BNR34 Fuel Pump, ECU, FPCM trouble shooting help...ugh :(

[ocd]

Hi all. I could use some help with a BNR34 fuel pump trouble shooting. There was a time when I had to drain the fuel tank and used the FP to purge the tank by collecting the old fuel at the fuel rail feed (redirected into a gas can). As a result, I may have crossed some wires because I've had this problem ever since. The fuel pump does not turn on with normal key ignition on. The FP relay bench tests fine. Jumping terminals 3/5 on the FP relay connector sends constant 12v to the pump with ignition on...it doesn't time out. Fuel pump runs fine. I haven't checked the running voltage. Tried 2 different FPCM's with the same result. Running a Nismo resetting ECU.

 

Does ECU pin out #18 take the FP relay coil (terminals 1/2) to ground? It looks like it does on the schematic. If the FPCM takes the 12v FP power circuit to ground, then I think the fault is the ECU or the wire from pin #18 to the FP relay.

 

I'm trying to get a head start on tomorrows ongoing battle. Thanks in advance.

[joshuaho96]

55 minutes ago, ocd said:

Hi all. I could use some help with a BNR34 fuel pump trouble shooting. There was a time when I had to drain the fuel tank and used the FP to purge the tank by collecting the old fuel at the fuel rail feed (redirected into a gas can). As a result, I may have crossed some wires because I've had this problem ever since. The fuel pump does not turn on with normal key ignition on. The FP relay bench tests fine. Jumping terminals 3/5 on the FP relay connector sends constant 12v to the pump with ignition on...it doesn't time out. Fuel pump runs fine. I haven't checked the running voltage. Tried 2 different FPCM's with the same result. Running a Nismo resetting ECU.

 

Does ECU pin out #18 take the FP relay coil (terminals 1/2) to ground? It looks like it does on the schematic. If the FPCM takes the 12v FP power circuit to ground, then I think the fault is the ECU or the wire from pin #18 to the FP relay.

 

I'm trying to get a head start on tomorrows ongoing battle. Thanks in advance.

Yes, ECU pin 18 controls the fuel pump relay using the ground switch method. The two other pins that come from the ECU to the FPCM are signals to the FPCM to switch the resistor on the fuel pump to ground. No signal on the two means full power, one is idle and the other is cruise. 

Edited August 13, 2023 by joshuaho96

[Duncan]

FWIW, I would not bother troubleshooting that setup.

Run a new wire suitable for 30a from the battery (and add a fuse close to the battery as you can) to pin 30 or 87 and 85 or 86 of a standard relay mounted near the factory one. Find which wire into the factory relay is earthed for 3-5 seconds when you turn ignition on (relay pin 1 from the diagram) and then earth is removed, and run wire that to pin 85 or 86 of the relay. 

Then cut the factory fuel pump wiring for pin 1 near the plug and run it to a good earth with a 30a suitable wire. Also run a wire from pin 30 or 87 of the new relay to pin 2 of the fuel pump plug.

No more factory complexity, stable, full running voltage to the fuel pump at all times and ready to go for a modern fuel pump it drop in when you want to double the power.

[MBS206]

4 hours ago, Duncan said:

FWIW, I would not bother troubleshooting that setup.

Run a new wire suitable for 30a from the battery (and add a fuse close to the battery as you can) to pin 30 or 87 and 85 or 86 of a standard relay mounted near the factory one. Find which wire into the factory relay is earthed for 3-5 seconds when you turn ignition on (relay pin 1 from the diagram) and then earth is removed, and run wire that to pin 85 or 86 of the relay. 

Then cut the factory fuel pump wiring for pin 1 near the plug and run it to a good earth with a 30a suitable wire. Also run a wire from pin 30 or 87 of the new relay to pin 2 of the fuel pump plug.

No more factory complexity, stable, full running voltage to the fuel pump at all times and ready to go for a modern fuel pump it drop in when you want to double the power.

Just on a note for this Duncan.

On the relay coil, your higher voltage should be on 86, your lower voltage (typically ground) should be on 85.

It "won't matter" right up until you install a relay that uses a diode instead of a resistor to kill the coils flyback.

When it's the wrong way, the diode allows a short circuit from power to earth. Great way to fry electronics / cables or relays and fuses.

 

Good way to remember is high number, to the higher voltage

[ocd]

Thank you all for chiming in. Before I run a new more robust circuit, I want to find out what's failed.

It looks like the ECU isn't switching ground on the FP relay. I don't think the issue is the FPCM.

1. I traced the FP relay signal wire (PIN 18 black/brown). There's continuity from the FP relay to the ECU harness on that wire.

2. I grounded the FP relay signal ground wire direct to chassis in the trunk. With "key on, engine off" the fuel pump continuously whines. It's not momentary like normal operation when you key up the car.

3. I swapped out the FPCM with a different one, although I'm not 100% the swapped unit is good. All results the same.

If the FPCM only changes voltage at idle and voltage at cruise, and controls nothing else, then the likely culprit is the ECU...but HOW? I've talked to a few people and no one has heard of the ECU crapping out is such a way.

