bypass fuel pump ECU with a relay
 

Good morning all, I'm looking to delete the fuel pump ECU and use a relay instead. I've seen a lot of posts/comments about this, but I'm not very good at reading schematics. I have the relay and I have bench tested it, about 8 hours with a 9 amp load. (I remembered later I have some resistors and I can run it at a higher load, I probably will. How much does the pump draw at full power?) What I'm asking is if someone can tell me simply which wires on the harness need to connect to which ones on the relay. I thought I had seen that information but I can't find it now. And also is there a connector I can get that mates with the one on the harness? This is the relay: and this is the connection diagram:
https://cimg9.ibsrv.net/gimg/www.clu...eb064f9ded.jpg

Thanks in advance!

I'm not sure how much the 2JZ-GE and 1UZ-FE stock naturally aspirated application Denso fuel pump draw in amps but it is less than the 18A-20A of (if I am recalling correctly) of the Denso high volume fuel pump that the Supra MKIV TT uses. With both OEM pumps there is a different computer that governs the 9V and 12V modes. The longevity of the higher flow pump is said to be something ridiculous like 18 years on average even running at full flow 12V-13.8V all the time. The same Denso high volume fuel pump was also stock equipment on the 3rd generation RX-7's I believe.

I've never looked into what the same max power draw of the NA Denso pump is at 12V-13.8V but it is less than the high volume version.

As to replacing the fuel ECU in either application (though I'd still recommend allowing it to act as an aftermarket relay trigger since the stock engine ECU communicates with it) I used a standard Bosch style 30A/40A 5-pin automotive relay using my SC300 Fuel ECU as a trigger on/off module only. I didn't run it very long like that before wiring in a Supra MKIV TT Fuel ECU but I had no issues whatsoever. Although soon I may be reverting back to that setup if I have to in case my new Haltech ECU isn't compatible with it.

As to which wires to connect to the relay I'll dig up the posts I made on my build thread for the setup you plan to do. It'll walk you through the process. I highly recommend you build a new fuel pump circuit with a 10GA wire in some extra heat shrink tubing of the supra long variety (for added safety) going from your battery to a fuse (in-line in the engine bay or into an extra fuse box in the engine bay) to your fuel ECU area in the back seat.

I also highly recommend excellent small additional mini fuse box options from the company LittelFuse (that is not a misspelling-- the "L" is indeed at the end of the first word) which ca be bought through the Mouser Electronics website. Please note that at Mouser you'll usually have to specify each little part that goes with the box. LittelFuse's website will show all the applicable part numbers and associated terminal ends and seals for each of their offerings.

SC300 Fuel ECU Bypass w/Relay notes from my build thread (you can ignore any wiring connector pinouts related to the Supra MKIV TT Fuel ECU but please know that the basic method for a bypass with a 30A/40A Bosch style 5-pin automotive relay is the same for both cases).

In my case I reasoned that the relay would be okay in the factory fuel ECU location. This would be a chore to get to for any service or repairs but that's not different from stock. The fuse for the new fuel pump power source wire I wanted to keep somewhere easy to get to under the hood.

In Supra MKIV Turbos and in factory turbo Soarers this is identified in the service manual electrical schematics as "EFI2". The reason they did this for higher current draw and higher flow fuel pumps (turbo applications) is to separate the current load onto its own circuit. In the stock SC300 and SC400 the fuel pump's current draw is on the same circuit that the ignition system is also on. This isn't an issue for the lower current draw of the non-turbo Denso fuel pump but it is with any stock or aftermarket high flow / high current draw fuel pump for turbo, hence the need for the "EFI2" circuit dedicated solely for the high volume 18A-20A fuel pump used in factory turbo models.

If you are keeping a stock NA Denso fuel pump you can likely avoid having to change any of that.

https://www.clublexus.com/forums/bui...l#post10245454

https://www.clublexus.com/forums/bui...l#post10249619

https://www.clublexus.com/forums/bui...l#post10256118

And for what it's worth in case you do happen to need the info... a couple of posts showing the LittelFuse aftermarket mini fuse and relay under-hood small generic OEM style boxes I mentioned. This is not required of course. I'm just sharing to let you know some options that exist for adding new circuits under your hood. You will have to fabricate your own chassis mounting brackets from cardboard test templates, cut them from some plate stainless steel, drill them, sand them, bend them into the appropriate shape and paint them yourself.

https://www.clublexus.com/forums/bui...l#post10976534

https://www.clublexus.com/forums/bui...l#post10976542

https://www.clublexus.com/forums/bui...l#post10976544

Thanks a lot for that. If the fuel ECU will reliably trigger the relay, that sounds good and I would like to know how to connect that. I understand, somewhat, that having it communicating with the main ECU is a good idea. But is it functionally better than using just the relay instead? (I'm a mechanical engineer with just enough electrical knowledge to be dangerous.)
My issue has been that the ECU fails to switch to full voltage to the pump when it should. I'm driving it now with a jumper from B+ to Fp in that diagnostic port under the hood, and I've been driving it successfully for several months like that. My goal here is to implement a more permanent solution. To be clear, I am not planning any performance mods, ever. I love this car just as it is and I just want it to run, as designed, as reliably as possible. So I don't think I need to upgrade the wiring, unless the trigger leg of the relay is a significant additional load. But I would think that fuel ECU is using some power itself (why else would it need that heat sink?) so if the relay replaces that completely, I'm probably using less total current.
I see your next post here and I'll check out those links later, and see if I have any other questions. Thanks again. Side note- I'm envious of your 5 speed SC, even though my left knee hasn't the stamina to drive a stick.

