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Fuel pressure relay question

BOOGI3 M@N

BOOGI3 M@N

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I just bought a 1991 k5 with a 5.7 in it I have been having trouble starting it. It cranks but will not start until oil pressure starts to go up. I have narrowed it down to the fuel pump relay, the person that I bought it from replaced the wire plug and all the wires are white I don't know where the wire that should be red according to diagrams connects to.
 
Where does the red wire go I don't have any loose wires around there to connect it to.
 
The red wire is a jumper. Use a jumper wire from the positive side of the battery to the red wire. It should kick your fuel pump on. It is a relay bypass to check power to the pump. The red wire is about 4 inches long and just hangs there. Its for testing purposes.
 
Ok thanks a lot that's a relief there is no missing wire. Is there a way to bypass the relay. I have a wire that has been tied into my orange wire. It is to dark to take pic. of it right now but I could post one tomorrow. It comes form a junction block on the fire wall on the driver side.
 
Here is a test. Have someone turn the key on while you listen at the fuel tank to see if the pump kicks on. If it does. The relay is working.
 
It doesn't I have done that. Start with replacing relay?
 
Well something is throwing me off. If the truck eventually starts when the oil pressure comes up, then the relay is kicking on. Something isn't right.
 
What I would do would be to unhook the fuel supply line and turn the key on. If gas spews out of the line, then relay and pump are good.
 
The relay on the firewall is needed to start the truck initially,then the oil pressure relay takes over,what sounds like is happening with yours is the fuel pump relay on the firewall is not working,and when you are cranking your engine it won't start till it builds oil pressure
 
snowdriver said:
The relay on the firewall is needed to start the truck initially,then the oil pressure relay takes over,what sounds like is happening with yours is the fuel pump relay on the firewall is not working,and when you are cranking your engine it won't start till it builds oil pressure
Click to expand...

That is not correct. Look at the wiring diagram (link in my signature). The oil pressure switch is on the hot feed to the relay, if it closes, the pump runs. Jumper the oil pressure switch with the key in run but engine off to test for yourself. Here is a link to the diagram: https://ck5.com/forums/attachments/eaglemark-gif.138958/ You can clearly see the oil pressure switch is spliced to the relay feed/ECM power straight from the fuse panel.

This is why the pump doesn't prime and there is a delay in starting his truck, but it still starts.

Likely a relay issue, but since the wiring has been monkeyed with, you need to ensure it is wired up properly. It obviously wasn't (no test lead) but you need to ensure that is the only problem, and not something that will cause you to waste time/money on parts that are not the issue.
 
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So I still have not figures this out went ahead and did a tune up replacing plugs and wire and dist. and several switches, sensors. It started up great the first time then wouldn't start tried to pull codes and I my check engine light will not come on. I thought I might have a bad ECM (which still might be the case) but I tested my orange wires (B1 & A8) coming into the ecm and they have no power. One of those wires go to my aldl port and the other is battery in which ties into my fuel pump relay which is has no voltage and my oil pressure switch, then test A6 which is pink and black that has power when you turn on ignition and it does which leads me to believe it is a short in the orange wires where they have been spliced together. The weather has been cold and snowy so I haven't had the motivation to trace those wires and see if my theory is correct. I put a few key words in here to hopefully help someone in the future I will post resolution when I find it. Any other thoughts would be appreciated.
 
Dyeager,thanks for setting me straight and and posting the correct info,schematic really helps as well,I was way off on this!
dyeager535 said:
That is not correct. Look at the wiring diagram (link in my signature). The oil pressure switch is on the hot feed to the relay, if it closes, the pump runs. Jumper the oil pressure switch with the key in run but engine off to test for yourself. Here is a link to the diagram: https://ck5.com/forums/attachments/eaglemark-gif.138958/ You can clearly see the oil pressure switch is spliced to the relay feed/ECM power straight from the fuse panel.

This is why the pump doesn't prime and there is a delay in starting his truck, but it still starts.

Likely a relay issue, but since the wiring has been monkeyed with, you need to ensure it is wired up properly. It obviously wasn't (no test lead) but you need to ensure that is the only problem, and not something that will cause you to waste time/money on parts that are not the issue.
Click to expand...
 
dyeager535 said:
That is not correct. Look at the wiring diagram (link in my signature). The oil pressure switch is on the hot feed to the relay, if it closes, the pump runs. Jumper the oil pressure switch with the key in run but engine off to test for yourself. Here is a link to the diagram: https://ck5.com/forums/attachments/eaglemark-gif.138958/ You can clearly see the oil pressure switch is spliced to the relay feed/ECM power straight from the fuse panel.

This is why the pump doesn't prime and there is a delay in starting his truck, but it still starts.

Likely a relay issue, but since the wiring has been monkeyed with, you need to ensure it is wired up properly. It obviously wasn't (no test lead) but you need to ensure that is the only problem, and not something that will cause you to waste time/money on parts that are not the issue.
Click to expand...

The TBI fuel pump wiring system is a little complicated, but I think SnowDriver may be somewhat correct. After looking at the schematic you posted, it appears, that when the key is turned to the "ON" position that it sends current to the ECM ORANGE wire terminals B1 and C16. The circuit is then completed through the ECM, and comes back out the ECM B2 terminal to the fuel pump relay "E" terminal, thus completing this circuit. Then when the starter motor is spun, and the oil pressure sensor reaches 4-PSI, it too completes its circuit (both circuits relay and oil pressure now completed), and then the fuel pump powers up. Where SnowDriver may have been wrong, is that both circuits must be completed to run, not just one then the other.

If I am wrong please explain it to me because I need to know for when I build a complete wire harness myself for my truck by hand.
 
From what I can see through the wiring schematics wire 440 .1.0 which is tied to your fuel pump relay ad oil pressure switch is a constant hot and should always have 12 volt and that is the B1 terminal in the ECM.
 
1-ton said:
The TBI fuel pump wiring system is a little complicated, but I think SnowDriver may be somewhat correct. After looking at the schematic you posted, it appears, that when the key is turned to the "ON" position that it sends current to the ECM ORANGE wire terminals B1 and C16. The circuit is then completed through the ECM, and comes back out the ECM B2 terminal to the fuel pump relay "E" terminal, thus completing this circuit. Then when the starter motor is spun, and the oil pressure sensor reaches 4-PSI, it too completes its circuit (both circuits relay and oil pressure now completed), and then the fuel pump powers up. Where SnowDriver may have been wrong, is that both circuits must be completed to run, not just one then the other.

If I am wrong please explain it to me because I need to know for when I build a complete wire harness myself for my truck by hand.
Click to expand...

If one end of the circuit has 12v (either the oil switch or from the relay) then the fuel pump will run. Doesn't matter if both are completed or not.

In this case, the relay is dead or incorrectly wired.
 
bp71k5 said:
If one end of the circuit has 12v (either the oil switch or from the relay) then the fuel pump will run. Doesn't matter if both are completed or not.

In this case, the relay is dead or incorrectly wired.
Click to expand...

It looks to me like the relay "A" terminal and the oil pressure switch are on the same circuit , and if either one of them is OPEN, then the fuel pump circuit is not complete.
 
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