Before you buy anything, jack the car up and inspect the wiring to the speedo sensor on the differential. The wires tend to fray near the connector causing the speedometer/odometer to not work. This is a fairly common problem and can be an inexpensive fix if you are handy. The wires will need to be cut and butt spliced together. I replaced the connector end on mine using the volvo connector end, heat shrink butt splices and Volvo pigtails.

http://www.brickboard.com/FAQ/700-900/ElectricalInstruments.htm#SpeedometerFluctuates

Has the AC been retrofitted to R134a? If so and it only needs a recharge it should be inexpensive to recharge. Who diagnosed the AC? I tend to be suspicious of AC claims when acquiring a car without it working.
--
Paul NW Indiana '89 744 Turbo 145K/ '90 745 turbo 127K

> The speedometer some times works,could it be a cable?

> I googled a cable for it but it just shows a small sensor that looks like a magnetic pickup and goes in the rear end is this correct? I guess the other thing it could be is in the cluster itself.

Yes, this car has an electronic speed sensor built into the differential. Two wires run from the sensor up to the instrument panel.

> Is this a common problem?

The "700/900 Series FAQ" (http://www.brickboard.com/FAQ/700-900/) has a page that addresses this. It claims that the speedometer itself can commonly fail, which is not easy to deal with:

http://www.brickboard.com/FAQ/700-900/ElectricalInstruments.htm

As it happens, I am dealing with an intermittent speedometer problem on my 1992 940. I am not 100% sure that the speedometer is at fault yet, so I cannot corroborate the information above. Just pointing you to some more information!

For what it's worth, I am seeing replacement speedometers for between $130 and $300 from automobile recyclers. My local Volvo dealer wants $800 for a brand new one.