mrmox

Sorry I have added confusion...mine is 1991: it has the 2 speed Fuel Pump scheme with a RELAY and a RESISTOR to choose the speed.

They revised this to a "black box" (Named Fuel Pump Control ECU for more confusion) hidden behind the panel beside the left rear seat, is this correct?

Anyone know what year?

Yamae

In case of a 1991, the story is not the same. The UCF10/11 and UCF20/21 have different power supply methods to the fuel pump and those 2 are not the same. Your UCF10/11=1991's fuel pump relay switches the power supply to the fuel pump directly to it or via the fuel pump resistor as is shown in the diagram below. The relay changes the current depending on the load driven by the main ECU. This method may cause a problem when the relay is failed because there will be a possibility of no power supply to the fuel pump when the contact is worn out. On the other hand, UCF20/21 has a different power supply method which only bypass the fuel pump resistor by the fuel pump relay shown like this. So, even when the fuel pump relay is failed, the fuel pump can get the power though the fuel pump relay and the car can run but not very fast.

In case your engine stops right after the start up, the important first step to check is the voltage at the pin #1 of the relay which is connected to the green wire. No voltage here simply means that the circuit opening relay is open/disabled by the main ECU or the relay itself is failed.

FloridaCam

96 LS does not have Fuel Pump Control ECU.

FloridaCam

I think you should spray starting fuid to see if the car run longer. If it does, then troubleshoot the fuel. If you have too, first with fuel pressure. You can intercept the fuel return line (from the fuel pressure regulator to the fuel tank) and connect with a hose to let fuel pour into a jar; if the fuel flows, then the pump is working.... That is just an example.
There are two fuses I would check first: 20 Amp yellow and IGN fuse. IGN fuse is the fuse box inside the car (driver side).

mrmox

@FloridaCam So it has the RELAY+RESISTOR 2 speed scheme? Are you sure?

I will post some observations I made on my 1991 LS400 in case they are useful to anyone. My daughter is driving it again, I think it is working. My problem and solution probably won't help anyone, though....the RESISTOR in the FP circuit had become disconnected (by me).TOP BLUE TRACE is Cigarette Lighter

BOTTOM YELLOW TRACE is FUEL PUMP VOLTAGE MEASURED AT THE FUEL PUMP

(NO Fuel Pump Controller ECU in this 1991 LS400! I attached special wires to the FP just to make this measurement! This car has RELAY AND RESISTOR for Normal FP speed operation.)




(I used the cursors on the scope to pin down the times & voltages below, not my eyeballs)
So on the date July 4, 2020, *my* 1991 LS400 behaved like this (times are approximate):
BLUE (cigarette lighter) Trace
0.5 seconds Ignition On
1.6 seconds CRANKING
20+ seconds RUN (alternator showing 14.6V)

YELLOW trace (Fuel Pump)
1.6 seconds "full power" (but limited by battery under cranking load)
1.0 seconds "full power" (boosted to 14.6 when alternator running)
20+ seconds Reduced to NORMAL POWER after the ECU thinks the motor is running (and kicks in the RESISTOR) until I turned it off.

Now, here is what I get if I DISCONNECT the Resistor (accidentally or otherwise):

With the RESISTOR path open, the FP is totally cut off...so then the motor will stop (evidenced by Cigarette Lighter voltage drop off).

Total run time turned out to be about 6 seconds, but NOT because of any INHIBIT sensing!
Amusing that the FP gets turned back on after motor stopped! Too late!

All this to say I was WRONGLY looking at those INHIBIT modes to explain my shutoff.

As a bonus I found this that might be useful to someone trying to figure out the Circuit Opening Relay behavior (google the phrases if you want more I guess):

or some other conditions.

FloridaCam

You need the resistor for normal mode. The relay only works when you start the engine and when the engine needs power. I think the resistor is about 30 ohm which drives the current to 1/2 of when it uses relay.

Yamae

The resistance is not that big. It's 0.73 ohm.

FloridaCam

Thank you Yamae! 0.73 ohm is the correct value.

mrmox

@kuality
and @FloridaCam

In this thread someone posted a see-thru (xray) drawing that shows placement of a Fuel Pump ECU (looks like in the trunk)!
He says it is for a 1994.
https://www.clublexus.com/forums/ls-...-pump-ecu.html

Here is the text (that describes a different placement location though!) :I watched a Youtube video (search for LEXUS SC400 Fuel Pump ECU Trick) (yes it is NOT an LS400 but still interesting) that shows where the Fuel Pump ECU is on that car.

Either way (Relay+Resistor or Fuel Pump ECU) maybe this IS your problem @kuality since ALL THE INHIBIT MODES SHOULD THROW A CODE, right?

What do you think @Yamae ?

FloridaCam

But. this thread is about a 96 LS400. A 96 does not have Fuel Pump ECU.

mrmox

I hear you, so do you know what they did in 1996? (If 1994 had Fuel Pump ECU but 1996 did not)
Engine ECM drives the FP directly?
No FP speed changing at all?
Or FP voltage modulation absorbed into the Engine ECM?
Is there still a Circuit Opening Relay?
Help us out, got any circuit diagrams?

FloridaCam

I don't know much about car. I only know that 96 model does not have a fuel pump ECU. Circuit Opening Relay is controlled by ECM directly. Fuel pump has two modes, normal (with the resistor) and without resister (when relay kicks on). Circuit Opening Relay is located right next to the FP relay.

kuality

been awhile since i worked on it but will check into that thanks