This is a common problem with 850s. The speedometer is driven electrically, while the odometer is mechanical.
There's a tiny little gear inside the odometer that runs it from a motor; it's no longer tied directly to a speedometer cable like older models. What happens is the gear gets old and brittle and loses teeth, at which point it can no longer turn the odometer.
My wife's '96 850 had the same issue when I got it, and a little internet research turned up the answer above. There are companies that make replacement gears you can buy fairly cheaply, but it does require a fair amount of mechanical aptitude to pull the instrument panel and disassemble the speedometer assembly.
One note of caution that I learned the hard way: you have to take the front cover off the instrument panel, exposing all of the gauges. Be very careful not to disturb the position of those little needles; I ended up messing up the gas gauge, speedometer and temperature gauge, and it took a lot of trial and error to get them put back in so they showed the right readings.
If I recall correctly, I found instructions for doing the gear replacement online, but it's been so long that I don't have the web page(s) I found any more. A lot of times the manufacturer of the replacement gears have instructions on their web sites.
IPD USA sells them and has instructions.
850 Odometer Gear
SOURCE: Speedometer work on and off on a 1997 850 Volvo
back of engine at the very bottom on the trans where the axle mount
SOURCE: My Volvo 94 850 odometer stop working, I bought
There are actually two gears that you need to replace, a large one and then smaller one that rides inside it. You can purchase them both at www.fcpgroton.com These can also sometimes affect the operation of your speedometer. Do you by chance have an ABS and TRACS light on? If so the speedometer issue may be related to your ABS controller.
SOURCE: 96 volvo 850 speedometer somtimes works then it drops to 0
Two questions: Is your odometer working? If not then it's probably a broken gear by the stepper motor that Emissionwiz mentioned. Is your ABS and/or TRACS light on intermittently or constantly. If so that is a common sign your ABS control module has gone or is going bad. This usually happens before the speedometer starts acting up. Either one of these will start intermittently and finally end up with the speedometer not functioning at all, and both are pretty common failures with this model.
SOURCE: 1995 volvo 850 turbo speedometer is working odometer is not.
You have a broken odometer gear wheel which is very delicate and hard to change it cost less than fifty bucks but it will take a volvo expert one hour or more to change if your not volvo acquinted dont attempt to fix it.you will mess the dash board
SOURCE: odometer and speedometer stopped working 1996
possible electric chk fuses or cluster problem
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