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Author: Subject: Fuel Gauge issues
snippy

posted on 16/7/18 at 09:06 PM Reply With Quote
Fuel Gauge issues

I`m using a Smiths Telemetrix fuel gauge and a Smiths fuel sender unit in the tank. The sender unit has 2 terminals on top. I have 2 wires to connect to these, the signal wire and earth. One of the terminals has a `W` marking next to it. Should the signal wire or earth wire be connected to this? It doesn`t seem to matter though with the issues I`m having?

The fuel level sender has an ohms scale of 10 to 180 Ohms. I`ve checked the resistance across the 2 terminals and I`m getting 15 Ohms. I know I have approx 3/4 tank of fuel so this reading is probably correct as some give opposite readings (low is full, high is empty). A full tank will probably read 10 Ohms.

I`ve checked the fuel gauge itself which has 3 terminals, 12v feed, earth and signal wire to sender. I`ve got 12v in and 9.5v out on the signal wire. The signal wire at the fuel sender also reads 9.5v so no broken wire.

The problem is my fuel gauge only reads just above empty? With the ignition on the needle moves to just above empty, ignition off the needles moves to stop position. If I disconnect the sender wire on the sender unit with ignition on the gauge moves up to the full/max limit. If I touch the sender wire and earth together the gauge shoots back down to empty due to the resistance across the wires as it should do. Everything seems to be working but the fuel gauge reading isn`t. I thought it was a bad earth so I rigged up another earth from the sender terminal to the chassis but the same happens.
There is also another earth wire running seperately from the tank to the chassis by the way.
Any suggestions?

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gremlin1234

posted on 16/7/18 at 09:15 PM Reply With Quote
is it the original tank?
or have you taken a sender unit from one tank, put it in a new one and the float range is different.

maybe possible to bend the wire the float is on?

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snippy

posted on 16/7/18 at 09:23 PM Reply With Quote
It`s the original tank & sender unit that were fitted in the car when I bought it but not wired up to any gauges. It`s been a full ground up rebuild as the car has never been finished or on the road. I tested the sender before refitting it (against the tank height) and the gauge seemed to work fine.
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big-vee-twin

posted on 17/7/18 at 07:06 AM Reply With Quote
You may need one of these, I had similar issues sorted with this.

link





Duratec Engine is fitted, MS2 Extra V3 is assembled and tested, engine running, car now built. IVA passed 26/02/2016

http://www.triangleltd.com

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mcerd1

posted on 17/7/18 at 09:58 AM Reply With Quote
is the senders float getting stuck on a baffle inside the tank ?

(just to ask the stupid questions before you spend any money )





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snippy

posted on 17/7/18 at 07:53 PM Reply With Quote
Came home from work tonight and decided I would remove the sender unit from the tank and check it. Having removed it I discovered the float arm was very stiff instead of being loose. I checked the arm across it's full range and the fuel gauge worked fine. It turns out where the arm pivots inside a plastic collar was tight so I gently filed it until it was nice and loose. Once refitted my gauge now reads 3/4 tank. Problem solved and free too!
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mcerd1

posted on 18/7/18 at 08:22 AM Reply With Quote
do you think it might have been designed to add a little friction to damp out any sudden movement on the float arm (ie to stop it from changing the fuel reading when you corner / brake)
i had to change the fuel sender on my old tin-top and the new one was very tight for this reason (the old one had basically no friction at all, at least noe left anyway)

obviously yours was too tight though - but too loose might also be an annoyance when your driving





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snippy

posted on 18/7/18 at 02:36 PM Reply With Quote
It was too tight but I haven't made it too floppy for the reason you say. It's got enough resistance.
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Macbeast

posted on 19/7/18 at 11:12 AM Reply With Quote
Thanks V Twin. I was just about to search back several years for this company. I have mismatched gauge and sender so that the gauge reads in reverse and only goes to about half FSD. It didn't bother me, as I know what the readings mean but I will be selling soon so it would be nice to get it right.





I'm addicted to brake fluid, but I can stop anytime.

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