Low voltage at fuel pump. Power FC warning light
 

Hi all. The warning light for my power FC came on and when I did a sensor check it's showing 5v to the fuel pump at idle. I verified with my multimeter and it's only 5v. I bypassed the resistor and get 10v at idle but the power FC is still showing 5v. 10v is still low and I'm assuming there is a wiring problem somewhere. Where does the power FC read the voltage from? Also how do I verify the high speed relay is working. I don't want to rev the car too much with low voltage but what rpm should it kick in at?
It's a 92 jap FD with a walbro pump and no other electrical mods to the fuel system.
Thanks for any help

Re-wire and be done with it.

I assume you mean to rewire by running a new wire straight back to the pump?
I don't have a problem with that but would like the power FC to read the voltage correctly. Having the warning light always on sucks if something else goes wrong.

The PFC reads from the fuel temperature sensor.

and if you have an aftermarket fuel rail setup there is no spot for the fuel temperature sender. but the CEL shouldn't illuminate for that anyways.

if that is the case you could install resistors into the harness for the sensors it is triggering for or install the sensors and wrap them up and lay them on the harness. resistors work a little better as the engine comes to operating temp it won't skew the figures from ambient heat.

What light are you talking about? Are you just saying that FUEL is reverse highlighted on the PFC's sensor check? If so, RotaryEvolution is right, that's just the reading for the fuel thermosensor.

Also, the only thing the PFC contributes to the fuel pump voltage is switching the resistor relay from 9v to 12v, that's it. The fuel pump wire doesn't go through the PFC or anything. Stock there is a good amount of voltage drop to the fuel pump, and with a weak battery and the car off, I could see a low voltage like that, but you should be seeing around 8v or so at idle at the fuel pump connector itself.

The fuel pump doesn't switch at a particular RPM, it switches with load. This is all in the shop manual.

Dale

I know the fuel pump wire doesn't pass through the power FC. I know I'm not explaining this properly. In the sensor check in the power FC where is says FUEL the voltage is reverse highlighted. It's causing the overheat light for the cat to light to show there is a problem. It's the same as the check engine light for the US spec cars. I tested the voltage at the pump while idling and it's only 5v I have no real way to test under load and see what it is when it switches to the high speed relay. If I bypass the resistor I still only get 10v at the pump but the sensor check is still only seeing 5v.
Are you saying the voltage reading in the sensor check is only for the thermo sensor? I assumed it was reading voltage to the pump since its getting the same voltage as its showing in the sensor check. I'll check my connections to the fuel temp sensor and will still run a new wire to the pump.
Thanks for the replies guys.

the fuel temp sender being lit up is not what is causing the light to come on. it is an error for the PFC itself. i honestly don't recall how to check to see what the fault is that is triggering the light to come on for the PFC though, i have never dealt with it before. some threshold warning settings will trigger the light.

as mentioned you also have an issue within your fuel pump circuit. it is not uncommon and most people eliminate it and rewire it from the battery through a fused relay.

FUEL is for the fuel thermosensor, that's it. nothing to do with the fuel pump or pump voltage matching what you are getting.

Thanks for clearing that up. I'm going to do the rewire today and I'll do a search to see what's throwing the light. The fuel was the only thing highlighted when I looked through the sensor check so I assumed that was the problem.
Thanks again.

The only thing that can light up that light, as far as the PFC is concerned, is high knock or high duty cycle. It doesn't actually light steady, it will blink. If it's lit steady, it could be the temp sensor for the cat - the JDM cars have a temp sensor that goes into the cat, that could be removed from the car, grounded out, whatever causing the light to come on.

Since that system is quite different from the US system most of us won't know how to troubleshoot it. But, if the light is steady, that's logical as to what's causing it.

Dale

I think I've had someone ask me about this CEL situation before on an RHD car. Fixing the fuel temp warning made it go away, iirc. In that case I think wiring in a resistor did it.

Fuel pump is totally unrelated, but yes it has a two-speed system with relay.

I have the cat removed and the temp sensor jumped so that's not what's causing the problem but I did think of that and checked it after your post Dale. I have the wire to run for the fuel pump so will do that tomorrow and will change the fuel temp sensor at the same time. I'll let you guys know what I find. thanks again

Hi Arghx, quick question, do you know exactly what resistor, and where? thanks.

Hi Bahamasair, I'm interested to know what happen! I also have the HEAT light all the time, and I think is the 5v on the Fuel Temp Sensor reading on the PFC.