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Thread: Mom need help!! 96 Chevy 1500 cranks but wont start

  1. #11
    Senior Member DaveDanger's Avatar
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    Jelaine3, I've discovered in troubleshooting fuel pump problems of my own (96 Silverado Pickup) recently that there's several possibilities that can make that engine crank over but not start.
    The following 2 items are not the only things that can affect your son's problem, but they need verifying.
    The fuel pump receives electrical power thru a parallel circuit; it begins when the key is turned to the run position by sending current thru a relay in the "underhood relay center". The relays (about 4 or 5 depending on how the vehicle is equipped) are labelled inside the top cover of the relay box. They simply plug in and pull out as necessary to test or replace. The fuel pump relay is a 5-pin relay, and powers the fuel pump during the start process. As soon as the engine does fire and run, oil pressure should build up almost instantly. When it reaches at least 4 psi, the oil pressure switch becomes the circuit that provides electrical power to the fuel pump, and supposedly the fuel pump relay goes to sleep, un-needed during the normal run. (I'm not 100% convinced the relay does this, but that's a whole 'nother story!)
    The oil pressure switch is considered the safety switch in the event of a crash. An impact that is sufficient to stop the engine also stops the oil pressure when the engine quits. The electrical power is removed from the fuel pump and fuel is not being pumped all over a hot, possibly sparking engine.
    Verify that the relay is providing electrical power to the fuel pump when the key switch is turned to the run (not crank) position. It should run the pump audibly for about 2 or 3 seconds. I've had old and new fuel pumps in my truck, and I've been able to hear them all if there was not a huge amount of ambient noise. The radio on at normal volume is enough to hide the pump noise.
    Verify that the oil pressure sending unit is plugged in properly. It's right down under/behind the distributor. It would be very easy to have disconnected that while working on the distributor, and missed reconnecting it.
    DaveDanger A&P/IA
    "I have a mind like a steel trap. Everything that goes in, comes out mangled and full of holes".
    '96 C1500 Silverado 4.3L '99 C1500 Suburban 5.7L '83 Honda GL1100A Aspencade

  2. #12
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    ok so the oil switch/sending unit is basically a sensor? and having had all sorts of oddball sensors in diff vehicles go bad over the years, if this quit working while we were driving would it have caused the pump to quit and therefore the truck to just die? I get the part about how it stops the pump after an accident--so along those lines when the engine unexpectedly quits for mechanical reason, does it also cause the fuel pump to shut down and perhaps not let it restart? how is the switch "reset"? We know the fuel pump initially starts up via the relay we can hear it. but if the oil switch isnt working, or is "off" is that preventing the fuel pump from making the changeover to the other circuit to keep it running?

  3. #13
    Senior Member DaveDanger's Avatar
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    [quote=Jelaine3 ok so the oil switch/sending unit is basically a sensor? and having had all sorts of oddball sensors in diff vehicles go bad over the years, if this quit working while we were driving would it have caused the pump to quit and therefore the truck to just die? I get the part about how it stops the pump after an accident--so along those lines when the engine unexpectedly quits for mechanical reason, does it also cause the fuel pump to shut down and perhaps not let it restart? how is the switch "reset"? We know the fuel pump initially starts up via the relay we can hear it. but if the oil switch isnt working, or is "off" is that preventing the fuel pump from making the changeover to the other circuit to keep it running?[/quote]

    (Pardon the length of reply, In the interest of clarity, I sometimes over-explain)
    Not exactly a sensor, as the other sensors in the system function. It is more accurately an on-off switch, providing or stopping electrical power to the fuel pump depending on whether it sees sufficient oil pressure or not. A sensor would typically send a "signal" to the ECM and allow the ECM to determine what to do, the oil pressure switch in this instance simply opens or closes the circuit to the fuel pump motor.
    Per the wiring diagrams I've studied on this system, the oil pressure sending unit actually has 2 circuits going thru it, one operates the oil pressure gauge at the instrument cluster, providing a variable current to send the gauge needle up or down. The other circuit is a simple open or closed circuit based on whether or not it sees 4 psi oil pressure.
    The switch does not have a "reset" function. It either sees pressure and closes, or it doesn't see pressure and it opens. (The default position is open, as when the engine is turned off intentionally). An accident (that stops the engine) would be one instance where the oil pressure would fall to zero, opening the circuit and stopping the fuel pump... another would be insufficient oil quantity, where the oil pump simply couldn't pick up enough oil to pump thru the system. Again, the oil pressure switch would see zero (or near zero) pressure and turn off the fuel pump. In either case, whether engine repairs are effected, or more oil is poured into the crankcase... when the engine begins to spin over and the oil pressure switch sees more than 4 psi pressure, it allows the fuel pump to operate. If you are in fact hearing the fuel pump run initially, during the cranking process, the possibility exists that the oil pressure sending unit may be damaged, or unplugged, or shorted internally in some fashion... not allowing it to either "see" the oil pressure, or to allow the circuit to complete to the fuel pump. (Remember, 2 circuits inside the sending unit) just because you see oil pressure at the instrument panel gauge, doesn't mean the other circuit is complete and sending current to the fuel pump. In theory, if you were to unplug the 3-wire connector at the oil pressure sending unit, you would lose BOTH the oil pressure gauge reading and the current to the fuel pump. The wire connector at that oil pressure sending unit is a 3-wire plug, and you either have both circuits connected, or both disconnected, can't easily separate them.
    My thought is that your Son's truck may have a wire cut, pinched, damaged, shorted or pulled out of the connector at that oil pressure sending unit, as a result of the work performed on or near the distributor.
    DaveDanger A&P/IA
    "I have a mind like a steel trap. Everything that goes in, comes out mangled and full of holes".
    '96 C1500 Silverado 4.3L '99 C1500 Suburban 5.7L '83 Honda GL1100A Aspencade

  4. #14
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    well from what its sounding like is the timing is a bit restarted or to far advanced as for doing anything to the valves id run a compression test across the cylinders before i went digging internally if at most it could be a bad crank position censer but i would think that a code would come up for it if all else fails id look into disconnection the remote start iv had many bad things come into play with diy install jobs if need be as a off beat idea to keep in mind is see if you cant strip the electronics back on it to run on the moter its self

  5. #15
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    Thank you to everyone who responded and offered help. Mechanic found that distributor had jumped 180 again, so further tear down found a bad cam that's allowing the distr to jump and slip, and thus the timing issue/no start. sooooo......now we're looking for a cheap motor to drop in it. thanks again!!

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