I did my water pump this morning and if you've ever done a M104, doing a M112 is a breeze in comparison. With the M104, all sorts of crap has to come off first but with the M112, the pump is readily accessible and nothing comes off besides the belt, idler pulley and water pump pulley.
I used a new Laso WP because from a picture, I liked the style of the impeller, but after getting it, it had the same impeller as the other brands (Graf, Meyle, HEPU). Note the different impeller on the old OEM Mercedes WP.
Here's how I did it. Any tips or corrections welcomed.
Work on a cold car. Release cap on coolant tank, remove cover under engine, 8mm screws, attach hose and drain radiator. The procedure calls for draining the engine block but I could not find the drain fitting. Got almost exactly 2 gallons of old coolant out from the radiator. Capacity of the system is about 9.5 ltrs.
Loosen 4 screws on the water pump pulley while the belt is still tensioned. Release the belt with a breaker bar on the torx fitting at the center of the tensioner. Put your breaker bar at about 9 o'clock, turn counter clockwise, release belt at idler pulley and your breaker will swing back clockwise to about 12 o'clock. Remove belt.
Remove idler pulley. Now is a good time to check this pulley. They are prone to cracking. I have some photos of some old ones that I replaced.
http://www.peachparts.com/shopforum/tech-help/300599-m112-idler-pulley-bolt-torque-value.html
Loosen 2 hose clamps and work off the 2 hoses.
Put your new WP on your work bench and note the location of all the holes. This is your guide to all the bolts you have to remove. As you remove the bolts (3-4 different sizes) , drop them in the corresponding holes of your new WP. Check the threads at the end of the alternator bolt. Some parts shops suggest a new bolt. The first 4-5 threads were barely there so I used a new bolt.
Check your new water pump and make sure you've gotten all the bolts. Should be only one empty hole at around 2 o'clock ( hole at 12 o'clock is for idler pulley).
Gently pry off old WP and clean up any dirt around the opening.
Transfer all your bolts from the new pump to the old pump.
The old pump used a rubber gasket. The new pump comes with both a rubber and a flat gasket. I used the rubber gasket. Dry fit the WP a few times to make sure you get it right. Make sure the gasket stays in place.
Torque on all your bolts except for the 2 upper bolts to the left of the idler pulley hole. This is the one problem that I ran into. These 2 holes on the old WP are tapped. The same 2 holes on the new WP are not tapped. There is mention in the manuals for different torque values, one for tapped holes and a higher torque when using self tapping bolts. I didn't understand what they were talking until seeing the new WP. These 2 bolts hold a plate to the WP housing, and do not go all the way thru to the engine housing, so I just zip tied the plate in place, until I can find self tapping bolts.
Attach hoses. The clamp for the smaller hose can interfere with the WP pulley, so have the screw facing you and pointing down so you can tighten it from below.
Attach idler pulley.
Attach belt. Fill with MBZ coolant mixture.
Run car and check for leaks.
Water pump pulley 9-10Nm, 7 ft-lbs.
Idler pulley, M8, 20 Nm
Alternator bolt, M10, 42 Nm
Water pump M6, 10Nm
Water pump M8, 25Nm