Wierd fuel delievery problem
Thread Starter
TheCorvetteBen
Joined: Feb 2007
Posts: 4,987
Likes: 144
From: Van Alstyne, TX
Wierd fuel delievery problem
Ok here is a little background. 1987 C4 convertible
Replaced the fuel pump and fuel pump relay, and got the car to start run and drive.
We figured we had solved the problem, so we decided it was time to get all the old gas out of the tank.
So we unhooked the sending unit at the tank, ran a line into a bucket and put 12 volts to the pump and had the car pump all the old gas out.
Filled the tank with fresh gas.
Problems:
When the fuel pump is jumped with a separate wire, it works like it should, but no power when the car cranks.
Fuel pump relay lights up the test light at the orange wire, but then loses voltage when the key is turned to on. I has assumed that it was supposed to get voltage for two seconds after the car was switched to on, and then shut off. Seems to be working backwards.
Thoughts on troubleshooting?
Power wire to the little junction behind the battery ohms out as well.
Replaced the fuel pump and fuel pump relay, and got the car to start run and drive.
We figured we had solved the problem, so we decided it was time to get all the old gas out of the tank.
So we unhooked the sending unit at the tank, ran a line into a bucket and put 12 volts to the pump and had the car pump all the old gas out.
Filled the tank with fresh gas.
Problems:
When the fuel pump is jumped with a separate wire, it works like it should, but no power when the car cranks.
Fuel pump relay lights up the test light at the orange wire, but then loses voltage when the key is turned to on. I has assumed that it was supposed to get voltage for two seconds after the car was switched to on, and then shut off. Seems to be working backwards.
Thoughts on troubleshooting?
Power wire to the little junction behind the battery ohms out as well.
Race Director
Joined: Apr 2002
Posts: 10,036
Likes: 346
From: Anaheim CA
The ECM turns on the fuel pump for 2 seconds (through the fuel pump relay) when you turn on the ignition. That pressurizes the fuel rails to prepare for starting. The turn off after 2 seconds stops the fuel pump from running if you turn on the ignition without starting the engine.
When the ECM sees reference pulses coming from the distributor (engine cranking) it turns the fuel pump on (through the relay).
If the ECM fails to turn on the fuel pump for some reason or the fuel pump relay fails, the oil pressure switch will send 12 volts directly to the fuel pump, bypassing the fuel pump relay.
You can force the fuel pump to run by putting 12 volts into terminal G of the ALDL connector. The connection goes through the fuel pump relay and it must not be energized to do this.
When the ECM sees reference pulses coming from the distributor (engine cranking) it turns the fuel pump on (through the relay).
If the ECM fails to turn on the fuel pump for some reason or the fuel pump relay fails, the oil pressure switch will send 12 volts directly to the fuel pump, bypassing the fuel pump relay.
You can force the fuel pump to run by putting 12 volts into terminal G of the ALDL connector. The connection goes through the fuel pump relay and it must not be energized to do this.
Team Owner
Joined: Jan 2003
Posts: 20,322
Likes: 248
From: Summerland B.C. Canada
Orange wire is direct to battery through a fusible link. Turning the key on should not cause no voltage where orange enters relay. Trace that wire back and see if someone has tied it into an ignition controlled power source, or if your fusible link is hanging by a thread, or poor contact at battery connection.
