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fattonysirocz28
New User
Apr 19, 2008, 9:32 AM
Post #1 of 2
(1660 views)
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no power to fuel pump
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1985 chevy irocz28 with the 305 and 700r four speed
just had a complete tune up, plugs,wires, cap, coil.
ran great for one week. Tuesday morning the fuel pump didn't hum. i checked fuses/relays all good. pulled pump tested still good. traced wires to the best of my ability no breaks. i do not know what to look for next, none of my chiltons shows schematics.
Thank you,
Fattony
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DanD
Veteran
/ Moderator
Apr 19, 2008, 2:48 PM
Post #2 of 2
(1654 views)
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Re: no power to fuel pump
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The fuel pump on this vehicle and a lot of GM’s are fed power in two different ways; one from the fuel pump relay and the other from the oil pressure switch (OPS) as a back up, if the relay fails. Both of these sources of power to the pump, receive there power from the same orange wire that comes directly from a battery source, through a fusible link. A fusible link is a short section of wire (2 or 3 inches long), two sizes of wire diameter smaller then the wire (circuit) it’s protecting. You’ll find this fusible link close to the battery positive terminal or on the firewall, at a junction block or at the large bolt-on connector of the starter.
How the systems are supposed to work is; when the ignition switch is turned to the run position; the ECM (computer) energizes the relay pull-in coil, via the dark green/white and black wires. The relay then closes a connection between the orange relay feed wire and the tan/white wire of the relay. Power then leaves the relay on the tan/white wire to the bulkhead (firewall) connector. The tan/white wire turns into a solid tan wire leaving the connector and goes to the fuel pump.
If the ECM doesn’t see an RPM signal within 2 or 3 seconds the ECM turns off the relay, shutting down the fuel pump. As soon as the ECM does see an RPM signal, it will reenergize the relay again.
Now if for whatever reason the fuel pump relay fails to energize and close or keep closed the connection, between the orange and tan/white of the relay, is where the OPS comes into affect.
As long as there is oil pressure to the switch, the contacts of the switch remain closed; making a connection between the same two wires; the orange and tan/white. All these tan/white and orange wires of this circuit are wired in parallel; two different paths for current to flow.
One of the symptoms of a failed relay, is that the engine has to be cranked an excessive amount before the engine starts.
Oil pressure has to build to approximately 4 PSI before the contacts of the switch will close turning on the fuel pump.
If both the relay and the OPS cannot turn the pump on you may have.
• An open (burnt) fusible link.
• An open in the orange wire to the relay and the OPS.
• An open in the tan/white, after they leave the relay and OPS going, to the bulkhead connector.
• Bad connection in the bulkhead.
• An open on the solid tan wire between the bulkhead and the pump.
• Both the relay and OPS could be bad (not likely but I have seen it in the past) and you’ve been running on one or the other until now.
If you need help with the tests, post back and I’ll try and help.
One thing to check is that the black wire(s) at the fuel pump sending unit have/are connected to a good ground.
All of this should be tested with a test light; a test light will add a small load on a circuit, where a voltmeter will not. Sometimes you need to have a load on the circuit for a bad connection to go open.
Hope I didn’t confuse you?
Dan
Canadian "EH"
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