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Here's the way we diagnose problems with the fuel gauge.
1st we remove the fuel sender/sending unit which is the transmitter in the gas tank. Attach a ground wire to the outside of the sender to provide a good ground and with the sender wire attached to the connection on the sender, turn the sender upside down on top of rag in the trunk so the arm is close the top of the under side of the sender. Turn the ignition key to on so that the dash warning lights come on and observe the fuel gauge. The needle should slowly climb to full or near full.
If the needle doesn't move, turn the key to off. Then remove the sender wire from the sender and connect the sender wire directly to the ground wire used in the previous step. Then turn the key back on and the fuel gauge needle should quickly move towards full. If the needle does start to move, quickly turn the key off. You don't want to let the needle travel all the way to full as it could damage the gauge.
If the fuel gauge needle still doesn't move, then go under the hood and install a ground wire to the water temperature gauge sender wire after removing the wire from the sender. That sender is at the back of the cylinder head behind the valve cover. Then turn the key on and the water temperature gauge needle should quickly move towards "H" and once again, quickly turn the key back off before the needle moves all the way to the right. Don't forget to reconnect the wire to the water temperature sender afterwards.
If the water temperature gauge needle moves, then there's a problem with the fuel gauge itself or the wiring to the the fuel gauge. It's not uncommon to need to check and sometimes clean the fuel gauge contacts on the back of the instrument cluster. There's supposed to be a couple of brass nuts that hold the gauge to the PC board on the back of the cluster and the nuts sometimes need to be tighten and/or the contact areas under the nuts on the PC board need to be cleaned.
If the water temperature gauge doesn't move, then there's a problem with power getting to the voltage stabilizer which is usually a fuse or fuse contacts or the voltage stabilizer itself is bad or the wiring to and from the voltage stabilizer is faulty.
The voltage stabilizer is attached to the back of the instrument cluster and provides 5 volts to both the fuel and water temperature gauges. The early version of the stabilizer is connected to the PC board via 3 wires that are soldered to the PC board that sometimes need to be repaired.
BTW, if you do pull the cluster, I strongly recommend changing the bulbs in the cluster. And with the cluster removed, it's way easier to access the 3 bulbs for the heater controls.
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Eric
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90502
hiperformanceautoservice.com or oldvolvosonly.com
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