I've recently picked up a 1991 RS 3.1 car. It has a new fuel pump and has pressure when cranking but I can't get it to prime. I tested the pump with a jump wire at the connector behind the seat and it does work. My constant power wire wwith the fuse is good. I have power at the relay plug on the orange but even if I jump to the red fuel pump prime wire at the relay plug I get nothing. any ideas?

 

Sounds like there is a break in the wire between the relay and the fuel pump.

Test with continuity on a DMM between the relay output and the fuel pump wire at the tank. Keep in mind that even if you do have continuity, there may be a corroded connection that won't allow power to flow. The fuel pump takes a lot higher current than it takes to simply test continuity.

 

After a hour or so of chasing wires with a test light I have power from the relay to the red prime wire, power to the oil pressure sending unit, but not coming out. Jumping a wire to the grey pump wire on the oil pressure sender plug it got power to the pump. I ordered a new sending unit that should be here tomorrow. Thanks for the response mate!

 

The sending unit won't keep the pump from running, it's in parallel with the relay.

If you can feed power in the test lead and get the pump on, then you will need to look at the power feed to the relay (and OPSU), the relay itself and the signal from the ECM (Green/white wire, that is a positive voltage trigger). The Green/White wire will turn on for a few seconds to prime, and then be on any time there is a DRP (Distributor Reference Pulse), AKA an "RPM signal" from the distributor.

 

I have fuel pressure while cranking, but no prime. I'll check the wire from the ECM next.

 

Well, the pump is now priming. but it doesn't seem like I'm getting a lot of pressure. the car has sat for a few years so I'll change the filter and see if that helps. I appreciate the help! I can't try and start it until tomorrow after my new dizzy gets here. the one in it now is extremely rusty. I can't even get the rotor off of the shaft.