View single post by edward_davis
 Posted: 01-05-2006 05:43 pm
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Joined: 07-06-2005
Location: Eugene, Oregon USA
Posts: 162
I've disassembled my tach, and it turns out to have much simpler circuitry than the old Sunbeam Tiger tachometer that the diagram comes from.  There's only one transistor and one capacitor.  All of the individual elements seem to work in my tach, but the capacitor is no longer connected to anything.  There's a place for a connection in the circuit board, but nothing there.  I didn't find any loose parts, either, but I might have lost the bit the first time I took the gauge out of the dash.  I didn't realize how easily the bezel comes off, so it slipped off while I was looking at it; could have lost the circuit element then.

I've bought a natty old '76 MGB tach on e-bay to "reverse engineer".  If it really is the same sort of design, hopefully I can make one working tach from the two units.  Once I get it worked out (if, I should say), I'll post the circuit diagram.

There is a site online that has a circuit diagram for a DIY conversion to a voltage trigger, so that the tach can be used with an MSD system.  If I can't get the old-school system to work, I might try that instead.  It apparently works fine for old Jags and MGB's.  The big problem with MGB's is that the tach has the ignition warning light in the lower part of face, interfering with the placement of the new circuit board.  Good ol' Jensen folks, putting our ignition light on the console...

Does anyone out there have a photo of the insides of their original Jensen tachometers?  If I saw a working unit (or a unit broken in a different way), it would help with troubleshooting mine.

Mark, thanks for the pointers; they were good leads that will hopefully lead to a happy DIY resolution to my problem.