Monovalve should be the same on all W126 bodies, it's held in place by four screws on a small plastic cover. No need to remove a diagnostic cover or anything else unless I'm spacing out again. It's up behind the bulkhead just in front of the firewall near the air intake for the climate control.

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!