Help! Truck dies after ~20 seconds

[Xenon]

Ok. Will look at the diagrams and see what I come up with.

[Xenon]

Well after looking at the diagrams the whole thing seems pretty simple.... Looks as if I must not be getting a 12v signal from pin a1 to the relay, either that or I've struck out and tried two bad relays, which is still possible I guess.

What conditions if any must be met for the ECM to trigger the fuel pump relay?

[RobertISaar]

probably the same as anything else GM did in the era:

at key-on, runs for a predetermined amount of time, then shuts off
after it shuts off, the only thing that will bring it back on are reference pulses or turning the key off and back on

that's the ECM controlled side of it anyways... a lot of engines also used oil pressure switches that would force the fuel pump on when above ~4PSI in case the relay failed. i believe that went away shortly after OBD2 was implimented?

[EagleMark]

What conditions if any must be met for the ECM to trigger the fuel pump relay?

Key turned on! May be a wiring issue still, or check your ECM fuses, I think stck truck has 2. If all is well just get rid of the gray box, 1project2many said a long time ago the box went bad and it drove him nuts to find it. Run A1, IIRC Green/White to turn on relay!

Here's a voltage checklist for ECM wiring.

[Xenon]

What/where is the location of this grey box?

So your saying if everything checks out good just run a wire from a1 to the relay so it can be activated by the ECM ?

[EagleMark]

That's the way it works on light duty without the gray box. It's under the dashboard somewhere? I have a harness out of a truck here with one on it. It has to be there or it would only prime 2 seconds not 20.

[Xenon]

Well I'm thinking I have a wiring problem of some kind... Not sure though. I have a wire running from he battery to the prime wire on the relay, and with the key off when you hook it to the battery t will run the pump as it should. No problem. Start the truck, after 20 seconds it still dies and the relay chatters. Unhook the wire and hook it back up with the key still on and it chatters again... Turn the key off and restart and it will do it all over again.

So I'm wondering what changes after 20 seconds that you can't manually jump the pump and make it run? Is something shorting out? Could bypassing the grey box solve it?

Kinda stumped, have to think this over.

[EagleMark]

Bad ground? The only reason I suspect the fuel module is it runs for 20 seconds, your having an issue at 20 second. Just have to check each wire.

[jim_in_dorris]

As a test, would it not make sense to wire 12 volts directly to the pump and see if it continues to run after the 20 sec. If it runs ok, then you need to check the A1 pin on the ECM for not only voltage, but on/off at the frequency of the relay chatter. Next I would check the relay ground for a floating ground. I would also check B1 and C16 for 12 volts steady.

[Xenon]

Ok. Will try that.

Thanks!

Edit: if I'm reading that diagram right, why doesn't it bypass the relay and run via the oil pressure switch as long as I have 4 psi oil pressure?

[EagleMark]

I'd swear that diagram is wrong and the wire from Relay should be on other side of switch. But is your switch wired right?

[jim_in_dorris]

IIRC I have had this discussion before, what the oil pressure switch really does is ground the fuel pump out when there is no oil pressure, shutting off fuel to the engine when you lose oil pressure in an attempt to prevent more damage. I ran my truck for 3 years without it even plugged in. Just cranking the engine should develop enough pressure to open the switch instead of grounding it.

[Xenon]

Well I unplugged the relay jumped 12v from the battery right to the tan/white wire wire in the plug and it ran great. Let it warm up ~15 minutes and it ran surprisingly well considering I'm just running the stock 350 tune for now...

Checked the voltages of the other wires while it was running and all were at a steady 14.2 volts... Put the ohm meter on the ground and it showed good...

So where do I go from here? My first thought is to get a continuous duty solenoid and run a keyed hot to it to switch it, and run the pump off that. Problem seemingly solved. There is probably a better answer, just don't know what it is yet.

[RobertISaar]

IIRC I have had this discussion before, what the oil pressure switch really does is ground the fuel pump out when there is no oil pressure, shutting off fuel to the engine when you lose oil pressure in an attempt to prevent more damage. I ran my truck for 3 years without it even plugged in. Just cranking the engine should develop enough pressure to open the switch instead of grounding it.

check the diagrams and you'll see this isn't how GM wired it. if you were to run the engine only based on oil pressure, you could just unplug the fuel pump relay entirely and the fuel pump will only get power when the pressure switch is above it's cutout threshold. cranking times will increase and you may or may not set a fuel pump voltage DTC.

modded version of the diagram Mark posted:



notice, ground is always present for the pump and +12V for the pump can come from AT LEAST 3 places:

path 1:battery->fuse->fuel pump relay(when relay activated)
path 2:battery->fuse->oil pressure switch(when above 4PSI)
path 3:ALDL G-terminal/prime terminal->fuel pump relay(when relay deactivated)

there MAY be a path 4.... the fuel module A pin could supply the 12V feed for the pump, but i don't know enough about it to confirm it.

in reality.... when the engine is running, if the pressure switch is acting normal, it's actually supplying some(probably close to half) of the current to the pump as well, since it will be a closed circuit when above 4PSI, so load will be split between the relay and switch.

[jim_in_dorris]

Robert, not trying to start an argument, just clarify things a little in my mind. If the Oil pressure switch did not cut the fuel pump voltage when oil pressure was lost, then why have it? After all, like I said, my switch was unplugged for years with no problems.