speedy_az

I Need help In locating the FUEL PUMP RESISTOR Relay Please. That is the only Trouble Code that I get when I run the self-Diagnostic. Or if anybody has any other info on this problem. Thanks Speedy_AZ

HAILERS

Remove your airfilter assy. Directly in front of the airfilter assy is the Fuel Pump Resitor Relay assy.

NZConvertible

Both the location of the relay and resistor and the error code troubleshooting procedure are in the FSM Fuel Systems chapter. I recommend you read it and do what it says.

If you don't have the FSM, click here and download it in PDF format for free.

HAILERS

I really don't see how the ECU can tell if the relay is kaput or not. There is but one wire b/t it and the ECU and that is a green/red wire that puts a gnd on the relay when there is no load on the system. It's possible the ECU also looks back thru the relays coil at the voltage to the coil OR is looking at the current at the green/red wire that runs b/t the coil and ECU. Got me. I've a series four that won't throw a code like that even if I cut the green/red wire.

If it were me, after I did the above, I'd tap into the fuel pumps input wire with a meter and look at the voltage going to the pump. It should be batt voltage during starting and just after starting the voltage should drop to about 9vdc. Then go for a drive. You should see approx 9vdc when cruising along. Stomp the pedal and it should go to full Batt voltage. Done.

I take it your car runs/drives ok because you did not mention it not working. Just the code 51.

NZConvertible

Probably the same way it monitors all the other 13 outputs (solenoid valves and injectors) that generate error codes. Current and/or voltage drop would be my guess too.

speedy_az

The car runs fine I'm just trying to get the check engine light off. Thanks for your info.

speedy_az

Thanks fot the PDF downloads and all the info. That is going to help out a bunch! I'll let you know how it turns out.

NZConvertible

This system runs the fuel pump at two different speeds depending on engine load; reduced speed at low load and full speed at high load. If the relay has failed off (most likely) then the pump will be running at full speed all the time, which will result in richer low-load mixtures and increased fuel consumption. If it's failed on (unlikely but possible) then the pump will be running at reduced speed all the time, which will result in leaner full-load mixtures. Neither will have noticeable effects, but obviously lean mixtures could result in engine damage. I highly recommend you check the fuel pump voltage as Hailers suggested.

HAILERS

There's a jpg attached of the Resistor/Relay assy. There should be a green/red wire coming into the large white plug from the EM harness. It goes to the ECU.

That relay even looks expensive. Have you ever tried to clear the codes from your ECU? Per the FSM?

Even if the relay failed, to me a worst case scenario would be your fuel pump seeing full batt voltage all the time and never seeing the cutback voltage of approx 9vdc. In other words it would fail *safe*. Much like a water thermo sensor failing to a safe 179*F or the air temp sensor failing to a safe figure.

NZConvertible

It probably is it you buy it from Mazda, but you could replace a failed one with a generic 5-pin DPDT replay for just a few bucks.

RETed had a case of the relay getting water in it and rusting everything, eventually causing it to fail in the "on" state, i.e. running the pump at reduced speed constantly. This is the only example I've heard of this happening though. Failing "off" like you said is far more likely.

slo

iTrader: (1)

the ecu senses resistance across the relay's solenoid, this can be replaced with a standard relay, and rewired so as the pump always see's 100% voltage.

HAILERS

Here's a Headlight Retract Relay from a early series four. It look nice in the wrecking yard, but pull the cover off and walla! It was located near where the fuel pump resistor relay is located. Same bulkhead but off to the right.

NZConvertible

This makes little sense. If you want to rewire the pump for full voltage, you would delete the relay not replace it. And doing that is silly anyway, because all you'll achieve is increased fuel consumption.

Fixing the problem is easy and cheap. No half-assing required.

HAILERS

Might as well give this thread some legs....here's a schematic of the resistor/relay. It's out of a series four FSM, but the series five works the same. Different pins on the ECU though.

I'm throwing in this jpg of a T-34 from WWII being pulled out of a lake in eastern Europe a few years ago.

slo

iTrader: (1)

BS, the reason for replacing it with a standard relay is to get rid of the check engine light, if the relay switching circuit is left open then the check engine light will stay on. So effectively the relay isn't switching anything, and yes this could be done with a resister, but many people have a very basic understanding of electronics that doesn't extend to the level of picking out the correct resister.

And as to increased fuel consumption the fuel pump on NA car's is no walbro (which arent that great either), I seriously doubt that there would be increased consumption.

Do you understand how a fuel pressure regulator works? I'm sure you do, and I'm sure you know that increasing the fuel pressure by a marginal amount before the FPR would merely bypass more fuel back to the tank.

Now if he put a rewired walbro on the car, it would probably overcome the stock FPR and increase consumption, but an OEM NA fuel pump I don't think so.

I have a stock 13b-rew fuel system yes this is different from his NA or your turbo2, I have a rewired modified supra pump that supposed to be good to well over 300+LPH, no resister.. Guess what, it makes no different to the base fuel pressure. The car has the same base fuel pressure as a stock FD.

NZConvertible

If someone is unable select a resistor to replace a relay then they're probably also unable to wire up another relay to provide a constant voltage, since it would have to be wired up different from stock. And if you're going to buy and install a relay, why would you do anything other than fix it properly so it works the way it's supposed to?

