Starting problem

[PlayingWithTBI]

Here's some reading material for ya, enjoy!

http://www.gearhead-efi.com/Fuel-Injection/showthread.php?3256-Tuner-Pro-FAQ-and-Tutorials

 

[texas tough]

None of what follows is important for solving the problem with the truck in this thread. However, I think it's important to correct a small misunderstanding or two.

Agreed.




No. I'm looking at the wiring diagram for my '88 K1500 (C-K , 1500-3500 is all the same) I believe what follows is "GM Standard" engineering, including vehicles other than GMT400, and model years other than '88.
There are two things that are not correct.
1. The normally-closed contacts on the fuel pump relay are not part of the active circuit when the pump is driven by the oil pressure switch. The normally-closed contacts do have a useful function, but NOT with the oil pressure switch. More on that later, and
2. 400 RPM has nothing to do with anything in the fuel pump power supply.

this is correct, I never looked at the wiring schematic, when I saw the fuel pump relay configuration, I assumed, it was a switched circuit., thanks for taking the time to verify.,., this is how we all learn.. as far as ignition side, the way I understand, is it is a bypass ignition system.. the module runs the firing until the engine reaches 400 rpms, then the ecm senses this from the module and takes over spark advances operation IF the timing set connector is plugged in. the KNOCK sensor can also retard timing thru the module mounted on the bracket next to the map sensor. The engine can fire on its own without the ECM being involved, but in order to get fuel, the ecm has to get a signal from the ignition switch initially to prime the pump, and a feedback signal from the distributor module to engage the fuel pump relay or the oil pressure switch circuit.. is this right?
Power for the fuel pump comes from the battery or the alternator, via the Junction Block on the cowl, under the lil' plastic cover on the passenger side. A fusible link supplies protection to Circuit 440, orange wire. This circuit splits into two branches. One branch supplies power directly to the oil pressure switch, the other branch supplies power to the Normally Open (NO) contacts of the fuel pump relay. The power to the oil pressure switch does NOT go thorough the Normally Closed (NC) contacts of the relay, in fact you could REMOVE the relay and the oil pressure switch would still turn the pump on when it sensed enough pressure to close it's contacts.

If the relay and the oil pressure switch (and the wire harness connecting this mess together) is properly functioning, the current that drives the fuel pump is split between the relay and the oil pressure switch based on Ohm's Law--the division of current may not be equal depending on which part(s) of the circuit have higher resistance.

If the oil pressure switch fails--or the engine has really terrible oil pressure, the pump will be powered by the ECM which energizes the fuel pump relay. The ECM must sense that the engine is running via signal from the pickup coil and ignition module. No ignition pulses, relay is turned off, pump doesn't run.

Conversely, if the relay fails--or is removed--you only need oil pressure to run the pump, but crank times are extended while the engine builds oil pressure.

I believe that the ECM needs to see 400 RPM to begin computer-controlled spark advance, but NOT to power the fuel pump.

What do the NC contacts in the fuel pump relay do? They're connected to a wire, Circuit 490, which if manually connected to system voltage, will run the fuel pump for testing purposes. The ignition does not even have to be "on".

For any operation of the fuel pump--powered by the ECM-energized relay, powered by the oil pump switch, or powered by the test wire, the fuel pump inline fuse must be good. A popped inline fuse will prevent fuel pump operation.

There is also the "Hot Fuel Handling Module" which is connected to the fuel pump relay and the fuel pump circuit on the protected side of the inline fuse. It's tied into the aux. cooling fan relay, and increases the "priming" time. I've never seen a vehicle that has this module. In '88, only LO5 and L19 got them, and only then in some markets. (High altitude? California?)

 

[texas tough]

there has to be a signal that comes from the ECM , that is momentariliy energizing the fuel pump, then energizing it while cranking, .. I suspect this is sensed in the ignition module, then cent to the ECM. ?????

 

[texas tough]

Im going to go unplug my fuel pump relay, and see if it runs good.. BRB.....

 

[texas tough]

None of what follows is important for solving the problem with the truck in this thread. However, I think it's important to correct a small misunderstanding or two.

Agreed.




