First, I want to say I’m aware of the fuel pump relay mod, but I think it’s unnecessary to run the pump at full 12V all the time, see the list below.

• At idle, injectors use less fuel, increasing return line pressure.
• Constant 12V operation wears out the pump faster.
• Increased fuel flow creates unnecessary vapor in the gas tank.
• Stock wiring allows the ECU to shut off the fuel pump in a crash, while the relay mod bypasses this safety feature. (I’d rather not burn to death in my car if I get knocked out in a collision)

With that out of the way regarding why I’m not using the popular relay mod, I’m upgrading my fuel pump to an AEM 50-1000, which draws more current than the OEM pump. As a result, the OEM resistor causes too much voltage drop, so I need to adjust the system to compensate. Basically, I need to lower the resistance in the circuit so I don’t burn the infamous fuel pump connector in the fuel tank. The new pump AND using the OEM resistor makes the circuit run too hot, which is why they always burn.

Plan:

• Measure OEM pump performance (voltage and current in low/high speed modes).
• Install AEM 50-1000 pump and test with OEM resistor.
• Compare voltage and current to OEM values, then adjust resistance to match the original operating conditions.
  (Using the resistor below as a parallel resistor in the circuit or just replace the OEM resistor all together with a lower value)
  
  
  

Has anyone done this before? Looking for input from those who have adjusted the resistor setup while keeping the speed relay functional.

 

You misunderstood what the relay mod does. All its doing is giving the pump a direct power source using the factory power source as a trigger to switch the relay. So in the scenario you laid out, the pump would turn off because it would lose the factory power. It's all the same electrically. The power is simply being diverted at the last step to trigger a relay instead of power the pump.

for those that have been DOING IT WRONG then it DOES work as you described and the scenario you laid out could come to pass.

aftermarket pumps are fairly robust and the increased wear you describe is not a concern or a scenario that requires defense. In addition, the other reasons you described are super negligible. Theres no need to innovate on this mod. Perform the relay install correctly and you will be fine. Bypassing the factory connector is a safety mod. If you intend to run an upgraded pump in a factory manner then you would be better off just running a factory pump.

I think the stronger ecus have the ability to vary voltage to a pump on their own but the aem 340 is really just an oem+ pump. Varying the voltage is not necessary.

 

I'm just going down a deep rabbit hole, and I can't find my way out. I have all the parts for the relay mod that everyone does: bulkhead, 18-inch fuel pump harness, Bosch relay, 12-gauge wire, etc. But it's hard for me to gather all the facts or even understand them well enough to make the right decision. I'm okay at reading wiring diagrams, but I’d need a few hours to go through the one above.

If fixing the fuel pump issue is as simple as adding a resistor, that would be great. I just think the relay mod became more popular over the years because different companies sell it as kits, and since it worked, no one ever looked back. Well, I’m looking back and now I can't find my way out.

 

You're over thinking it. Everything functions exactly the same. The power wire that would normally go to the fuel pump and run it variably is getting the switcheroo at the end of its journey and goes to a relay instead. That's it. You bypass the resistor because the duty of the power wire is changing to trigger a relay instead of run a fuel pump so it doesn't need to nor should it vary its voltage.

Its not a matter of just adding a resistor to reproduce the function with a larger pump, its the fact that it makes no sense to do so. Like running a gt40 with a stock exhaust. The configuration doesn't make sense.

What is reason for putting an upgraded pump in handcuffs when you can just use a stock pump in stock configuration, modify nothing and achieve the same result? The 340 isn't going to perform better at 9v than the stock pump. Depending on your injectors duty, it won't perform better at 12v either. There's nothing to gain from doing this. It's just a complicated electrical venture for no reason.

