payne

What does the little silver thing at the bottom of the fuel tank do?

Its mounted on the fuel pump, about the size of a pinkie, it is connected by a green wire...

help?

-Jason

TT_Rex_7

If you have a picture, that may help.

-Alex

NewbernD

iTrader: (1)

I believe it's the sensor for the "Low Fuel" warning light.

payne

I have heard a few things,

1. low fuel light
2. capacitor to calm the signal
3. fuel temp sensor
4. fuel alcohol sensor...

DamonB

If you're talking about the little silver can shaped thing with wires coming out of it that's a capacitor. You put them across the leads of motors to get rid of electrical noise.

NewbernD

iTrader: (1)

2. Capacitor would be located somewhere else, like the dashboard. Why put it in the tank?
3. Fuel temp sensor is in the secondary fuel rail.
4. Now this is just rediculous.

NewbernD

iTrader: (1)

Is this what you're talking about?

DamonB

Because that's where the motor for the pump is.

NewbernD

iTrader: (1)

I was translating the vague "capacitor to calm the signal" into "capacitor to calm the signal that the variable resistor sends to the gas gauge." I see no other signal to calm back there.. perhaps that further explains my comment.

In this case, I don't believe Mazda installed anythign across the power leads of the motor. Besides, wouldn't you use some sort of inductor (coil of wire like a crossover) to reduce motor noise? A capacitor would help to even out voltage dropouts but would need to be considerably larger to do that effectively.

DamonB

No. Capacitors can be used as low pass filters. Capacitors across the power leads of a DC motor are very, very common as they keep the high frequency electrical noise from the motor brushes from getting into the rest of the car's electrical system. The cap filters out high frequency electrical noise from the motor. Ever heard alternator whine through your stereo? That's electrical noise.

There are many other uses of a capacitor. In this case the value of the capacitor is small and does nothing to smooth the input current. You'd need a much bigger cap for that.

In the old days when cars had distributors with points in them there was a little silver can with a wire called a "condensor". A condensor is just a capacitor and it was present for the exact same reason: To keep the electrical noise from the points constantly opening and closing from contaminating everything else.

Of course you could always check the schematics too...

payne

NewbernD - thats it, thanks for the pic!



I have the mazda diagram, and I could not find it in the fueling section.

DamonB

I just looked at the microfiche and it's hard to tell but it looks like that cap is part of the float assembly. I'll look at the schematics when I get home.

It could be a cap acting as a buffer on the fuel level sensor so the needle on the fuel gauge doesn't go everywhere as the fuel sloshes or it could just be carried on the float assembly and actually wire into the circuit elsewhere. The schematic will say for certain.

JebenKurac

i have opened up my fuel tank a few times before because of the floater getting stuck, and have seen that sensor also. i was always under the impression that once the fuel level went below that the low fuel light would come on.

DamonB

There is a cap across the fuel pump power leads but it's the little black one that's bolted next to the fuel pump resistor (component B1-10 on page Z28).

Page Z44 shows the fuel level sensor and it shows a variable resistor driving the needle on the gauge and a switch that turns on the low fuel light. I can't tell for sure without the unit in front of me but it appears the low fuel level switch is built into the float assembly. The buffer for the needle on the fuel gauge is inside the speedometer cpu in the gauge hood.

All that said I'm now not completely certain what the little silver thing is without the unit in front of me. My best guess without taking mine out to look is that it is in fact the low fuel warning light switch or just a filter on the float circuit. Which of those it is should be easy to figure out by looking. Seems to make sense that it's the low level switch because the went to the trouble to mount it at the bottom.

Shad Laws

The cap is likely for what was said above - to smooth out the fuel tank reading.

A cap by itself is not a low pass filter. However, a capacitor and resistor certainly can be setup as a low pass filter... the tank level sender is a variable resistor. There you have it.

If you've ever driven in a car without something like this (either in the tank or in the gauge), then you know what fuel sloshes look like. It's kinda neat to see the tank level change. You can even watch it rise as you fill up at the gas station :-). But, almost no cars are built this way anymore.

Take care,
Shad

payne

Shad-

you a Stanford student? I am.

What do you roll in?

-Jason

Mazda99Nikon

Just curious, so please excuse my obvious ignorance on this issue, but with all those electrical connections running through highly volatile gasoline, has anyone ever heard of an RX-7 blowing itself up at the gas tank? I remember a similar case of a 747 blowing up from just sort of a problem. A bit unnerving.

Shad Laws

Yup, I sure am :-). I have a '93 Montego Blue FD. You may see it parked around Rains or Terman quite frequently... shoot me an e-mail at shadlaws@stanford.edu and we can chat offline.

Take care,
Shad

rex u.k

The fuel suppreses the electrical input.

akiratdk

the fuel suppresses the electrical input???? What does that exactly mean??? Isn't fuel super sensitive to spark.... quote n quote.