Fuel pump relay and resistor: the 60 second test

[John E Davies]

I have kind of been wondering if the fuel pump resistor is working right on my '96 LX450... I read up on the test procedure in the manual and checked the resistance, which should be around 0.7 ohms, but I also wanted to see if the relay was switching correctly. The fuel pump is supposed to go full rpm at startup, then switch immediately to low rpm for idle and low load conditions. Then at high load it should switch to full rpm again.

Here's the 60 second test:

Unplug the fuel pump resistor connector. The resistor is the aluminum heat sink thingy located to the rear of the cruise control black box on the right inner fender. It's just forward of the power antenna access hole. You need a small flat screwdriver to release the retaining clip.

Start the engine.

It should start and rev toward high idle normally, then die within a few seconds.

Reconnect the resistor.

Start engine - it should be back to normal.

It sounds like an idiotic test, but this proves definitively that the resistor is working and also the pump relay is switching between high and low speed. The test does NOT in any way test that the ECU is correctly telling the relay to switch to high rpm under heavy load - you would need a mechanical gage or factory tester to verify that. At least I think so - does anyone have a shortcut to test this mode?

The 60 second test is not harmful in any way - it merely removes the ground for the power circuit to the pump. I didn't get a Check Engine light after the test.

BTW, has anyone come up with an easy way to connect a test gage to the fuel rail or regulator without using a "special" test banjo fitting as per the manual?

John Davies
Spokane WA

 

[RavenTai]

does anyone have a shortcut to test this mode?

having been on the recieving end of a bad fuel pump realy I can say that there is an easier way to test,

engine stats and runs well?

No -> check fuel pump realy.

Yes -> fuel pump relay and resistor are good.

As far as the fuel pump and resistor are concerned with fuel pressure all you are worried about is whether the pump is pumping more than enough fuel for that particular condition, you could determine this by observing the return fuel coming out of the fuel pressure regulator

The actual fuel pressure is controlled by the fuel pressure regulator not the pump relay or resistor. The later three control volume.


I too wish that Toyota had provided a fuel pressure nipple like those seen on most other fuel injected vehicles, unfortunately without that nipple you will have to use some form of bolt in pressure tap like a banjo fitting