 

I have had the original fuel pump mod in place since 2008, which has worked very well. This is different than other mods than came later. The original mod was to bypass the pump fuse in the main cabin (take it out) by splicing in a 10-guage wire directly from the battery to the fuel pump relay input under the hood. Put a 20-30 amp in-line fuse under the hood near the battery connection to add back a fuse to protect the circuit. This bypassed the lower amperage wiring in the original route. Then, run a 10-guage wire from the speed relay (along the front bumper area, near the airbox) output directly to the back fuel pump and splice into the original stock wiring near the fuel pump connector. Also, run a new 10-guage ground wire in the back from a frame screw and splice into the original stock ground wire, near the fuel pump connector (later, I actually ran a 10-guage ground wire directly from the battery to use for the pump and other things I added in the hatch area). This mod left the stock 14-guage wiring in the resistor system, which it keeps and the stock 14-guage wiring going from the main fuel pump relay output to the speed relay input. As I remember, my voltage went up at the pump significantly since the higher gauge wire was used over most of the circuit. Also, the heating of the wiring would conversely go done (less resistance). I also had a parallel resistor along the fuel pump resistor (a smaller one at that time). I had an upgraded Supra pump then, which is not much greater than stock amperage draw. This keeps all the original circuit design in place and only upgrades the wiring along most of the path and lowers the resistance on the resistor circuit.

Since then, I have added the Radium surge tank and a main Walbro 450 pump and the AEM lift pump (quieter than Walbro for lift since it can be heard more outside the surge tank and needs less output as lift). I kept the original mod for the main pump, except I upgraded the parallel resistor under the hood. I added yet another 10-guage wire directly from the battery to the lift pump in the hatch area. Then, I added a second dual 30-amp relay in the hatch area triggered by ignition on (need to run a wire from an ignition on feed at the main fuse box by your feet), to relay off-on both pumps. I didn't need it for the main pump (still have main stock fuel pump relay under the hood) but only for the lift pump, but I liked the idea to relay both off-on there too. Make sure to add a 20-30 amp in-line fuse for the lift pump too, under the hood. This time for the parallel resistor I just bought an available one from Rotary Performance. They sell a plug and play one with the stock connectors that simply plugs into the stock harness at the fuel pump resistor under the brake booster. So much easier than messing with it yourself for $30. They don't have it listed on their website, and you need to call them. Here is the invoice listing:

Resistor pack, fuel pump × 1 $30

IMHO, this is best for me. It keeps an upgraded resistor circuit in place, as was one of your original concerns, and upgrades the vast majority of the wiring for a reliable fuel pump setup.

Mike

 

You're right, I am overthinking it, but I always go back to the fundamentals of why Mazda designed it this way and weigh the pros and cons. The relay mod is simpler, safer, and more reliable, I agree with that. However, if done right, I don’t think variable voltage is useless. That’s where we disagree, but hear me out.

A pump running at 12 volts at idle, especially in Florida, will generate more heat, which will only worsen the longer you sit in this damn Orlando traffic. The return line wasn’t designed to handle that much fuel at idle, so in theory, I’d need to upgrade it with the relay mod to avoid excess pressure buildup. Probably not necessary on stock injectors, but still something to consider. Even though fuel pump technology has improved over the years, running it at 9 volts at idle reduces wear and helps extend its lifespan. Lastly, there’s the noise and keeping the pump at a lower voltage at idle helps reduce the whining sound, which is big for me since I'm making my FD a daily driver.

I’m not saying you’re wrong, I agree with the benefits of the relay mod but for my taste, I like the advantages of keeping the fuel pump speed relay functional. Even though you probably think I'm smoking crack with making things so difficult.
This is great. I'm only running the AEM 340, so I'll test the voltage at the bulkhead connector, both on the tank side and outside the tank. If the readings are good and there’s minimal voltage drop, I'll keep the OEM wiring. Otherwise, I’ll consider upgrading it. Thanks for sharing your idea. Glad someone else is using the variable speed relay, I don't feel so alone.

 

The AEM will pull more amperage, so I would at least do the resistor upgrade, since you may overwhelm the stock resistor and get a more erratic pump voltage due to overheating the stock resistor (particularly in your hot climate in a daily driver). It is very easy with the plug and play one from Rotary Performance for just $30. This does require some retuning of the fuel maps since the voltage does go up slightly prior to the speed relay kicking in.

Mike