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Discussion Starter #1
I have a 91 S10 4.3 that the fuel pump won’t prime when turning the key on. It does not get voltage when the key is on but when it’s cranking it gets voltage and will start after some cranking. I have replaced both ECM fuses with 10A I had from other projects. I would like some other opinions before I rip apart the entire truck trying to find the problem. Thanks!
 

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Well...if you haven't gotten any opinions in two days with three different threads...it might take a little longer. I think you've pretty much covered all of the forums that pertain.
 

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Find the fuel pump relay and see if you get 12V on the grn/wht wire for 2 sec to see if a prime signal is being sent by the ECM. Maybe you can swap in one of the other relays if it's the same to see if the relay is bad. The reason it starts after extended crank is because on the older models, power is supplied to the pump as a backup when you build oil pressure.




 

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I'd guess it's your fuel pump relay. If you crank it long enough to build oil pressure, the oil pressure switch will send power to the fuel pump. But the relay has to be working for the initial prime.
 

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Discussion Starter #5
I did switch out the relay last night with a brand new one from autozone and it still doesn’t prime. I will look into the signal wires this morning.
 

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Discussion Starter #6
Find the fuel pump relay and see if you get 12V on the grn/wht wire for 2 sec to see if a prime signal is being sent by the ECM. Maybe you can swap in one of the other relays if it's the same to see if the relay is bad. The reason it starts after extended crank is because on the older models, power is supplied to the pump as a backup when you build oil pressure.





The grn/wht wire does get 12v when the key is cycled. I don’t know if it matters but the 12v hangs around longer than 2 seconds. This is with the relay pulled out of the socket and the multimeter probes in the terminals where the relay plugs into. I did plug in the relay again and probe the other side of the connection. When I cycled the key with the probes on the back side I got -0.08v. I’m pretty sure that my probes were making a good connection. I am truly stumped.
 

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Are you getting 12V to and from the relay when it activates? The relay might be good but has no power supplied. ECM B fuse. Broken wire? No ground on the black wire?
 

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Discussion Starter #8
Are you getting 12V to and from the relay when it activates? The relay might be good but has no power supplied. ECM B fuse. Broken wire? No ground on the black wire?
Is there some way to test all of these things? There is constant 12v from the orange wire, and I did replace the ECM A and B fuses with ones that I know work.
 

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The orn wire should be steady 12v and the blk/wht should be a ground. The other wire, not sure of color, maybe grey, is pump power and should read a few ohms thru the pump. That wire should also be common with the unconnected pump prime test connector that is usually located near the center of the dash panel in the engine compartment.


 

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Discussion Starter #10
The orn wire should be steady 12v and the blk/wht should be a ground. The other wire, not sure of color, maybe grey, is pump power and should read a few ohms thru the pump. That wire should also be common with the unconnected pump prime test connector that is usually located near the center of the dash panel in the engine compartment.


The orn wire has steady 12v, the blk/wht wire is a good ground, and i do get ohms from the grey wire to the test connector.

Does the prime have a separate wire going to the pump? If it does then that could have a break but the main power wire is still good and that’s why it only pumps while cranking?
 

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Re: Fuel pump won’t prime

No, one wire to the pump. Usually grey.
If you have power (orange), ground (black), signal 12V on green and when key is first turned on, and no voltage to the grey wire which goes to the pump there is only one thing that could be wrong. That Chinese replacement relay. It is supposed to connect the orange and grey when it receives the signal on the green and white by completing a circuit to the black ground which energizes the magnet that connects the orange and grey. If it it's not doing that it's not working. Maybe (I know this sounds impossible) they gave you the wrong one or somehow a defective relay got past the strict quality control in Shenzhen. :yikes:

Other than the oil pressure redundant power wire that is the whole fuel pump circuit. Besides the ground on the pump at the tank, which we know is good or it would never pump fuel.

