternator or even the AL129x from the Saab.

Even so, to make sure there isn't an internal problem that I'm missing, I will take it to Napa or an electronics place a week from now to get it spin tested. I thought that most times you can spot bearing failure if it makes noise when you spin it, if it catches or seizes while spinning, or if there is free play when you try to wiggle the pulley. This alternator has just 62k miles on it and it's a Bosch part from a 2000 E430. I guess it's 11 years old though.

Again, I appreciate the responses...they have been very helpful on my end.