Please help me :(

[rixskgw]

Hello, i have a 1991 Lexus LS400, heres the problem, i know this car has a dual speed fuel pump, well one day, the car was acting like it was low on gas when u give it gas it acted as if it was struggling to get gas, i check the voltage at the fuel pump and it read 9 volts, well i know when the car is floored it should go back to 12 or so volts then back down to 9 when in idle and such, well its stuck at 9 volts, like a relay is stuck, i replaced the fuel pump relay and the capacitor thing with metal fins on it that cuts down the voltage.. i did a voltage test at the capacitor thing and its constantly getting voltage, its like theres something telling it to always go threw the capacitor to lower the voltage to my fuel pump, i hear there is another relay somewhere called a fuel pump speed control relay , is this true? and if so , where is it? and what could be causing this problem, i would really appreciate some help thanks

 

[Zuffen]

You are right. There is a fuel pump speed control relay.

An LS400 owner should be able to tell you where it lives but I would look in the fuse box under the hood, thats where the Crown has it. From memory they are green.

Also try looking in the owners manual it should be listed there.

You're diagnosis sounds correct to me.

 

[rixskgw]

thanks , i checked under the fuse place and couldnt find the speed control relay, unless its called something else one the diagram..

 

[Lextreme II]

I think this link might help

 

[rixskgw]

Thanks for that pdf file, i saved it but i still dont find the speed control relay only the main one, is there even a speed control relay on this car of mine?

 

[rixskgw]

would anyone else have anymore pdf files to my car? they are helpfull, thanks,,

 

[Lextreme II]

Repair Diagrams

 

[rixskgw]

I have replaced the Ecu, the main relay the circuit opening relay, the fuel pump relay, and the fuel pump! and still it wont switch back to 12 volts, there has to be a reason for this! what else controls the fuel pump!!! and i am going to order those diagrams soon, will they tell about this?

 

[Lextreme II]

I tried to find the diagram for you, but seems like the speed fuel pump relay might not exist.

 

[rixskgw]

hmmm, what in the world else could be wrong then? i replaced the Ecu of the car, so i dont know what else it could be...

 

[rixskgw]

I just noticed the relay will open and close when u really make the car go vroom vroom with fast throlltle movements, so could there be a sensor dirty? i replaced the throttle position sensor already too..

 

[Lambo]

Fuel Pump Slow

Hi, maybe i can help, the fuel pump ecu is controlled by the main engine ecu, which ecu have you replaced, the engine ecu outputs a signal on pin 22 and 21 that control the fuel pump ecu, which relay can you hear clicking there does not appear to be a fuel pump relay only a efi main relay, also check there is no damage to the wires going to any of your oxygen sensors they are part of the fuel pump circuit and if damaged would not return a signal to the engine ecu.Also check that the main connector to the engine ecu is not full of oil vapour like mine was, try disconnecting it and clean the pins, hope this helps
Best regards
Lambo

 

[Dreamkiller]

The fuel pump speed control relay is green on the crown.
It doesnt say on the relay that it is a speed control relay!
I think it was located next to the right hand kick panel, where the bulk of the relays are found for abs, steering etc.
I can take a pic of the relay and post it if you like.

 

[rixskgw]

I took the cap off the fuel pump control relay and when the engine is reved high enough it clicks over, which is what its suppost to do i have read in a diagram, but i dont think its switching in the right times or so, but i changed the main engine Ecu... also, i have a used fuel pump which could be bad, because when i leave the car off for a min or so its hard to start like the check valve in the fuel pump is not holding the pressure, so it could be a bad fuel pump...

 

[Lextreme II]

Its possible the pressure switch behind the gas padel is stuck? For the LS Model, there is a pressure switch. When u gun it, this switch will activate.

 

[rixskgw]

Just checked, Nope, the switch isnt stuck....

 

[rixskgw]

Can someone do me a favor? take the little lid off of the control relay and rev the motor and tell me how much rev does it take to make it switch back and forth? this would be a great help and would tell me whether its the electrical side or the fuel pump

 

[Lextreme II]

Possible ur gas filter is dirty since u ran low on gas or possible ur injectors are dirty too?

 

[rixskgw]

I changed the gas filter and ran injector cleaner a couple times before, still didnt help

 

[sideshow]

there is no change over relay


the soarers have a fuel pump ecu
its not really and ecu but its called fuel pump ecu

this is a little unit at back of car under the left side i think
open the boot and stick yr hand under the left hand trim / panel


the fuel pump relay sends power to this unit

the engine ecu sends a signal to it to turn on f/pump
the engine ecu then sends another signal to make it go from 9 to 12 volts

this unit sends the power to the pump

its a small heat sink style unit

it has hi and low speed relay incoporated inside it

i just realised yr car is an ls400

not to sure if its the same as what i explained