dimondjack

Well, I can't post a picture at the moment, but I did see it come on last night.

Unfortunately, I ran out of gas on the highway about 4 miles later. I put 1 gallon in courtesy of a good samaratin and then put in 18.5 gallons at the pump, for 19.4 gallons total. The car was REALLY done with gas, I limped about an extra mile by basically idling alongside the road.

Does the gas tank really hold 20.5 gallons? If it does, I'm guessing there is something wrong with my fuel pickup. All told though, I will IMMEDIATELY get to a gas station if I ever see the light come on again.

DaleClark

iTrader: (55)

Could be the fuel level sender or the fuel pickup is boogered up for some reason. I'd pull the fuel pump assembly out of the tank and give everything a look-see, it's not hard to do.

At least you know the light works .

Dale

wstrohm

According to my 1994 RX-7 factory workshop manual Section C-1a, page Z-42, the only chassis ground return for the "Fuel Low" warning light runs through the fuel sender unit switch. If the switch is open due to sufficient fuel level, that light cannot light, even if ignition switch is ON. (FYI, the eventual ground point for the return is Ground Point 3, in the engine compartment, and the return wire runs through five connectors to finally get there.)

Denorx7

It should come out when you turn the car on and then gone when start .
Just don't let the fuel below 1/4 is not good to suck all crap in tank .

wstrohm

No, not according to the '94 U.S. FWM, and not according to our '94 FD's operation. There is no way for current to flow through the low-fuel lamp unless the in-tank low fuel switch is closed. There is no self-test of that particular light during ignition turn-on. You might be correct if the wiring of non-U.S. models (or other year models) is different.

RedRevolver

iTrader: (5)

So yeah it definitely works. Being poor lately I've been running around low on gas a few times. It doesn't come on until the needle is pretty far below E. At least on my car, then again I can put 18 gal in and it only says 3/4 of a tank...so, probably varies from car to car. Point is, when it does come on, you are VERY low on gas. like get some gas ASAP. Only seen it twice, ran out once.

chicagozer

iTrader: (3)

FWIW, mine came on this week just as I was pulling in the gas station. Filled up with 16.1 gallons; so there seems to be a fair amount of headroom.

SA3R

This is a little embarrassing. I found the light in the gauge. Well, actually my girlfriend in the passenger's seat found it.. Turns out its more noticeable from the passengers seat angle... When I looked at it closer, there's the circular fuel light, between the E and the fuel pump symbol.

*My girlfriend is not a car person at all..

dimondjack

Whilst this may be true on your car, it is decidedly not on mine. I put in 19.5 gallons after the car died shortly after the light went on.

I think the moral of the story is to use the gauge, measure how much you put in, and then slowly inch lower to see if your light goes on. Just don't blindly try and get your light to come on, as your car could be like mine and you'll end up stranded.

Sniper_X

Let's establish the facts here and separate the ambiguities and assumptions about how this works/ is supposed to work.

1. As far as any FD RX-7 NOT having been built with a fuel low light, that's incorrect, no stock/production FD (JDM or otherwise) RX-7 was created without one. I have looked at every part available from Mazda over 5 years, and they ALL have the lens in the gauge face, and the bulb actually available from MAZDA.
   
   
2. Yes, the Fuel low light IS SUPPOSED to come on with the car in the "ON" key position with the engine off.

Now, the confusion about when the light will illuminate comes from the wacky stuff that happens depending on what parts are used, like the sender design, bulb type used, circuit constructions, and other things.

Here are the factors I have observed that impact how this light works (or doesn't):

Contributing factor 1:
The use of a common positive (+) in circuits vs. a common negative (-) as is usual.

Mazda uses a common POSITIVE circuit here so (as it does so often) instead of a common ground. This make cause a current leak through the filament of a traditional bulb, but due to the wattage and resistance, MAZDA deemed it "ok" and allowed it because the filament won't light using this leaked current.

Contributing factor 2:
Different fuel sending unit designs and manufacturers.

Depending on how yours is built, (original or aftermarket, etc) the sending unit MAY use the rheostat (variable resistor) at 99.9% or higher resistance to emulate the effect of the level switch being open instead of the rheostat for the gauge + switch for the light as indicated in the manual. in the end, this never COMPLETELY stops sending current, and once again we have a current leak to the warning light socket.

Contributing factor 3:
Bulb wattage / Technology (LED vs. Filament bulb)

A filament bulb, will likely still get a tiny amount of voltage from a 99.9% resistive rheostat unit in the tank, (or any other current leaked across it), and yet you will never see the light illuminate because a filament needs much more wattage to actually glow, or at least be visible through the dark amber lens in the gauge face.

This being said, here is why I think we get such different results from these low fuel lights on occasion...

A low wattage filament bulb, (lower than expected by the MAZDA guys), may indeed light very early as fuel diminishes in the tank, especially for sending units that fake-out using a switch + rheostat system with a 99.9% resistive method.

A high wattage filament bulb, (higher than expected by the MAZDA guys), may light very LATE (or not at all - which is unlikely) as fuel diminishes in the tank.
An LED used here (like I do) will likely light relatively brightly when low current/voltages are applied. (I remind you of the current leaking scenarios)
This LED will illuminate when the key is "ON", and may be dimly visible during the entire time the tank is full, yet get brighter as fuel level lowers even perhaps too soon.

When two or more of these factors combine, other odd behaviors can be seen.


