The difference between those two is in the resistance produced by floating device in the sending unit and number of connectors in the fuel gauge. I have pulled sending unit and instruments cluster from 93 turbo few months ago, but recently when I run out of gas in my 92 turbo about 2 km from home I decided to finally get my hands on it. I did the swap and everything is working fine. I have replaced sending unit. I decided to drop the tank, because I didn't want to wiggle with it in the sedan
limited space. The original 92 was short tube type compare to the 93, but the tube slides in the 93 unit so it works OK. The problem was with the gauge. The 92 gauge is four connectors type and the 93 is only three connectors. I have studied the wiring diagrams, the gauges and the print on the back of instrument cluster. Here is what I have found: (see the pictures) http://www.brickboard.com/GALLERY/volvopic.htm?id=10441
http://www.brickboard.com/GALLERY/volvopic.htm?id=10440
in the 92 gauge connector 4 is for the sensor, 1 and 3 power up the gauge and number 2 is an option between two different tank capacities. In the 93 gauge 1 and 3 power up the gauge and number 2 is the sensor. Number 4 on the circuit print is left unconnected. Number 1 and 3 in both gauges are the same, they lead to the same places: 1 to the metal box on the top of instruments cluster and number 3 goes to number 4 on the connector bar. Number 3 on the connector bar is for the sensor, so the
only problem is to fix the sensor circuit. You have to get rid of capacitor from the cluster circuit board (20W soldering iron, higher wattage will melt the plastic circuit) between number 3 and 4 of the gauge connectors. This capacitor is already on the 93 gauge circuit. Now you have to separate with a piece of plastic with hole in it number 2 gauge connector screw from number 2 connector on the instrument cluster, there must be no connection between the screw ( the gauge connector) and the
cluster circuit board at this point. Take a piece of wire, make a loop and now the screw goes through the loop on top of the plastic piece you have cut and then to the connector 2 on the gauge. Solder the other end of the wire to the lower point where the capacitor was seating previously. Verify that the screw from 2 on the gauge has connection with 3 on the connector bar and there is no connection between 2 and 3 on the gauge itself ( we have separated screws 2 and 3 on the gauge with the
plastic piece). Enjoy!

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'91 740 turbo SW, '92 940 turbo, '94 940 Regina SW (PL)