Speedometer erratic/intermittent 1991 non-turbo 740 wagon, +200K miles. speedo usually sits at zero. speedo will indicate correctly from 0-45mph, where it stops while i'm over 45mph. it will indicate correctly again as i decelerate below 45mph, falling from the 45mph mark. speedo occasionally goes up to 90mph when i'm driving over 45mph. speedo occasionally indicates the correct speed. tachometer and odometer work. if speedo stops working while i'm driving and it's indicating more than zero, it will slowly fall to zero. i have noticed no correlation between temperature or humidity. how may i troubleshoot to determine whether problem is speedo, wiring, or sensor? or may i simply judge from the symptoms that it is the speedo? thank you ben in ohio, USA

I don't know what is causing this problem, but I know phil140701's answer is completely wrong on two counts. First, 740 Volvos starting at least in 1986 have electronic speedometers and no cable. There is an electronic sensor on the differential which sends a signal to the speedometer cluster. Second, if it were strictly mechanical with a cable then the voltage regulator could have no effect on it at all.

If i remember right from prior experience with volvos you'll have a speedometer cable on that model, the cable is made from wire which is square shaped on both ends and then a blob of solder is added to make them stronger(so they stay square), you will probably find that one of these soldered ends has broken of and that the cable is no longer gripping the square shape hole as it should at speed, best bet is a replacement cable if the solders broke off, if it's not this it could be either a faulty dial or the voltage regulator on the back of the instrement clusters wareing out