I currently have to use a filament bulb, because when I use an LED, my Fuel Low warning light is dimly lit at all times, and never turns off , but gets very bright when i am low on fuel.


So, any thoughts on this?

wstrohm

No, at least not on a U.S. 1994 FD. See posts #28 and 30, or better yet, look at the schematic in section C1-a on page Z-42 in the factory workshop manual.

bajaman

I am still on the fence on this one, having reviewed the FSM repeatedly. It does indeed say that the fuel light should illuminate with the key in the 'on' position. This is logical, otherwise how would one know if the light was working or not? This is the FUNCTION of all dash lights, on all cars...it is the ONLY way to know if they are working properly.

Yet...some very well respected members with working lights have proven that this lamp..this ONE lamp, seems to NOT illuminate (at least on their cars) when the key is turned to the 'on' position.

Weird.

wstrohm

Don't know where the FSM says that, but the schematic is the last "word" in the manual, and it shows no connection on a stock FD that would allow current to pass through the indicator's filament other than through the low-fuel sensor in the tank.

Look at the schematic below, from the 1994 FWM (page Z-42). Note that the return paths for the "Overheat Exh System," "Charge," "Coolant Level," and "Oil Level" are each diode-OR'ed into the white/black wire going to the alternator. That point will be at a zero voltage level until the alternator begins charging the battery, which lights those lamps when ignition is ON but turns them off when engine is running. Now look at the "Fuel Low" light. There is no such connection to the alternator, so there is no self-test mode for that light. There would be one only if two diodes were added to its return line: one to the fuel sensor switch, and one to the white/black wire going to the alternator.

bajaman

But...how illogical would it be to have a warning light that you don't know is working or not? Doesn't make any sense at all.
Look at the FSM, C2-4, it SPECIFICALLY STATES: " fuel-level warning lamp, illumination ON when ignition switch is ON, engine stopped. Illumination ON when fuel level low - engine running." It simply doesn't get any clearer than this as far as what the function SHOULD be.

bajaman

a scan of the page:

Metan

in my is 14-16l when the light goes on

wstrohm

Bajaman, I agree there "oughta be" a self-test mode for the low fuel indicator when ignition is ON, and page C2-4 of the '94 Body Electrical Troubleshooting Manual does state that, as you posted. However... look at the schematic! The Fuel Low indicator is not connected to the self-test circuit (i.e. the white/black wire to the alternator). Its return line goes only to the fuel tank sensor switch.

The light will be on only if ignition is on and fuel is low enough to close that tank switch. And that is the way our '94 FD works.

Mazda just didn't implement the self-test function for that indicator.

I am only repeating myself to prevent someone from trying to troubleshoot the self-test mode for that light, when there isn't any!

wstrohm

Just to flog this dead horse once more, if you wanted to mod the "Fuel Low" warning light circuit to have a self-test like the other functions, you could do it by adding a diode as shown below, in the return line of that light. That would make its circuit the same as for all the other lights. (Note I show two diodes, but probably the one pointing down toward the fuel sensor is not needed... just the one to the left.)

Just a thought...

bajaman

SO...the FSM says the low fuel light WILL illuminate when the key is on and the engine is off. Yet we seem to have conflicting actual experiences.

Once again...weird.

SA3R

In one corner we have:

"Schematic wiring shows no ability for self test, due to how its wired" and wiring in the car matches this.

In the other corner of opinion we have:

"The manual states in text that it should come on with other dash lights in test mode when key is switched to On position"
Perhaps this is a misprint in the manual, or an excerpt taken from another Mazda manual like MX6, MX5, and cut/pasted in there during the writing of the book and not closely proofread?

You can screw up some descriptive text in the manual, but you cant really screw up a wiring diagram. I would tend to go with the wiring diagram, and I can also confirm that my fuel light does not light up in the ON key position.

If a diode was added, it would self test like the rest of the lights. Otherwise, there is no current flow, according to that diagram, to make it light in key ON position. Which follows with what a lot of people are saying?

I did check my other R1 car and all the other FDs I have. Out of 13 FDs, Australian market or Japanese market, all do have a fuel light. Its just hard to see. I believe that all FDs do have one. I'm a firm believer of this now.

bajaman

I don't know man...I just don't know. The FSM is 'the Bible' for these cars. I find it extremely hard to believe the description from the FSM is incorrect.

wstrohm

The FWM contradicts itself... text in the 1994 Body Electrical Troubleshooting Manual says one thing, schematic shows something different. (The schematic in the 1994 Workshop Manual does agree with the schematic in the BETM.)

Sniper_X

wstrohm is indeed right!

I never looked at it from the point of the diagnostic circuit not being implemented.

To verify this, I went to my 1995 FD and checked the circuit, nothing is there to allow it to light as part of the start up diag. lighting system.

My light being off when using a bulb, is normal.

My light coming on when using an LED is merely due to current leak.

adam c

iTrader: (2)

My low fuel light does not come on with the key on. I dont think I have ever run the tank low enough for the signal to be activated.

SA3R

I finally ran the car low enough yesterday to see the low fuel light come on.

It does not come on like the other lights when they key is clicked to ON position- IE no self test.

But with very little fuel, it does operate fine. In fact with the sloshing of fuel on slopes and uneven ground, it comes on and off as the fluid moves in the tank.

Here is a photo for anyone who may be searching in future: