In the UK there is sometimes another surprising incompatibility. The sensor version on the axle crown wheel housing has to match the speedo head, or else it doesn't register at all.

There are more differences between the various years of electronic clusters than are initially visible. The speed difference between the 10042 and 9800 speedo heads is minimal. I think it works out to about 1.3 mph at 60--somebody out there remember the math for final drive ratio? As Paul mentioned, it has to do with the larger tires Volvo originally spec'd for the wagons for load handling. IMHO, the tire stores stopped stocking the oddball sized tire when the general quality and load recommendations increased with technology and the 185-70-14 tires could hold as much as the taller, older size wagon tires. I think the wagons took 175-R14 from the factory. A good swap is, I think, 195-75-14. Your tire store has software these days to convert the size.
As to interchange on the cluster, all of the clusters from 86 through 91 will work and give you basic functions in all of those years. That is, speedometer, clock or tach, fuel and temp gauges and odometers will work. The warning light positions on the left half of the cluster change considerably. Starting in '88 there is a service reminder light on the far left and a reset lever in the rear. For 89 Volvo added halogen illumination lamps for the cluster. 89 is the first year for LH 2.4 with the check engine light also. In 90 the service reminder reset has a remote button on the front, lower right corner so you don't have to reach in back of the dash to reset the light. 90 is also the first year for SRS (airbag) so there is a light for that. 92 on is a completely different cluster. If you have cruise control, you should know that you must use a cluster from a car that uses the same cruise system as yours if you want that to work. An 86 cluster will read fine in an 89, but you will not get the cruise to work. Feel free to mail me at dpsam@sbcglobal.net for more details, but you can indeed put just the speedo head from the 86 into the 89 cluster. You won't have the service reminder though.
Best for the holidays!
DS

The wagon speedo is not the same as the one on a sedan. The wagon has larger tires (unless you let Discount Tire tell you what size goes on your car) and the circuit board is slightly different. With that said it will fit and work in a 89 model, just not accurately.

I had fixed my odometer a couple of years ago and was about to finally go back to the right tire size from TireRack Tuesday night (see the part about Discount Tire). I was going to have an accurate speedo finally. Filling up on Monday night the mileage read 159 miles and it took 10 gallons to fill up. I was about to post a email asking for help with a sudden MPG drop. You guessed it's broke again and you would be right. This time it is not the gear, put one of those BB recommended ones in. May be an old chunk of the old gear stuck someplace, but bet you it is not the replacement.

Next sunny day in the 70's (supposed to be that way this Saturday) time to pull the dang cluster one more time.

Good Luck,

Paul