Ed Hanna: Like many of the systems in your car, the instrument cluster has microprocessors on the circuit board. When the fuse that controls the circuit is pulled and re-inserted you are re-booting the circuit and the item will work OK for a while. I think that cleaning the fuses and fuse clips worked well on the 126 models, not the 140 models. I would hesitate to take the soldering iron to the circuit board if the problem is a defective microprocessor.

Napoleon: You need to replace the 2 watt bulb on the left side of your cluster to illuminate the fuel gauge. Someone has a repair for the illumination of the outside temp display. There is a small LED that can be soldered in with a tiny bulb from Radio ShackThe temp display is on a small rectangular circuit board that can be unplugged from the main instrument cluster circuit board.This is the board mentioned by Ed Hanna. I can access the instrument cluster wiring diagram on my Alldata subscription for my '93 300SD.. My '93 300SD has the soldered in bulb wheras my '95 S350 has a replaceable bulb.

The following post was from Benzworld forums:
http://www.benzworld.org/forums/messages.asp?messno=49782&id=10&view=t

Adjust the steering wheel to lowest position. Disconnnect the battery. As stated above, use the tools to gently nudge out the cluster. It's held in place by 5 pressure wedges. Remove 4 plugs on the back. There is no speedo cable to deal with. Take the cluster into the house and set it down on a clean towel to avoid scratches The temp LCD unit is on a plug in circuit board and comes out the front of the cluster. Remove ONLY 2 screws which hold the clear front assembly to the main unit. You'll see which ones when you have it in front of you. Pry at 5 plastic snaps to remove front. Grasp the LCD and gently pull forward; maybe wiggle a bit. The lamp is soldered into the circuit board. Use solder sucker and very low wattage iron to remove old bulb. It's a very delicate circuit board. Replace bulb with 60 ma wire lead bulb from Radio Shack ( 2 for $1.29 ) It's a bit smaller physically, but it will work just fine. Now put it all back together. About a 1 hour job. ( Not counting the trip to Radio Shack )