 

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Since you say that A1 has power when you first turn on the key. That does tell us A1 is the source for both the prime and run to activate the relay.
Is A1 providing 12V during early cranking? If it is doing it during prime, but not when cranking then the problem is that the ECM is not receiving an ignition pulse to tell it to turn on the fuel. On later models a bad ICM is a common cause for no fuel / no start.
If you are not getting 12V out of the relay on the grey during the 2 second prime, it's the relay or wiring because ignition doesn't enter in at that point. If you are getting power on the grey during prime, but not while cranking it's either not receiving the ignition pulses to the ECM or in rare cases it could be the ECM. It is 28 years old.
Just thinking out loud here...
 

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Discussion Starter #13
Since you say that A1 has power when you first turn on the key. That does tell us A1 is the source for both the prime and run to activate the relay.
Is A1 providing 12V during early cranking? If it is doing it during prime, but not when cranking then the problem is that the ECM is not receiving an ignition pulse to tell it to turn on the fuel. On later models a bad ICM is a common cause for no fuel / no start.
If you are not getting 12V out of the relay on the grey during the 2 second prime, it's the relay or wiring because ignition doesn't enter in at that point. If you are getting power on the grey during prime, but not while cranking it's either not receiving the ignition pulses to the ECM or in rare cases it could be the ECM. It is 28 years old.
Just thinking out loud here...
If A1 is the Green wire coming from the ECM I am not getting anything during early cranking. I don’t get 12v on that wire at all during the key on and cranking process. This is with the relay plugged into its socket and multimeter probes sticking into the wire terminals from the back
 

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I thought in post #6 you said it was showing 12V when the key was first turned on? That would be for a few seconds only and not during cranking. If it is doing that then we know the ECM is functioning properly for the prime cycle. Try testing the grey wire out of the relay when you first turn on the key. It should duplicate the reading of the grn/wht. You have to wait a bit between tries for the ECM to reset for prime to occur.
You can't just go on off on off.
 

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Discussion Starter #16
I thought in post #6 you said it was showing 12V when the key was first turned on? That would be for a few seconds only and not during cranking. If it is doing that then we know the ECM is functioning properly for the prime cycle. Try testing the grey wire out of the relay when you first turn on the key. It should duplicate the reading of the grn/wht. You have to wait a bit between tries for the ECM to reset for prime to occur.
You can't just go on off on off.
The grn/wht wire gets 12v only when the relay is unplugged. When the relay is put back on there is no voltage on both the grn/wht and grey wires. It seems like i have power in all the right places with the relay unplugged but when the relay is plugged in, all hell breaks loose.
 

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That's odd since the relay low amp side doesn't draw much current. Under 200mA. Recheck your ground as that is the other side of the low amp circuit.
Only other possibility that comes to mind if the primer driver in your ECM is bad.
You could avoid replacing that by connecting a momentary switch on or below your dash via a fused wire to an always hot wire and to the grey wire at the prime connector under the hood. Then you could manually prime it when needed. Just hold the switch for 2-3 seconds before turning the key to start. Manually doing what the ECM is supposed to do and skipping the relay. Maybe a bit ghetto, but it'll prevent wearing out your starter.
Don't try feeding 12V into the grn/wht. That could damage the ECM.
 

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Discussion Starter #18
That's odd since the relay low amp side doesn't draw much current. Under 200mA. Recheck your ground as that is the other side of the low amp circuit.
Only other possibility that comes to mind if the primer driver in your ECM is bad.
You could avoid replacing that by connecting a momentary switch on or below your dash via a fused wire to an always hot wire and to the grey wire at the prime connector under the hood. Then you could manually prime it when needed. Just hold the switch for 2-3 seconds before turning the key to start. Manually doing what the ECM is supposed to do and skipping the relay. Maybe a bit ghetto, but it'll prevent wearing out your starter.
Don't try feeding 12V into the grn/wht. That could damage the ECM.
Before I go completely ghetto on this thing where is and how would you go about cleaning the ground. Thank you for all of your help trying to figure this out with me, it’s greatly appreciated!
 
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