Certainly!

If you are not planning any performance modifications with forced induction then there is no reason to use any higher flow fuel pump or change to a new dedicated thick wire power feed direct from the battery. So those aspects of my post you can also ignore.

If you go to the posts I linked you would want to copy just the aspects where I took the power from NA Fuel ECU direct to the fuel pump and instead allowed the Bosch aftermarket 30A/40A 5-pin relay handle that. Then the wire from the NA Fuel ECU that previously went to the NA Denso fuel pump is the wire that now becomes the relay trigger signal. The aftermarket relay won't care if the trigger is 9V or 12V. It's just a trigger to energize its internal switch so that 12V-13.8V can flow to the NA Denso fuel pump through it rather than through the SC300 NA Fuel ECU.

If you look at the supplied general (not Toyota specific) circuit diagrams showing how the bypass works you'll see that it's not that difficult to hook up. The relay's ground pin just needs to be affixed with a ring connector to a chassis bolt somewhere convenient.

As to why the NA Fuel ECU should be retained but used only as a trigger signal for the relay... this is for the purposes of safety. This preserves the automatic fuel cutoff feature whenever the main engine ECU shuts off. In the event of an accident or really in ANY situation where you'd *expect* the fuel pump to turn off... this makes sure that still happens and retains a minimum measure of main ECU control.

What you're doing is bypassing the NA Fuel ECU's internal 9V or 12V switching and letting the automotive Bosch relay do that at 12V-13.8V. The OEM Denso NA fuel pump will technically last a bit less time in lifespan but it is such a long lasting pump to begin with this would be shaving maybe 2 years of lifespan out of 15-18 years or so at a constant 12V operation. Not a big deal.

This would be a much better and more permanent solution than constantly keeping a thick wire on the diagnostic port's FB & +B terminals.

Many thanks! I sadly haven't been driving it for many months while I've had to deal many other things since it had a sunroof leak issue. It has been garaged while all the repairs have been worked on a bit at a time. I've got almost all the new parts needed and the goal is to have it back together and revived with the Haltech in 2026.

I hear you on being wary of a clutch. The OS Giken twin plate in my SC is what I consider daily drive-able but it has heavier pedal effort than stock and a tricky to learn bite point. Driving on the much lighter stock clutch (conventional single disc) in my manual GR86 for the last couple of years is night and day to the clutch in my SC300. I have zero discomfort with my knee on the light 86 clutch but daily driving the SC300's heavier twin plate is something I would occasionally get a little fatigue with. Not too bad but once I started daily driving the 86 I realized how much I had become accustomed to a heavier clutch with slightly aggressive behavior as being totally normal.

Again... unlike some seriously heavy single plate aftermarket clutches the OSG STR2CD twin plate is a lot easier on your knee and really is a decent daily-drive capable clutch once past the initial break in period (since it spreads its torque holding capacity over two discs rather than just one) but it is still different from driving on a lighter factory clutch in something like an 86, Miata or other not so overpowered car.

Our cars are amazing machines regardless of what transmission they have :)

Ok, I see now. The hack I'm currently running, jumping B+ to Fp, is bypassing that safety function isn't it? I didn't realize the relay would trigger on less than 12V, but now I get it. (I can even test what the minimum trigger voltage is to be double sure. I have a variable power supply.)

So I'm tapping into B+ while leaving it connected to the controller, then disconnecting Fp and using that as the trigger, then the power output from the relay goes to the pump via the green wire, right? Just one other thing I'm curious about, that Fp wire is branching to go somewhere else. I guess it doesn't matter but I'm curious where it's going and what it's for.
https://cimg0.ibsrv.net/gimg/www.clu...edd2e3432a.png
Regarding the clutch, I can drive a short distance or on highways with one, but in traffic my knee will fatigue quickly and start hurting pretty bad. It's all scarred up from a tibia repair long ago. Doesn't matter how stiff the clutch is. That's ok, this car is still tons of fun. The envy is mostly because I hear the stick is very rare from the factory.

Thanks again. Now I just have to get my old self back there and do that job.

the green wire goes to the Data Link Connector 1. i think, this is the port under the dash or in the engine room


https://cimg1.ibsrv.net/gimg/www.clu...6ab4fb072f.jpg

That's it. I'm actually using that, in that diagnostic port under the hood, with a jumper to run the pump now. I know that's not as safe as doing it the right way, that's why I want to put in a relay. So is the connection scheme I described in my earlier comment correct?