With the Earthquakes trailing 2-1 and time winding down, Cronin collected the ball after it was knocked down by Chris Wondolowski's header and fired a volley from 20 yards out that got past Impact goalkeeper Troy Perkins.
Adam Jahn began a comeback from a 2-0 deficit with a 59th-minute tally for San Jose (2-3-5), which logged a draw for the fourth time in five games.
Justin Mapp registered two first-half goals for Eastern Conference-leading Montreal (5-2-1), which extended its unbeaten streak to three contests.
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