fuel sender connections

[kenwhiteside]

Pulled the tank out to fit one that doesn't leak.

The old tank fuel level sensor had a yellow wire attached to the plug and a black wire attached to the tank. Looks as though a poor repair to the black wire. I assume the black wire is the ground.



Why is there three connections? I assume I only need the two wires?

[ian65]

if the black wire is fixed to the sender unit, it's been bodged. Quite often on these senders, the earth fails and a fix (bodge) is to take an earth directly off the sender body rather than use the connecting plug.
You're right, you only need the 2 wires... the third wire is for an unused low fuel level sensor, you can see it here next to the float arm...




The revised dash models which were only available in the US in left hand drive had a low fuel level light but all UK spec cars across all 3 series didn’t have this feature. I’ve often thought this was strange given the cars appetite for fuel and the rest of the car being so well equipped for its day.
The fuel sender unit on the series 3 has 3 male terminals on it in a plastic housing and the plug from the car loom is also a 3 way but has only 2 wires going into it. The 3rd space in the plug is left empty.





I had a go at connecting the low fuel sensor up on my old car.....I got an old piece of loom with a female plastic plug on it, cut the plug open and retrieved the female terminal connector…. This then just pushed into and locked in place in the spare space in the terminal block on the sender wire. You can see the additional wire here in orange....





My guess is that if the fuel level drops, to say 1 gallon left, the sensor will go to ground and complete a circuit so if I connect a bulb or led to +12v that comes live with the ignition and the other end to this sensor, I should have a working low fuel light.
My thoughts are that when the new tank was introduced for the Series 3,( and the stateside GSL-SE) Mazda just used the US spec sender and ignored the fuel level sensor part of it as the UK cars weren’t wired for it, as they still used the old dash.

[kenwhiteside]

Thanks for the detailed response.

If connecting the earth to the tank directly is the bodge how do I fix it properly? Can I pull apart the sender and re-connect the earth or do I need to get a new sender.

Thanks,

Tim

[ian65]

these senders are very delicate and temperamental Tim.... if yours works as it is, I'd just splice or connect the black wire that's been added to it into the wire from the loom leading to the plug..... they are so easy to break completely it isn't worth trying to fix your's if it's got poor connectivity..... the only way forward is to get a good replacement sender from a parts car. Make sure it's from a s3 though as they are slightly different.

[codge]

these senders are very delicate and temperamental

You can say that again....I changed my tank about 4 years back and the sender stopped sending halfway through the first trip out.

I've run round brimming the tank and calculating fuel against mileage ever since. BUT.....it suddenly jumped into life again 2 weeks ago. .... WTF!

[kenwhiteside]

Hmm well so far I have removed the sender and very carefully put in on the bench so whats the bet that it will not function when I try to reinstall it! Usually is the way.

Any tips for cleaning the rubber compond off the bottom of the tank before a re-seal? Hesitent to use a grinder on it so was scraping and wire brushing but not really getting it to the condition I wanted. Think it needs some paint stripper.

Tim

[Steve-A]

I used a wire wheel on a grinder on my tank. It ate right through the underseal/stone guard/rust on the outside of my tank, took a while though, don't think the neighbours enjoyed it!

[codge]

Remember these tanks perforate on the top Tim, just by the filler neck. So to preserve the tank you really have to give it extra dollops of sealant on that part.......though it is argued as well that they rust from the inside due to water condensing on the top inside surface.

Dave