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Recently, I purchased a VDO 7k tach (older 3 3/8" Cockpit series) from Ebay...the deal was good and all that, but then I noticed that it states it's for a 4cyl. on the box (argh!) <love complete descriptions>...it has no switches, etc. on it and clearly states this (4 cyl.) on the back of the unit also...it's a transistorized tach, if that makes a difference? one last thing that may have nothing to do with it is that it has UPM and RPM on the front...

Anyway, is there any way to get this thing to work on the chevelle (8 cyl.)? I guess the pulse would have to be cut down by half...is that correct? (or maybe I'm backwards ;) )...at any rate, can this be done with a resistor or whatnot?

Thanks for any help
I'd hate to have to resell this thing, as I do like it...
 

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Well, the UPM is a new one to me...have never heard that before. Your four cylinder tach would be expecting to see 4 ignition pulses for every 2 crankshaft rotations. Your V-8 would be putting out 8 pulses for every two rotations, so the way I see it, you are correct...you would need to divide by two to get the RPM's right.

This isn't so simple as hooking up a resistor, but a frequency divider is not too complex to build from Radio Shack parts. If you need more info, let me know...I probably have a schematic and parts list for a /2 laying around somewhere. Can't guarantee it would work, but the theory is sound and if you like that specific tach...
 
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