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Discussion Starter · #1 ·
Hi I decided to post here instead of the "electronics" forum because this has to do with the operation of the engine.

On the "EWD" document I have found this:



it shows how the fuel pump relay is connected to the fuel pump. The strange thing is that in one case the relay gives direct power to the pump, in the other it goes through a resistor.

Anybody knows why? I thought the pump was always running unless the ECU had a reason to stop it, like in case of crash or some other engine trouble.
 

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Discussion Starter · #3 ·
STA is for the starter signal, so looks like the fuel pump operates in "reduced" capacity during cranking?
well I wonder if that is to "protect" the battery during that phase or to not "overload" the engine with too much fuel until it's started.
 

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Im thinking its to keep the fuel pump from going into full blast mode during cranking... I dont know why its necessary as the ecu should be controlling the injectors. Oh wait, arent FJs returnless fuel supply? Then its probably to keep the psi from surging.
 

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You need triggers for crank (engine not running) and run. The relay switches the load to run position when it senses the engines is actually runnng, usually this can be achieved by an air flow meter or off the ignition module/computer. Pretty shure that's the Yellow lead for relay's magnet coil. If the engine does not have spark or other critical function, it wont' let the fuel pump run on.
 

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On other Japanese vehicles I'm familiar with the fuel pump is resisted to run at 12 volts until there is a high demand for fuel...WOT, etc. Then under the extra demand the relay trips and full charging power 14v is sent to the pump, which is kinda like an "overdrive" mode if you will so the pump can be boosted and supply more pressure since engine demand is higher.

This doesn't look too different from those designs. However if you want to know for sure follow the wiring diagram and find out where the other end of the switch side of the relay goes. Knowing what source triggers the relay will help you find out when/why the voltage changes.
 

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Low speed/high speed. I had trouble with lean codes, and am in middle of T.S. I took a mini-fuse (15A) and shorted the connactor that went to the resistor. This got me 50PSI at the underhood Non Domestic market external fuel filter quick connects. Without this shorted, I was getting +35% fuel trims both engine sides. After this, my fuel trims were 0%.
Intelligent tester has this function :FUEL PMP SP CTL Fuel pump speed control ON (low speed)/ OFF (high speed). The repair manual states: "DESCRIPTION
As shown in the illustration, when the engine is cranked, current flows from terminal ST1 of the ignition
switch into the ECM and the ST (starter) relay coil and also current flows to terminal STA of the ECM (STA
signal).
When the STA signal and NE signal are input to the ECM, Tr1 (power transistor 1) is turned ON, current
flows to the coil of the circuit opening relay, the relay switches on, power is supplied to the fuel pump, and
the fuel pump operates.
While the NE signal is generated (engine running), the ECM keeps the Tr1 ON (circuit opening relay ON)
and the fuel pump also keeps operating.
The fuel pump speed is controlled at two levels (high speed or low speed) by engine condition (starting,
light load, heavy load). When the engine starts (STA ON), Tr2 (power transistor 2) in the ECM is OFF, so
the fuel pump relay closes and positive battery voltage is applied directly to the fuel pump. The fuel pump
operates at high speed.
During idling or under light loads, Tr2 goes ON, and then power is supplied to the fuel pump via the fuel
pump resistor. The fuel pump operates at low speed."
What is nice about this - you cannot overdrive anything, as the relay just passes current thru the resistor or straight to the pump.
 

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Yep, a simple two speed pump control.

The resistor is that gray metal thing with fins mounted on the driver's side inner fender wall aft of the fuse box.

If you are worried about someone steeling the truck, just unplug it to kill the fuel pump.

G
 

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This is great info and I would like to ask you if that's okay. I would like to add a fuel pump kill switch for dicey areas when parked in let's say, Chicago. The wiring seems to give 2 power sources to the pump. So to disconnect the pump, the best option is to interrupt the ground and not the hot side. Which wire in the pump harness plug is the ground wire? From a diagram I found, the plug has 5 terminals. . Which one is the ground wire?



Is there a point where these wires are running inside the passenger area or is it routed under the truck somewhere?
 

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