How can I test the solenoids if they are good or bad, and if bad can they be ordered from the dealer?

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2008 ML320 CDI
2007 GL320 CDI
2005 Audi A6 (sold)
2001 E320 (sold)
2004 lexus RX330 (sold)
2004 Honda CRV (sold)
93 190E 2.6 (sold)
97 E320 (sold)
2000 Honda CRV (sold)
91 350SD (sold)
90 E300 with amg body kit (sold)
83 300sd aqua marine (sold)
81 300sd dark blue (sold)
81 300sd gold (sold)
81 300sd brown (sold)