No. I'm looking at the wiring diagram for my '88 K1500 (C-K , 1500-3500 is all the same) I believe what follows is "GM Standard" engineering, including vehicles other than GMT400, and model years other than '88.
There are two things that are not correct.
1. The normally-closed contacts on the fuel pump relay are not part of the active circuit when the pump is driven by the oil pressure switch. The normally-closed contacts do have a useful function, but NOT with the oil pressure switch. More on that later, and
2. 400 RPM has nothing to do with anything in the fuel pump power supply.

Power for the fuel pump comes from the battery or the alternator, via the Junction Block on the cowl, under the lil' plastic cover on the passenger side. A fusible link supplies protection to Circuit 440, orange wire. This circuit splits into two branches. One branch supplies power directly to the oil pressure switch, the other branch supplies power to the Normally Open (NO) contacts of the fuel pump relay. The power to the oil pressure switch does NOT go thorough the Normally Closed (NC) contacts of the relay, in fact you could REMOVE the relay and the oil pressure switch would still turn the pump on when it sensed enough pressure to close it's contacts.

If the relay and the oil pressure switch (and the wire harness connecting this mess together) is properly functioning, the current that drives the fuel pump is split between the relay and the oil pressure switch based on Ohm's Law--the division of current may not be equal depending on which part(s) of the circuit have higher resistance.

If the oil pressure switch fails--or the engine has really terrible oil pressure, the pump will be powered by the ECM which energizes the fuel pump relay. The ECM must sense that the engine is running via signal from the pickup coil and ignition module. No ignition pulses, relay is turned off, pump doesn't run.

Conversely, if the relay fails--or is removed--you only need oil pressure to run the pump, but crank times are extended while the engine builds oil pressure.

I believe that the ECM needs to see 400 RPM to begin computer-controlled spark advance, but NOT to power the fuel pump.

What do the NC contacts in the fuel pump relay do? They're connected to a wire, Circuit 490, which if manually connected to system voltage, will run the fuel pump for testing purposes. The ignition does not even have to be "on".

For any operation of the fuel pump--powered by the ECM-energized relay, powered by the oil pump switch, or powered by the test wire, the fuel pump inline fuse must be good. A popped inline fuse will prevent fuel pump operation.

There is also the "Hot Fuel Handling Module" which is connected to the fuel pump relay and the fuel pump circuit on the protected side of the inline fuse. It's tied into the aux. cooling fan relay, and increases the "priming" time. I've never seen a vehicle that has this module. In '88, only LO5 and L19 got them, and only then in some markets. (High altitude? California?)

HOUSTON WE HAVE A PROBLEM.. ha
started my 89 up, let it run for abt 1 minute.. shut it off.. unplugged fuel pump relay, cranked for abt 12 seconds.. no start.. retry,, no start.. has a brand new A/C delco oil pressure switch and gauge works.. so we need to explore this further.

 

[texas tough]

looking at the wiring schematic, it looks like the truck SHOULD still start without the fuel pump relay,,,, im going to go pull the realy in my 94 and see what happens,, BRB

ok same results on my 94k1500,., unplug relay, cranky, cranky,, no start.

 

[Mark Gilbert]

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According to this drawing it should be able to run without the relay, but does the engine create enough oil pressure to trigger the switch during cranking, or does it need more.

I wasn't aware that the PCM provided signal to the relay during anything more than the Prime cycle, but the only way to verify that would be to pull the oil pressure switch connector while the engine is running and see if the engine dies. or check PIN D on the relay while the engine is running. I don't have a TBI engine to check myself.

 

[texas tough]

OK looked at a factory wiring diagram for my 94 k1500.. the OIL PRESSURE SWITCH is not in the fuel pump circuit,.., the reason for the open and closed position in the fuel pump relay, is for the fuel pump prime circuit when you initially turn the key on. then when you crank, the ecm signals the relay to switch to the normally open terminal, allowing power to go to the fuel pump..

https://www.justanswer.com/chevy/2yofu-find-1994-chevrolet-factory-electrical-wiring.html

 

[Mark Gilbert]

So, it changed. That would explain why we all have a different explanation. Like the image I posted which is used from 87-91 trucks the oil pressure switch is in the circuit. 16147060 PCM which was used until 93 also used the oil pressure switch.

It seems like 94 changed things. Now I am curious to verify if the relay is triggered by the PCM for anything other than priming for 93 and older vehicles?

 

[texas tough]

only needs 4psi to close the switch