The resistors in the circuits rarely ever fail. What is the usual culprit is failed solder connections or electrolytic capacitors that have dried out and failed. A resistance reading won't tell you much since a poor solder connection can still test "good" but fail to pass any current causing a "fault". Any circuit with a transistor in there will give *a* reading, but the value is meaningless. You're time/money ahead to pull a faulty/intermittent tach amp and repair it. If nothing else, you'll retain more hair on your head.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

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