Volvo 740

Fuel pump relay probably. Take it out, remove it out if its case and resolder the connections on the small circuit board that contains two relays. Next, clean the relay contacts with fine grit paper. Glue two pieces back to back and cut a small strip from that. It should be stiff enough to insert between the contacts. The contacts that are situated in the center of the board are pump contact and see most of the wear. If you are really diligent you can unsolder and swap the two relays.

Could be the Hal sender also.

Regards,

Boris Mohar

Got Knock? - see: Viatrack Printed Circuit Designs (among other things)

Common problems are the Hall Effect unit in the distributor, and the fuel pump relay behind the fuse panel.

Both the brakes and steering will keep working, but they won't have any power assistance - you need to work harder at them, but they don't stop.

If it truly stops in an instant, and the tach plummets like a stone, it is almost certainly the ignition rather than the fuel.

Besides the Hall effect sensor in the distributor, I understand there is a radio suppression relay in the ignition circuit that can become intermittent. I've looked for mine a few times since Mike F mentioned they can be trouble, and for the identical relay beside it that can be swapped for troubleshooting, but I haven't actually seen them... or looked very hard.

The "power stage" mounted to the fender above the battery is also a frequent complaint, but most people report a more gradual loss of power rather than a switch-off. If you get bored or ambitious or desperate you can unmount it, clean under it and remount it with silicone grease (I like the dense white "heat sink compound" electronic and computer stores carry). I don't know if they ever actually fail completely, or just overheat and get tired of working.

Finally, it's worthwhile to clean up the battery terminals. If that separate ring terminal on the positive post doesn't make good contact the engine will cut out just as you describe. I scrape it every time I clean the posts.

You'll probably do the easy things and leave the more expensive Hall effect sensor for last. The sensor is hard to pin down as the cause because they typically fail on their own schedule. A replacement from a wrecking yard may be an affordable alternative.

Mike