I think the subject of jumping tach/speedo needles came up not too long ago
and I have some more info. I have a TR4 with Jaeger tach. The needle was
jumping around and often a new cable is the suggested cure. I did that and
no difference.
I then took the tach apart for a look-- The end play of the magnet shaft is
determined by a pressed on bronze bearing. With wear the magnet moves
closer to the cup disc and begins to physically brush the disc (needle
jumps). Wear of the disc spindle may also cause the disc to move closer to
the magnet. I took the magnet and shaft out and with a small deep socket
over the shaft pressed it further onto the shaft in a vise. We're only
talking .010"-.020" so be careful. If you tighten the vise and it does't
seem to move, try tapping the vise jaw with a hammer. When doing mine it
moved a little the first time but I wanted a little more. The second time
it didn't budge and then popped and moved a little too much and I had to
pull it back with pliers. I lubed the disc spindle with WD40 and put it
back in the car. -- It works very nicely, steady as a rock.
I just looked at a speedometer and it of course is similar except that the
bearing is the brass gear for the odometer but I presume a fix would be
similar.
Allen Hess
62 TR4 CT13678L
Allen Hess and Elaine O'Neil
Http://www.netacc.net/~aephoto
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