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Fuel Pump Not Working, Low Voltage (4v)


Raymond Chen
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I currently have a 1991 Lexus LS400 that will not start, even though it cranks. I have narrowed down the problems to be fuel system related, and I have already removed the fuel pump from the portal behind the rear seats. When I attached a multimeter to the terminals on the fuel pump, and turned the key to crank the motor, I found that I am not getting the full 12 volts as I should, but instead, I am getting only 4 volts. Can someone please tell me what would cause this?

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The fuel pump supply is controlled by the fuel pump relay and has 2 outputs.

One supplies the fuel pump under cranking conditions and the other supplies it during engine running mode but is via a ballast resistor.

I would 1st check the supply voltage with the ignition on but not cranking the engine.

As you are getting some supply to the pump I would go for the fuel pump relay 1st ( easy to change, passenger side fuse/relay block) then check the ballast resistor.

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