Is there anything else at play here that I may be overlooking?

[joshuaho96]

8 minutes ago, ocd said:

Thank you all for chiming in. Before I run a new more robust circuit, I want to find out what's failed.

It looks like the ECU isn't switching ground on the FP relay. I don't think the issue is the FPCM.

1. I traced the FP relay signal wire (PIN 18 black/brown). There's continuity from the FP relay to the ECU harness on that wire.

2. I grounded the FP relay signal ground wire direct to chassis in the trunk. With "key on, engine off" the fuel pump continuously whines. It's not momentary like normal operation when you key up the car.

3. I swapped out the FPCM with a different one, although I'm not 100% the swapped unit is good. All results the same.

If the FPCM only changes voltage at idle and voltage at cruise, and controls nothing else, then the likely culprit is the ECU...but HOW? I've talked to a few people and no one has heard of the ECU crapping out is such a way.

Is there anything else at play here that I may be overlooking?

Open up the ECU. Verify that nothing is burned, no caps have dumped their dielectric or burst. To eliminate the FPCM to narrow down the ECU as the fault you can bridge pins 4 and 6 on the FPCM connector directly to ground. If bypassing the FPCM like this does not change the behavior then you know the ECU has failed.

As for how someone could have damaged the ECU this way I don't really know. It is possible someone grounded something improperly and is putting way, way too much current through the ECU ground planes which can cause things to break. Not all ground points are equal, see this explainer to get an idea for what I'm talking about: https://www.haltech.com/news-events/ecu-grounding-the-dos-and-donts/

Edited August 28, 2023 by joshuaho96

[GTSBoy]

Testing whether the ECU is actually setting pin 18 to ground is really simple. Just pull the FP relay out of its socket so there's no actual power going anywhere. Put a mutlimeter set to continuity onto pin 18. Turn on the ignition. If it beeps, you got ground. If it doesn't beep....you got problem, as you suspect.

[ocd]

On 8/27/2023 at 7:47 PM, joshuaho96 said:

Open up the ECU. Verify that nothing is burned, no caps have dumped their dielectric or burst. To eliminate the FPCM to narrow down the ECU as the fault you can bridge pins 4 and 6 on the FPCM connector directly to ground. If bypassing the FPCM like this does not change the behavior then you know the ECU has failed.

As for how someone could have damaged the ECU this way I don't really know. It is possible someone grounded something improperly and is putting way, way too much current through the ECU ground planes which can cause things to break. Not all ground points are equal, see this explainer to get an idea for what I'm talking about: https://www.haltech.com/news-events/ecu-grounding-the-dos-and-donts/

ECU looks perfect inside. I see nothing apparently burned, leaking, or damaged...traced the PIN 18 circuit board path as well. Does the ECU case need to be grounded to the chassis? I don't think so, but it's currently sitting on the passenger floor.

Just to confirm, are you saying to disconnect the FPCM and ground both PIN 4 and 6 on the connector? Or, are you saying to keep it connected, bridge PINs 4/6 and ground the jumper wire?

Does the ECU need to be queued up by the FPCM or other sensor(s) for it to send ground to the FP relay?

Thanks for the Haltech link. I'll take a read.

Edited August 29, 2023 by ocd

[ocd]

On 8/27/2023 at 8:26 PM, GTSBoy said:

Testing whether the ECU is actually setting pin 18 to ground is really simple. Just pull the FP relay out of its socket so there's no actual power going anywhere. Put a mutlimeter set to continuity onto pin 18. Turn on the ignition. If it beeps, you got ground. If it doesn't beep....you got problem, as you suspect.

I did this both at the FP relay connector, and at the ECU...no beep in either location. Does the ECU need to see other sensor information (FPCM, etc) to set PIN 18 to ground?

[Duncan]

Yes, it will only earth the Fuel Pump when the ECU first turns on for about 3-5 seconds, or if the engine is running (rpm signal > xxx)

[joshuaho96]

33 minutes ago, ocd said:

ECU looks perfect inside. I see nothing apparently burned, leaking, or damaged...traced the PIN 18 circuit board path as well. Does the ECU case need to be grounded to the chassis? I don't think so, but it's currently sitting on the passenger floor.

Just to confirm, are you saying to disconnect the FPCM and ground both PIN 4 and 6 on the connector? Or, are you saying to keep it connected, bridge PINs 4/6 and ground the jumper wire?

Does the ECU need to be queued up by the FPCM or other sensor(s) for it to send ground to the FP relay?

Thanks for the Haltech link. I'll take a read.

Basically yes, if you disconnect the FPCM and ground pin 4 and 6 on the connector you have bypassed the factory FPCM. If the pump gets power from the fuel pump relay it will run full power immediately. You can see the same idea discussed for Z32s here: https://conceptzperformance.com/wiki/index.php/FPCU_Bypass_(2-Seater)

If the pump isn't running then it's safe to say that something is wrong with the ECU. 
 

