My 1991 volvo 940 2.3 sedan fuel pump is not working. I have a button that comes straight from the fuel pump (previous owner made that) and when i press it, the fuel pump works. It used to work with ignition too, but it doesn't anymore. Fuel pump fuse and relay is fine. Sometimes it doesnt even crank while trying to start it with ignition swith. What could be the problem?

 

there's a reason the pump is wired to the ignition... sounds like a good time to get your car back to stock (and safety). Since you have home made wiring you don't know if the wiring is bad or your pump is bad so you'd need to test the pump with a reliable 12V source. What happens if you bypass your button? Even if it is a bad pump, I'd say its time to fix the wiring back to stock (ie remove the button, correct with wiring, make sure the ignition switch is providing power etc).

 

How can i make sure ignition switch is providing power? Where can i find the diagrams to know how the wiring should be?

 

if it doesn't crank over when you turn the ignition switch, there's a pretty good chance the ignition switch itself is worn out.

by design, the fuel pump only runs about 1 second when you first turn on the key, then when the engine is turning over it runs until the engine stops turning over. this is to reduce the risk of engine fires is a collision.

the Volvo wiring diagrams were published in a series of 'green books' (so called because they have green covers), one for each model year (as there were wiring changes through production). The 1991 940 wiring diagram is TP-31781-1 ... these are all out of print now, the only way to get them is from the Volvo Owners Club, they used to be on a DVDrom, but it looks like its a download now with a usb access key, https://store.otpubs.com/940-960-s-9...-1990-to-1998/