Start by diagnosing if you have 12V being applied to the monovalve. Make sure it is connected to the coil when you're testing (it's a transistorized circuit). If you do not have 12V being applied, you have a pushbutton unit problem. If you do have 12V being applied, you have a bad monovalve cartridge or a bad coil. The coil can be tested for resistance, it should be somewhere around 14-15Ω if good.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]