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Use a Jeweler's loupe, reader cheater glasses, or magnifying lens to inspect the solder welds for cracks and reflow the solder welds. Add some new solder.

Spares are cheap. Have a known good fuel pump relay in the boot and in the garage. Savage yard that have RWD Volvo may have the relay you want, or several, yet the Volvo 240 era is at a sunset.

Your 1985 is K-Jetronic injection and ignition, so get more green

May want to verify the current draw on the high current side of the circuit using an ammeter feature of your electric multimeter.

To this sites version of the Volvo FAQ:
https://www.brickboard.com/FAQ/700-900/EngineFuelinjection.htm

Right up your Volvo 24o Turbo in 1985 as Turbobricks helps:
https://turbobricks.com/index.php?threads/burning-through-fuel-pump-relays.342274/

Dave Barton's Volvo relay page:
https://www.prancingmoose.com/volvorelays.html

Verify a balanced potential to ground. Or restated, you may want to treat ground points for the fuel injection harness. Loosen, some lithium grease spray, tighten. Actually, all ground points. There exist Volvo service manuals that indicate where the ground connection are throughout the 240 unibody. Include the alternator to engine block ground and also the b attery negative to body and engine block.

See here on this page:
https://ozvolvo.org/archive/

Find and download the PDF file from the page for ground points:
Taliessin Penfound - TP30678-1 1984 240 Wiring Diagrams.pdf

You may find some other pubs you want here.

Mitchell Service Volvo electric system single line diagrams (SLD):

https://v8volvo.se/mekartips/Volvo/index.html

Also, you have a 1985 so smack dab in the dumb Volvo deteriorating insulation on the wire harness. Though your 1985 240 T would not be on the road without a new engine bay and maybe other wire harness replacement from Dave Barton's site or the manufacturer in Belgium (?) or Netherlands (?) unsure.

You do not mention fuses blowing. Have you checked for a hot ignition switch after a long drive or hot fuses in the fuse panel? Corrosion can form at any electrical contact. Like the fuse box fuse holder contacts and also the quick disconnects at the back of the fuse box.

Get a spray can of Deoxit-D and treat electrical contacts. If your Volvo 240 resides in maritime, high humid, polluted, or heavy tonnes of salt with a snow flake area, the electrical system suffers. You may see a white film on soime contacts.

If you inspect and service the Volvo 240 fuse panel back and front, the wire harness quick disconnects a frail things. So apply some Deoxit. I had to replace quite a few of the factory installed quick disconnects on the fuse panel back side.
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