I swear the fuel pump ECU is such a mystery box you'll never know what it's thinking. I thought my FPECU was dead because the car would only start and run for maybe 3 minutes? on cold starts and then immediately shut off, so I jumped it and the car ran consistently once it was just the main ECU feeding the pump. Then I replaced my main ECU and removed the jumper out of curiosity and all of a sudden it started working again and even made it to florida with no problem. I still keep a jumper wire in the glove box if it finally decides to cash out for good.

 

It's not really how this works, if that was to be a thing, it would defeat the entire purpose of the Fuel Pump ECU to begin with.

Generally the Fuel Pump Bypass involves feeding the Pump directly from the EFI Relay and not from the ECU, said Relay becomes Active the moment you turn the Ignition to the ON Position, turning the Pump On along with it regardless of the Engine ECU Output, meaning that with the Bypass implemented, the Engine ECU has absolutely no control over the Fuel Pump, as all it can do is send the FPC Signal to the Fuel Pump ECU, which then converts it into the Output for the Fuel Pump. If you read earlier replies, this subject was discussed at length, including the safety implications of forcing the Pump to stay running regardless of the ECU Outputs.

What most likely happened in your case is the Engine ECU kicked the bucket at some point, for whatever reason, and stopped sending the FPC Control Signal to the Fuel Pump ECU, therefore causing the latter to shut down the Fuel Pump. Now that you implemented the Bypass, the Fuel Pump was being fed directly from the EFI Relay, bypassing both ECUs regardless of their condition, meaning that the Pump no longer needed the FPC Signal to function. Next you replaced the Engine ECU, fixing the Initial Issue, the loss of the FPC Control Signal, which is what disabled your Fuel Pump in the first place, hence when you pulled the Bypass Jumper out, the system returned to its Normal Operation with your perfectly functional Fuel Pump ECU accepting the Engine ECU Input and generating the Output for the Fuel Pump.

It seems like you really dodged a bullet here, it could've been the Ignition System that failed, preventing the car from running completely without getting a new Engine ECU, in this case you managed to get away with just a Jumper in the Diagnostics Connector, at least for the time being seeing that you ended up replacing it anyway. Though said luck is indeed up to the argument seeing the confusion it created..

Hope this helps and best of luck!

 

Not a bad idea! It's been running perfectly since Saturday, but I have to admit I'm a little wary. I've satisfied her need for attention for now, but...

 

The SC/Supra uses a pulse width modulated signal from the main ecu to the fuel pump ecu to tell it high or low. (note this is different than most other toyotas that use a regular voltage on fpc)
The reason why we keep the stock fuel ecu in place when it is still working and use it to trigger a 12v relay is because you cannot reliably trigger a standard 12v relay using that sort of PWM signal.
Now you can build a small circuit with a transistor that uses the pwm ecu output as a trigger to provide a grounded output, and then use that to trigger your relay instead of the stock fuel ecu.

Wilbo released the circuit a while back to do that, but most people don't build this circuit and just use the stock fuel ecu power output to trigger the relay on the power side.
That way you retain all the safety features but the pump runs at 12v whenever it should be running.
Either way works and does the same thing, it is essentially what the stock fuel ecu is except it is also has a relay built in and also a circuit for reducing 12v to 9v that tends to fail (other toyotas use a simple resistor).

https://www.toymods.org.au/forums/th...-of-newer-ECUs
Post 4 has the magic circuit you guys are talking about.

If you want to geek out and do a deep dive it is very similar to a tachometric relay which triggers a relay off a pulsed rpm signal, and you can just order those online and make a few modifications as well.
I would have to imagine there are countless threads on this by now though.... good luck old engineer!! if you worked on older cars then you have probably used a tacho relay before.

 

I came to the dead main ECU conclusion after the fact as well, I'm just not a fan of the FPECU in general. I was ready to just leave it bypassed since I thought why should there be another chef in the kitchen when normal cars just feed 12 from the main relay and this works perfectly fine. But at the end of the day I still hold on to the belief of not screwing with something that isn't broken so I decided to plug it back in. I knew the main ECU would need a rebuild eventually all along but was under the impression the only issues it had was faulty fuel pump control and lack of overdrive control. Restoration of the FPECU was just a semi-pleasant surprise if anything but given the capacitor issues I don't trust it to not give up one day out of the blue.

 

Love the info i get from these threads, I replaced my fuel ECU and had the main ECU brain rebuilt and all works great again. Price was more than cheap enough , easy to install everything on my own, and didn't have to change any of Lexus great engineering , my 96 sc300 still running strong ...