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Technical question re: ECU/PCM and fuel pump wire (green/white)

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Old 05-06-2012, 04:03 PM   #1
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Default Technical question re: ECU/PCM and fuel pump wire (green/white)

I am still problem solving around my fuel pump that runs continuously. I checked voltage with ignition ON and running directly at the wiring on the fuel pump (i have a trap door install).

I was getting a reading of:

11.65v at fuel pump power wire (grey) with just the ignition on
14.30v at fuel pump power wire with the engine running

9.20v at the ECU/PCM green/white wire (fuel pump) with just the ignition on.

I have done some reading and it sounds like the ECU sends a signal to the fuel pump relay (driver side engine bay) for a few seconds for the fuel pump to prime, and then should shut down with the ignition on.

WHAT EXACTLY SHOULD BE HAPPENING AT THE ECU DURING THIS FEW SECOND INITIAL PRIMING PHASE? Would the voltage from the ECU Fuel Pump (green/white) wire "drop" to shut down the fuel pump?

I am wondering if i don't have a bad ECU/PCM...and i don't know how to test that part.
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Old 05-06-2012, 05:51 PM   #2
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The ECU allows the pump to prime at ignition on, then will shut down the voltage signal after 2 seconds if the engine isn't running...if the engine is running, the ECU will send a signal to the fuel pump relay which will allow continuous 12v current to the fuel pump until you turn the engine off.
You still sound like you think the fuel pump should shut down when the engine is running, or am I misinterpreting what you are saying?
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Old 05-07-2012, 09:59 PM   #3
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No i get what you are saying. I don't think the pump will shut down, but with less voltage, (in theory) it should become "less" noisy?

I mistyped my voltage reading at the green/white pcm fuel pump trigger wire...it reads at 11.4v with just the ignition on.

The voltage doesn't drop at all (i left it running for up to a minute) with just the ignition on....the voltage stays between 11.3 and 11.4.

I'm guessing there is something wrong with my PCM/ECU....

What would your thoughts be on this?
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1998 6 spd manual Camaro Z28 with STS turbo setup; Turbonetics 75/75 turbo, Moser 12 bolt rear end, Forged Crate Champion Motors LS2 402ci engine (this one likes boost) - dyno (700+rwhp/700+rwtq @ 14# boost)
2010 Audi S5 Coupe 4.2L V8 6spd manual only one produced on current color combo for 2010... Mods to come (my new ride)
2011 Lexus RX 350 (wife's ride)
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Old 05-10-2012, 12:55 AM   #4
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Has anyone actually tested their ECM voltage with "just" the ignition on to see what their ECM fuel pump wire is doing? If anyone has or is willing to please respond or pm me...i can't move on until i figure out if my ECM is the culprit or not.
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1998 6 spd manual Camaro Z28 with STS turbo setup; Turbonetics 75/75 turbo, Moser 12 bolt rear end, Forged Crate Champion Motors LS2 402ci engine (this one likes boost) - dyno (700+rwhp/700+rwtq @ 14# boost)
2010 Audi S5 Coupe 4.2L V8 6spd manual only one produced on current color combo for 2010... Mods to come (my new ride)
2011 Lexus RX 350 (wife's ride)
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Old 05-10-2012, 12:10 PM   #5
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You should ideally be seeing 12v during prime, AND while the engine is running, anywhere along the path from the battery all the way to the pump. Your car is not a variable voltage system. If you are seeing less than 12v, you have a voltage drop issue somewhere, bad connection/ground. If you are seeing more than 12v, the only way is if you have a boost-a-pump hooked up. If after prime, and the engine does not start running after 2 sec's, the 12v signal to the pump should shut down. If your pump does not shut down after 2 sec prime with engine not running...then yes, you have an issue somewhere. Could be wired direct to ignition (bypassing PCM and relay), fuel pump relay stuck on/closed, oil pressure sensor stuck on; possible PCM issue but least likely. This is not the end of the world unless you get in a wreck and the engine stops and your fuel pump keeps pumping-spewing fuel everywhere and causes a fire.
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Old 05-10-2012, 12:10 PM
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