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Removing and reinstalling the cluster from the dashboard is actually quite easy, but you may not even have to do that. The first thing to check is the condition of the wires to the speed sensor on the differential housing. If they are frayed, or the connection is otherwise bad, you will have neither speedometer nor odometer, and the ABS warning light should come on. In normal operation, the ABS light should come on with all the other warning lights when the key is turned on, then go out a couple of seconds after the engine starts. If it doesn't come on at all, it is likely that the previous owner removed the bulb, or it has burned out.
If the ABS (and the light) are working properly, the sensor is not at fault, and you probably need a speedometer, though it's possible the connections on the back of the instrument cluster are at fault. I believe you can use a speedometer or instrument cluster from a 90 740 sedan or wagon with ABS. You may also be able to use one from an 89, again with ABS, as long as it has four terminals on the back of the speedometer part of the housing. 88 and earlier 740 speedometers, whether ABS or not have only three terminals, and 89 and 90 non-ABS units, though they look identical and have the correct number of terminals, require a different number of impulses from the sending unit and are not compatible with your car. 91 and later 740s have the new style dash and cluster, which are totally different from yours.
I once had a 90 cluster that I rigged to work in my 88, (it was never completely satisfactory) that was temperature sensitive. When it was below about 60 degrees, the speedometer and odometer would not work. On cold days, they would do nothing until the car warmed up, then they would suddenly spring to life.
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