Working Temperature of Fuel Pump Resistor
 

My request: I would like to know, when your car is up to operating temperatures, what is the temperature of the fins on the fuel pump resistor.

I would like to get a baseline temperature for future reference since I will be converting to relay-powered fuel pumps soon and using the stock wire as the trigger.

In hunting down my fuel pump issues (you can read about the saga here: https://www.rx7club.com/3rd-generati...relay-1149692/), I noticed something concerning about my fuel pump resistor. Within 5 minutes of a cold start, the temperature at the fins gets very hot, in excess of 275F! I do not know exactly how hot it actually gets, since I have avoided keeping the car running since discovering this, but I suspect it gets even hotter than that since I have been operating the car before I noticed this. I need to get it to the local shop this Monday, so I will have my spare resistor and a few fuses for the fairly short drive.

you got me curious, so i looked it up in the fsm.. the fuel pump resistor is 0.65 ohms. the oem pump probably draws about 6A at 12V.. if you do the math it pulls about 4.5 amps through the resistor, dissipating about 13 watts. pump voltage ends up being about 9V on low speed mode with the oem pump.

you are running two walbro 450s, which pull about 30 amps full blast at 12V. doing the math, it ends up being about 11 amps through the resistor, dissipating over 80 watts. so that is why it gets so hot. doing more math, the pump voltage ends up being about 5V on low speed mode, which is what you measured. the more current drawn through the resistor, the lower the pump voltage. what may be happening is the pumps just cant operate at 5V, or the overheated resistor messing with the power delivery

you could bypass the resistor, but two walbro 450s running full blast all the time will cause a lot of heat to build up in your fuel tank, wears out the pumps faster, and lots of noise. lots of people do it this way anyway

you could try replacing the resistor with smaller value, that way less heat is dissipated and more voltage to the pump. this one should get you about 8-9V at the pump:
https://www.mouser.com/ProductDetail...252BDOBipmQ%3D

you could also try switching to a PWM style controller, which basically controls fuel pump speed with a pwm signal from your ecu

You could also not run twin walbros 🤔🤔🤔

Also, as I mentioned in your other thread you need to remove the resistor entirely. You're putting an unnecessary strain on the system by having it in the circuit.

Thanks for the very thorough reply!! Such a great level of detail. I cannot run the Walbros at any less than 9V, so I will be removing the resistor and running the FP system with relays using the original voltage path as a trigger.

Advice taken. I was just posting this thread to get an idea of what resistor surface temperature of normal FDs was. I will be running a relay for the fuel pumps with the original voltage line as a trigger. As for the dual pumps - I bought the surge tank used, and the package came with the pumps. As stated in the other thread, I will move to appropriate pumps if/when the Walbros crap-out. I always appreciate your input (not being sarcastic here)!

The stock wiring is fine for a single Walbro pump, resistor and all. Even more so for the low pressure (and low amp) lift pump in a surge tank. A single Walbro will run perfectly fine with the stock wiring and 9-12V switching. I ran a Walbro 400 for years on stock wiring with no issues. Check out the Walbro myth busting thread.
When running multiple pumps depending on what you're doing you either need the second pump or both pumps independently wired.
In a surge tank setup, and I'm using Radium Engineering's, the pressure pump (or pumps) need dedicated wiring. The lift pump can run off the stock wiring.
https://cimg5.ibsrv.net/gimg/www.rx7...ef98b69b21.png

The B2A is an in-tank tank. The surge tank is not pressurized, and the lift pump just keeps it filled. I am concerned about running it the way you suggest, as I want to make sure the surge tank keeps up with what the send pump is sending to the rails. Also, TI Automotive says that the Walbro 450 pump should not be run any anywhere under 9V. Theoretically, the resistor will deliver more voltage with less current through it for a single lifting pump, but I really want to simplify the system (and I have lost faith in the low-load voltage path in the stock wiring). Since my car really is not a street car, pump longevity is not my concern. Plus, I will be switching to smaller pumps whenever the 450s crap-out. By then, I may even have a different direction altogether. I was drawn to the B2A because it did not require anything to be external to the fuel tank.

The Radium setup I'm running is also an in-tank surge tank, the lift pump only fills the small surge tank and the return also goes to the surge tank. I'm running a 450 as lift with stock wiring and a 525 as pressure pump with its own relay and wiring, very simple. I can add 2 more pumps if I so need in the future. Nothing wrong with having both pumps independently wired, in that case you can use the stock wiring to trigger the relays instead of powering the pumps.

Since it is a return system the majority of the flow returns to the surge tank, the actual fuel flow into the engine depends on injector sizes and duty cycle at a set base pressure thus the lift pump flowing at 0 pressure will outflow the pressure pump.