If the fuel pump speed is higher than normal (i.e. full speed instead of reduced speed), fuel pressure will increase and fuel flow through the injectors will increase correspondingly. The mixture will be richer as a result.

I do know how FPR's work, and I know that they can't compensate for upstream pressure changes. That's not what they're there for. It has been shown before that manually changing the pump speed on the engine while it's idling results in an AFR change. I believe Hailers was the one who proved this.

slo

iTrader: (1)

Nope, compensating for upstream pressure changes is exactly what they are there for, upstream is everything in line with the fuel pump up to the FPR itself.

I have tested this myself with a fuel pressure gauge. (yes I have a 13b-rew, but the principle is the same)

If its not there to compensate for upstream pressure changes then what exactly do you think its there for?

HAILERS

The resistance of the circuit from the green/red to the black/white is given in the FSM, Fuel section...... 68-92 ohms.

The last time I looked, if you have the engine running, then disconnect the green/red from the ECU, the afr changes and I'm going to have to go look tomorrow, I do believe the fuel rail pressure changed also. Just from memory of the above from a couple of yrs ago. I had made a bet about this.......and lost.

Actually, that is what the poster of this thread should do. Pull the relay/resistor out and ohm out the coil/resistor and see how close to 68-92 ohms it is. That should give you a clue if the relay is good/bad. There's no contacts that open/shut for this coil, so what you read is what you get. If it reads like the above, maybe, just maybe, unplugging the connector and cleaning the contacts on the PLUG, and then reattaching the plug could make things ok again. Just clear the codes and restart the engine and see if the code reappears ....or not.

slo

iTrader: (1)

Turbo or NA and S4 or S5?

On the 13B-rew the circuit controlling the fuel pump resister also affects a vacuum solenoid inline with the FPR, that would cut vacuum to the FPR if there was a malfunction in the system. Since at idle there is lots of vacuum pulling the fuel system pressure down this causes the fuel pressure to go up (not allot) but if you where to directly connect the FPR vacuum nozzle too the manifold so that this solenoid doesn't come into play this wouldn't happen.

I have never looked that deep into the stock NA fuel system but I suspect that this may be whats happening.

Next AFR's should not be the basis for this test, it should be fuel pressure.

NZConvertible

C'mon, this is basic EFI stuff. The FPR is there is keep a constant pressure differential across the injectors, so that their flow rate remains constant irrespective of load. As manifold pressure goes up, fuel pressure goes up by the same amount. As manifold pressure goes down, fuel pressure goes down by the same amount. That's all it's there for.

Do you have any other explanation for the proven example I gave above?

slo

iTrader: (1)

Varying fuel pressure based on manifold pressure, is secondary to keeping the fuel pressure constant with a source that varies in flow with voltage, age and load (as in the injectors at WOT removing pressure from the lines) the FPR increases or decreases the amount it bypasses to compensate for fluctuations in pressure that happen, yes you guessed it upstream of it.

So you really don't understand what an FPR does.

NZConvertible

Answer my question. Explain how the measured AFR gets richer when you increase the fuel pump speed, because this is contrary to what you're claiming.

slo

iTrader: (1)

Two things are possible, the ECU may be reacting to what it perceives as a dangerous situation (which would be the fuel system stuck in low pressure resister mode). If it actually where the car would lean out under heavy load because the pump wouldn't be pumping enough.

Note that the pressure at low load in resister mode will always exceed the base fuel pressure.

It may react to this by increasing the injection duration, or possibly switching and electrical solenoid in between the FPR and manifold causing it to not see vacuum. The FD has this solenoid and if resistance is not present on any of the 2 fuel control relay's it goes into limp mode (making the car super rich and preventing any boost) and trips the solenoid.

Second the FPR could be overwhelmed by the flow of the fuel pump. I have heard than NA's will do this is a walbro is wired in without the fuel pump resister, but I seriously doubt this will happen with the stock NA pump simply rewired.

An added rewired walbro won't change the base fuel pressure on a T2 or FD and my supra pump doesn't change the base fuel pressure on an REW.

Again this test should only be testing actual fuel pressure and not AFR, seeing as there are many other things which can affect AFR.

NZConvertible

The test was done on an S4, and the S4 ECU has no way of knowing the state of the relay or reacting to the fuel pump's speed being manually changed.

As above.

If the FPR solenoid was activated the idle fuel pressure would jump from ~29psi to 37psi, which would result in a much greater AFR change than is seen when simply changing the pump speed.

The FC doesn't do that.

We're talking about the stock 2-speed pump system, not an upgraded pump. Why would the FPR be overwhelmed by the stock pump's full-speed setting?

Yes it does. Plenty of people have posted about this. A direct swap on a TII sees base fuel pressure raise from 37psi to 42-43psi.

slo

iTrader: (1)

I could provide a line by line retort to each of your statments but that getting off the real subject your statement:

That is exactly what an FPR does.

Second just because the S4 doesn't have a CEL doesn't meant that it doesn't have and react to engine codes.

And finally, a fuel pressure regulator in some form is on every single Gas fuel injected vehicle ever made, and most carbed vehicles with a fuel pump.

A fuel pump resister is not on most fuel injected vehicles.

You are failing to understand what an FPR does.

If pressure and consequently flow increase with greater upstream pressure as in from a larger pump, then this is most likely a result of the FPR not keeping up with the pump, this does not change the basic nature of the part or how it works, it ca only bypass so much fuel.