need help fuel pump running
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Joined: Dec 2004
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From: michigan
need help fuel pump running
ok i have a 91 f150 4x4 the thing is the fuel pump keeps running i have replaced the relay for it but is still keeps running i shut off the truck take the key out and the pump keeps running untell the battery dies i really need help on this one
rael
rael
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Joined: Sep 2004
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The ignition switch is probably not the culprit. There's no fuse for the fuel pump.
The fuel pump is powered by the fuel pump relay. The relay contacts are powered off a fusible link attached at the starter solenoid, which in turn is "hot at all times" straight off the battery.
The FP relay coil is grounded by the PCM in order to cause the fuel pump to run. The PCM is powered up by turning on the ignition switch. The PCM should energize the fuel pump relay coil for two seconds, then turn it off. It will turn the relay coil back on once it sees pulses coming from PIP on the TFI module.
The wire labeled "FP" goes from the PCM, to the fuel pump relay, and to one of the pins on the 6 pin diagnostic connector. This wire should have 12 volts present with the key on and engine not running. If it doesn't, the FP relay coil is grounded somewhere, and the fuel pump will not shut off.
The first step in diagnosing this is to remove the fuel pump relay you just replaced. If the fuel pump STILL runs, "Butch" the electrician has been in there and hot-wired the fuel pump it somehow. If the fuel pump does not run with no relay, then it is almost certain there is something wrong with the FP line off the relay coil.
Probably the easiest way to avoid battery drain, until you get this fixed. is to use the inertia switch to interrupt power to the fuel pump.
The fuel pump is powered by the fuel pump relay. The relay contacts are powered off a fusible link attached at the starter solenoid, which in turn is "hot at all times" straight off the battery.
The FP relay coil is grounded by the PCM in order to cause the fuel pump to run. The PCM is powered up by turning on the ignition switch. The PCM should energize the fuel pump relay coil for two seconds, then turn it off. It will turn the relay coil back on once it sees pulses coming from PIP on the TFI module.
The wire labeled "FP" goes from the PCM, to the fuel pump relay, and to one of the pins on the 6 pin diagnostic connector. This wire should have 12 volts present with the key on and engine not running. If it doesn't, the FP relay coil is grounded somewhere, and the fuel pump will not shut off.
The first step in diagnosing this is to remove the fuel pump relay you just replaced. If the fuel pump STILL runs, "Butch" the electrician has been in there and hot-wired the fuel pump it somehow. If the fuel pump does not run with no relay, then it is almost certain there is something wrong with the FP line off the relay coil.
Probably the easiest way to avoid battery drain, until you get this fixed. is to use the inertia switch to interrupt power to the fuel pump.
Elder User
Joined: Sep 2004
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If you hadn't replaced the relay, I'd say that it had sticking contacts. This is a known failure mechanism of relays. Whacking the relay would free up the contacts and shut off the fuel pump. But you say it is a new relay.... Even if the fuel pump had a low resistance and was overloading the relay, it is unlikely that the new one would be sticking right away...
If there is some chafed insulation near the fuel pump relay, it is possible that the FP line is shorting to ground. This will hold the fuel pump relay engaged, and keep the fuel pump from shutting off. Whacking the relay, or the area around it, may be moving the wire and interrupting the short circuit, and allow the relay to drop out. You really need to measure the FP line on the diagnostic connector with a voltmeter or ohmeter or test light or something. Find out why it is at a low voltage / low resistance to ground when it should not be.
If there is some chafed insulation near the fuel pump relay, it is possible that the FP line is shorting to ground. This will hold the fuel pump relay engaged, and keep the fuel pump from shutting off. Whacking the relay, or the area around it, may be moving the wire and interrupting the short circuit, and allow the relay to drop out. You really need to measure the FP line on the diagnostic connector with a voltmeter or ohmeter or test light or something. Find out why it is at a low voltage / low resistance to ground when it should not be.
Elder User
Joined: Sep 2004
Posts: 941
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The yellow wire on the relay is the hot feed from the fusible link and the starter solenoid. This will be 12 volts all the time.
The red wire is the relay coil hot side; it goes back to the EEC power relay. This wire should be 0 volts with the key off, 12 volts with the key on. If this wire gets shorted to the yellow wire, it is possible for the fuel pump to run with the key off, though you should see other strange symptoms like the check-engine light staying on.
The light blue/orange wire is the relay coil ground side. This goes to the EEC to enable/disable the fuel pump from running. It also goes to the diagnostic connector. This should be 12 volts with the key on and engine not running. It should be 0 volts with the key off.
The dark green/yellow wire is the wire going to the fuel pump. It should be 0 volts with the key on and engine not running. It should be 12 volts when the engine starts. This wire also goes to the "FPM" input to the computer -- the computer uses this input to see if the fuel pump relay is responding correctly.
The red wire is the relay coil hot side; it goes back to the EEC power relay. This wire should be 0 volts with the key off, 12 volts with the key on. If this wire gets shorted to the yellow wire, it is possible for the fuel pump to run with the key off, though you should see other strange symptoms like the check-engine light staying on.
The light blue/orange wire is the relay coil ground side. This goes to the EEC to enable/disable the fuel pump from running. It also goes to the diagnostic connector. This should be 12 volts with the key on and engine not running. It should be 0 volts with the key off.
The dark green/yellow wire is the wire going to the fuel pump. It should be 0 volts with the key on and engine not running. It should be 12 volts when the engine starts. This wire also goes to the "FPM" input to the computer -- the computer uses this input to see if the fuel pump relay is responding correctly.
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