Tony is right, the diagram I've studied in Haynes shows the voltage stabilizer powering both the fuel and temp gauges. If I recall its 10.x volts, but that's only significant if someone wants to test the voltage in that circuit. If it failed, then there's no voltage in the circuit and the gauges will be at zero.

Anyway, first definitely check the fuses, and spin them in their holders to try to get better contact. The fuses with gray-silver metal material are notorious for getting corrosion and can shut down nearly anything in the car. It's good to clean them with emery cloth or whatever is handy.

If cleaning up the fuses doesn't do it, somebody may as well pull the cluster. What follows should be doable for a do it yourselfer with a bit of courage.

First pull the battery ground cable to disable the airbag (Steering wheel says "SRS", right?). You'll be working near it and don't want to set it off, into your face.

The plastic panels on either side of the cluster pop out, use a flat blade screwdriver after you pull the knobs off the left side panel Leave the rocker switch there and pull the wires off it afterwards.

Cluster has 2 screws at each side holding it in the dash, that's all. Slide it out of dash, up onto the steering column plastic cover shroud. Reach behind and pull off wires BUT make good notes of what goes where. There's a red + white wire for a tachometer; it is there even if there's no tach. Be sure you DON'T hook it up later unless you have a tach; it will ruin anything else. Speedometer wire set is attached with a white plastic clip; you have to break the clip to get it off. No matter, you can toss the clip afterwards and you'll be fine.

Loose mount screws can disable the fuel and temp gauges. To get to them, pull the 7 screws that hold the board on the back of cluster. Pull back assembly off. Now you can get to the gauge mount screws; tighten as needed. Just be careful of the odometer reset and clock setting knobs when you reassemble. See the end of this post for other good stuff you can do before closing up the cluster.

If tightening the screws doesn't fix things the problem could be the voltage stabilizer, but I can't give you a method to test it. If it's obviously damaged you don't need to test it. Damaged would be swollen and distorted, or burned looking.

Looking at rear of cluster, there's a galvanized metal plate at the left end, held down by two screws at corners. The voltage stabilizer is somehow attached to that plate. Screwed to it? Plugged into terminal socket mounted on it? I can't recall exactly but you will see it. Stabilizer is a small flat shoebox-shape maybe 3/4 inch long, has three contact pins sticking out one end that go into a socket. I think there's one screw at the stabilizer holding it in place.

If you need one, fcproton.com sells the stabilizer. On their site you can go to Volvo - 240 - Miscellaneous. I'm pretty sure that's where it's listed. If you don't find it you can call them; I know they list it. Installation is pretty straghtforward once the cluster is out of the dash, you'll see it pretty clearly.

Other very good things to do while you have the cluster open:
http://www.brickboard.com/RWD/index.htm?id=1128612
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Sven: '89 245 NA, 951 ECU, open-front airbox, E-fan, 205/65-15's, IPD sways, E-Codes, amber front corner reflectors. Wifemobile '89 245 NA stock. 90 244 NA spare, runs.