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Old 09-14-2010, 01:05 PM
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1990 Gmc truck fuel pump

I have recently converted my 1990 gmc TBI to a built 5.7 carb.! I want to keep my stock electric fuel pump for now, can someone tell me how to wire the fuel pump to bypass the relay switch the right way! So I have a steady fuel flow to my pressure reducer valve instead of the pump kicking off?? Any help is appreciated!

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Old 09-14-2010, 03:04 PM
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If you have eliminated the PCM you can use a standard "Bosch" type relay to run the fuel pump.
The coil of the relay (terminals 85 & 86) are connected to ignition ON and ground.
Contact (terminal 30 ) is connected to fused HOT AT ALL TIMES.
Contact (terminal 87 ) is connected to the fuel pump.
The fuel pump must have pressure switch to shut down the pump if the pressure gets to high.
Also, excess fuel must be able to return to the tank by way of a pressure relief valve similar to TBI setup.
I'm sure there are a number of products designed just for pressure reduction and relief situation.

vicrod
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Old 09-14-2010, 03:49 PM
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Thanx for the help vicrod, I'll look for the aftermarket products!
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Old 09-14-2010, 06:03 PM
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you could easily use the existing pump relay and pressure switch that is allready in the harness.The dark green with white tracer colored wire at the fuel pump relay is the 12 volt + from the pcm. Just hook it to a momentary switch to power it when first starting to prime the fuel system.Or better yet hook it to the purple wire that feed the starter its start energize and it will run the relay to prime the fuel system. After the vehicle starts the oil pressure switch will close and continue to run the pump. All the components are allready there ... nothing to by. Just change one wire. One other option would be if you have (which you dont) is to use the "I" terminal from the solenoid to energize the relay.Pull up the wiring diagram and you will see what I am saying.

Last edited by latech; 09-14-2010 at 06:10 PM.
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Old 09-14-2010, 07:40 PM
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That's more of what I was looking for! I knew there was a way to use the existing parts just was unsure how to!! This information is excellent !! Thank you very much! I'll keep u posted on how well it worx!!
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Old 09-15-2010, 03:22 PM
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Just a reminder that on the TBI cars and trucks, the relay is only used to run the pump for two seconds before the engine starts. Once the engine has oil pressure, the oil pressure switch bypasses the relay and keeps the pump running. The relay allows the ECU to run the pump for the "prime" function only. I'm not sure if the stock relay is rated for continuous duty, but I guess if the oil pressure switch is closed, the relay doesn't matter anyway.
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