Sorry, David, I never made the effort to chase down the mode of failure Russ spoke of, but I have mailed out a couple chips I've found in the junkyard. They're easy to find around here, because someone has already pulled the cluster and board diving for the (usually broken) gears. Maybe you could verify the signals on yours before shotgunning it. You don't need a signal generator; any power line frequency source-- like a wall transformer, will do, provided you play it safe and build a voltage divider network. Maybe you don't even need that; I haven't reviewed the protection VDO gave it.
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Art Benstein near Baltimore