The problem is most likely to be the speedo drive gear slipping on the transfer case output shaft. The gear is not keyed to the shaft but is clamped by the nut holding the output flange on. I would check this first, although the other suggestions may be the problem. Note that this problem is likely to be exacerbated if the speedo cable is hard to turn, for example if it is kinked or has loose strands and is about to break.
Disconnect the front of the rear prop shaft and tighten the large nut on the end of the shaft that this uncovered. Reassemble. Cost $0.
John
John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
Bookmarks