23 minutes ago, ocd said:

I did this both at the FP relay connector, and at the ECU...no beep in either location. Does the ECU need to see other sensor information (FPCM, etc) to set PIN 18 to ground?

Initial key-on should cause it to beep as Duncan mentioned. Then it will shut off after an initial prime. I would also verify that this isn't a NATS issue or something.

[MBS206]

Guys, STOP using continuity beeps as your test. They'll lead you astray something shocking.

 

For example, high quality Fluke Multimeters state they'll only show continuity if circuit resistance is between 0 and 50ohms.

Cheap multimeters I've seen much much higher resistance and still getting continuity beeps.

 

To give you an idea, if you have 40 ohms of resistance on a wire that's either giving power to the relay coil, or taking power from the relay coil to ground, then adding that in with the typical 140ohms of a coil resistance, when you try to activate said relay you'll end up with a roughly 3v drop in the 40ohms, leaving only 9v, and a lack of current at the coil, meaning it may not trigger.

 

Please measure actual resistance not "continuity beeps". 

 

 your first post, you state you have had this problem since you drained your fuel tank. You also state "you may have crossed some wires"

so my question is, how did you actually "possibly cross some wires"? What exactly did you do, and at which plugs, to get the fuel pump to let you drain stuff.

 

I have a small idea of how you could have killed the ECU if it is 100% the issue...

[GTSBoy]

33 minutes ago, MBS206 said:

Guys, STOP using continuity beeps as your test. They'll lead you astray something shocking

Meh. When used as I prescribed, we're talking about a mosfet and a couple inches of wire. If there's any difference between absolute short and what you get from that then there's bigger problems anyway.

But....valid point nonetheless.

[MBS206]

21 hours ago, GTSBoy said:

Meh. When used as I prescribed, we're talking about a mosfet and a couple inches of wire. If there's any difference between absolute short and what you get from that then there's bigger problems anyway.

But....valid point nonetheless.

I agree and disagree.

If the issue for example is on the pin into the ECU, it could definitely throw a red herring for it. Also I think others suggested checking continuity everywhere... ha ha

I've even seen auto sparkys fall victim to it too.

A mechanic diagnosed a car of ours as bad fuel pump. New pump in. Same issue. "But there's 12V at the plug and it has continuity too!

Turns out the power cable to the fuel pump had 4 hot joints. Not enough to break continuity, and you'll always see the full voltage when you unplug it at the pump and test power. Even at only 10ohms, max current our fuel pump could see was 1.44amps, and that's IF the car was already running at 14.4v

Try and crank and max current is <1amp.

 

Very very different to this fuel pump issue, only raise it as a good reason people need to be aware of it.

Many I've spoken to think if they have continuity, shit should work.

 

On the flipside, cheap $10 multis on continuity and be beneficial to check my PCBs when I'm soldering them for horrible shorts super quickly. Then I sometimes just go with a smoke test... That reminds me, I should order a few spare cans of electrical smoke to keep on the shelf... Ha ha ha

Edited August 30, 2023 by MBS206

[ocd]

The ECU is at fault. It no longer takes the FP relay to ground. I ended up replacing the ECU with a Power FC, and the FP primes on key up as it should. Mystery solved.

 

Thanks all

[Duncan]

22 minutes ago, ocd said:

The ECU is at fault. It no longer takes the FP relay to ground. I ended up replacing the ECU with a Power FC, and the FP primes on key up as it should. Mystery solved.

 

Thanks all

Thanks for closing the loop...you should check that the earth is only being used to trigger the fuel pump relay in your case as if the pump is trying to pull the full earth through the ECU that would cause a problem.

[joshuaho96]

2 hours ago, ocd said:

The ECU is at fault. It no longer takes the FP relay to ground. I ended up replacing the ECU with a Power FC, and the FP primes on key up as it should. Mystery solved.

 

Thanks all

You sure it wasn't NATS or something?

[ocd]

3 hours ago, Duncan said:

Thanks for closing the loop...you should check that the earth is only being used to trigger the fuel pump relay in your case as if the pump is trying to pull the full earth through the ECU that would cause a problem.

Not sure I follow you. ECU pin 18 is solely used send ground to the FP relay, switching on power to the FP. From what I've gathered, that's all pin 18 does. The FP relay tests fine. The FP grounds through the FPCM, which also tests fine.

Am I missing something here?

[ocd]

1 hour ago, joshuaho96 said:

You sure it wasn't NATS or something?

I've never had a problem with NATS. It's always worked. I have the original transponder key and the ECU (Nismo) is assigned to it (I believe that's the process). I remember reading somewhere that the Power FC doesn't need to be assigned to the NATS in order for it to operate.

 

What's making you think it's could be a NATS issue?

[Duncan]

I just mean, if you have modified the wiring and used the ECU ground pin to fuel pump earth, that could be a cause for burning out that circuit in the ECU. If it is wired as you said, that was not the cause.