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Date registered: Sep 2015
Vehicle: 1976 450SL
Location: CT, USA
Mentioned: 2 Post(s)
Quoted: 71 Post(s)
Alternator swap 55A to 70A, single piece pulley v split pulley
I thought this was going to be one of the simple jobs...
I'm replacing the (working) 50 Amp Bosch AL80X alternator with the 70 Amp AL68X Alternator on my '76 450SL. The original had a slight bearing noise, and I thought I might as well replace it and get some more Amps into the bargain...
Because shiny things are nice (!) I also decided to fit a new pulley and fan at the same time.
The pulley I bought is a single piece pulley - but has no keyway for a woodruff key. However, it did come with a large flange nut and split washer (note that the photos of this 6033GD5100 online show a large flat washer with key way, instead of the split washer mine included). The pulley is listed as being compatible with the AL68X alternator, and the 450SL.
The original alternator has a pulley that splits into two halves, front and back, with a keyway for a woodruff key.
The new 70A alternator does include a woodruff key, but there's no way to use it, because the single piece pulley doesn't have a slot, so you couldn't put it on the shaft with the key in place.
So...I decided that presumably one wasn't needed. I've fitted the new fan, pulley, split washer and nut onto the new alternator. I used the air gun. All I can hold is the fan or pulley while tightening (I see some reference elsewhere to some bosch alternators having hex sockets on the end so you can hold them, but nothing on this one..
What I've discovered is that with that arrangement, the fan is not held that tight, even when the nut is tight on the shaft. I can spin the fan on the shaft, if I hold the nut, though not easily.
The pulley will also slip, if I try and turn it anti-clockwise, again while holding the nut (when running, the motor will turn it clockwise). It doesn't seem to slip going clockwise, presumably because of the spring washer digging into it slightly.
If I spin the pulley or fan by hand, without trying to hold the nut, nothing slips, and the alternator spins freely. My concern is that there will be more torque when the motor is running, and the alternator is generating current, and the pulley will spin on the shaft (my guess is that there is very little torque on the fan, so that won't be an issue).
Does anyone know if this is normal? I have found other other threads indicating that some bosch pulleys don't need a woodruff key, as the turning of the motor will naturally tighten everything).
I could, of course, fit my old (rusty) split rim pulley (and the woodruff key) to the new alternator, but I prefer the shiny one!
Also....a quick Google search and the EPC didn't show the two piece split pulley - I was considering buying one of those, so I could use the woodruff key. Anyone know the part number (s)?
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'76 450SL 'Yard Find' Project
Last edited by TheFixer; 06-08-2019 at 05:54 PM.