FStanley

I have a 1984 Olds Ninety-eight, the speedometer jumps around but cruise control works great at all speeds..


I put in a new speedometer cable because in cruise control there seemed to be some variation in speed and also mph read on speedometer.

OK now very smooth in cruise control at all speeds,


NOTE: in 1984 they went to an electronic control "eye" at the speedometer.



But now the speedometer jumps around crazy up to 80 mph, down to 20, steady at 45 for awhile, then crazy again. I lubed the speedometer gears with a plastic gear lube, seems smooth when I turn it by hand. nothing binding... Always returns to the same "zero" spot consistantly.

Put back in car and still jumping like crazy..


so I think the issue is the speedometer thing the senses the speed and makes the needle move..


As I would think the cruise control would not be as smooth as it is.


No clue on how to fix...

Any help appreciated.

Fred

joe_padavano

Your speedometer is still mechanically operated by the cable. The optical sensor in the speedo (which first appeared in the 1980 model year) is only used to generate a speed pulse for the ECU and the cruise control. It has nothing to do with the functioning of the actual speedo.

Your troubleshooting is pretty much spot-on. Replacing the cable (and I assume you lubed it properly when you slid it in the sheath) eliminates what would have been the most common cause of this. Since the cruise works properly, that pretty much only leaves the speedo head itself.

FStanley

OK found the problem took speedometer out, found a piece of a gasket like material interfering. Gasket like material is on the back of the speedometer head... now all OK again..