If the water pump is similar to the M103, this is what you need to do:

Remove fan and clutch -- 8mm Allen, use some coat hanger wire to lock the fan clutch to unscrew the bolt (I don't know if the clutch on the 190 is the same, it's electric...).

once you get the fan off, remove it and the shroud. Remove the bolts that hold the water pump pulley and PS pulley, three each.

Release the tension on the belt by loosening the 19mm bolt in the center of the tensioner BEFORE you attempt to "unscrew" the tensioner sticking up beside the water pump -- if not, you will break the cam on the tensioner, big bummer.

Unscrew tension while watching the pointer on the main bolt. If it doesn't move back to the start of the "ramp" mark on the housing when the belt is loose, you need to replace the tensioner (assuming it's the same as the M103).

Remove belt. Loosen heater core pipe retainer nut and pull pip back. Don't remove it, it's slotted.

Water pump is held on with four bolts, two hex head, two allen. Note where they go. You may need a univeral joint extension to get two of them out.

Clean head surface, install new WP and large o-ring. No sealant, it goes on dry.

I'm not sure of the differences, you may not need to remove the tensioner (I did, because it was shot and I had to replace it). Less room on hte M103, too, I think.

Took me 6 hours exactly, hopefully less for you.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!