I had this problem twice on the 940SE wagon and once on the 940SE sedan. In ecah case it was very cold and only lasted for one day. It has been four months since have had the problem. It might be the circuit board, but I'm not ready to take the instrument panel out just yet.

I also had this problem on my 92 960 and took it to a shop to fix. The mechanic took it out and couldn't find anything wrong and advised against buying another unit. Put it back in and never had another problem with the speedometer.

Well, the crank sensor is used for engine speed not vehicle speed. The sensor you should be concerned with is in the differential housing and it is the vehicle speed sensor. Your best bet for diagnosing it is probably going to be in getting it to duplicate and then have an oscilloscope hooked up to the sensor and see what it says.

Why it shoots up to 70 and sticks is anybody's guess. I would fault trace the sender first and if it checks out ok while the problem is duplicating and all of your power and grounds are ok at the speedo you will probably need a new speedo head to fix it. I have occassionally found problems in the printed circuit cards on the instrument clusters but they usually don't show up as an intermittent problem.

Mark

I think the 700 series speedometers are prone to have weak solder joints on the PC board. I had an erratic speedometer for years and eventually sent it off to Overseas Speedometer (in Texas I think). They did whatever they had to do regarding re-soldering the board and it has worked properly ever since.

Had it done about 5 years ago or so. Can't remember the exact number, but it may have been around $100. I wanted to keep the original cluster instead of swapping in another that may have suffered the same fate.
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'86 745T, '72 144E, '70 145S, '68 220S, '60 544