Fuel sender and fuel pump ECU interfering with eachother- 1JZ conversion

[Neal_411]

Hey guys,

I wonder if you can help me with a weird and annoying problem.

I have done a 1JZ conversion on an A70 supra and used the fuel pump ECU out of a soarer.

The problem is that the car will only start if the fuel sender is unplugged from its 4-pin connector. The fuel sender works fine when the fuel pump ecu is grounded at the same point in the boot.

The fuel ecu, fuel pump and fuel sender are all grounded to the same point in the boot.

I tried to run a ground directly (FP ECU) from the boot to the battery in the engine bay and still it turns over but won't start when the sender is connected as usual in the boot.

Any ideas? I can't seem to figure this out. They are on totally independent circuits so how can they affect each other like this?

Thanks

[dumbass]

does the fuel pump wires and the fuel sender wires live on the same plug
maybe u got thbe wires mixed up on that plug
i never use those fuel pump ecus
they do tend to fail

[Neal_411]

Hi, thanks for that. Yes I may have to end up canning the FP ECU idea since it's been a bunch of problems but I'd like to explore other possibilities first.

The thin ground wire for the FP ECU was originally tapped into the gnd wire for the fuel sender, just before the actual ground junction bolt in the boot and after the sender. From looking at the diagrams for the MA70, this wire is only for the low fuel level warning light that is triggered by the in tank sensor. Because the car currently has enough fuel in it to sit at a level above this sensor, the light should be off and therefore the circuit should be open. Therefore you would think that they could not interact under these conditions. I mean the problem is obviously something to do with the grounds, because each circuit works fine on its own - just not together. Like I said above I tried isolating the FP ECU ground by connecting it directly to the battery and it still gave the same issue.

Fuel sender 4 pin plug:
#1: GND (For low fuel warning light)
#2: Batt voltage (For low fuel warning light)
#3: + 4.5V terminal (for sender - goes back to dash)
#4: - terminal (for sender - goes back to dash)

Just not sure what else to test or try at this point.

[Neal_411]

So after sleeping on it I decided to have another look today and had some success in narrowing down the issue.

Experiment #1:

Ground the fuel pump sender directly to the battery to bypass boot wiring.

Outcome: Still no go.

Experiment #2:

Unpin the ground wire (pin 1) from the fuel sender connector. Leave everything else plugged in. This should disable the low fuel level warning light but the fuel level should still work as it uses pins 3 and 4.

Outcome: Still no go.

Experiment #3:

Unpin both the ground wire (pin 1) and the +12V feed (pin 2) from the fuel sender connector. This should disable the low fuel level warning light even more but the fuel level should still work as it uses pins 3 and 4.

Outcome: Car starts! FP ECU is now happy to turn on again.

So somehow the ignition switched power to pin 2 when it is connected to the fuel sender plug, even when not grounded (grounding here was found to be irrelevant), causes the FP ECU not to turn on properly.

This makes me think there is something going on upstream near the dash or even the engine ecu area. For now I can get away with not having a low fuel level light and I really won't miss it, but will have to keep hunting for the mechanism of interference with the +12V wire.

Heres a pic:

[dumbass]

not sure if u mentioned it but is the 1j from a jz30
so does your 1j ecu work with a fuel pump ecu or does it use a circuit opening relay

if it does need a fuel pump ecu u should have fpc and di connected from ecu to fuel pump ecu
and how have u powered the fuel pump ecu did u use orig fuel pump power?
the relay that powers ecu +b and also powers auto and idle motor etc etc this should also goto fuel pump ecu black red wire
then green white wire should goto fuel pump
white green should goto any ground
then 2 thin wires goto fpc and di

u must have something causing a backfeed or something

[Neal_411]

Hi, yes it is from a JZZ30 which uses the FP ECU from factory. I have connected all the pins as shown in the diagrams including the DI and FPC going back to the ECU.

Yes FP ECU is wired to original fuel pump power on the 1JZ loom (pin 12 of CF2 plug).

[dumbass]

pin 12 cf2 plug is that black/red

basically the efi relay will power engine ecu and also fp ecu
then output from fp ecu goes to fuel pump power
make sure no other wire is connected to the power to fuel pump except for u can connect the wire from diagnostic plug called fp
this is incase u want to bypass fp ecu but for now leave it unconnected till u figure out whats wrong

maybe these is issues somewhere under the panel between the tank

i would run a new wire from fp ecu to fuel pump + with no other wires attached to that power wire