Author Topic: Fuel gauge  (Read 490 times)

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Offline Gary t

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Fuel gauge
« on: Thursday,April 29, 2021, 03:03:26 PM »
When I did my first drive a couple of days ago one of the snags was fuel gauge  not working.
Now it sort of works but is wildly out of calibration, the heater coil on the bimetal strip inside the gauge  was open found the break and soldered it back together.  The needle now moves yea!! but indicates full with about  2/3 of a tank. My long awaited question is:  is there a repair/ calibration service in North America?
It is amazing what you can learn if you take something apart i now know how smith fuel gauges work  and why they might quit.
Gary Toffelmire
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Offline BDA

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Re: Fuel gauge
« Reply #1 on: Thursday,April 29, 2021, 03:31:37 PM »
I don't know if they do calibrations but Nisonger (http://nisonger.com) does repair, restoration, and sales of new Smiths gauges.

Offline jbcollier

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Re: Fuel gauge
« Reply #2 on: Thursday,April 29, 2021, 06:11:19 PM »
Take the float off and immerse it in very hot water.  If it blows bubbles, there's a hole (very common).

After that, follow this flow chart:

Key on, W or W/B wire disconnected at ignition coil

- at the tank, disconnect the sending unit wire and touch to a known good ground.

Does the gauge needle go all the way up?

If yes, go A.  If no, go to B.

A:

- now touch the sending unit wire to the sending unit ground wire.

Does the gauge needle go all the way up?

If yes, remove the sending unit and hook up its wires.  Lift the float all the way up.

Does the gauge needle go all the way up?  Then you have a float issue or you accidentally fixed a bad connection.

No?  Sending unit open, poor connection between resistor and moving contact.

B:

Go to the gauge and ground the where the sending unit wire hooks up.

Gauge goes all the way up?  Corroded connectors, connections, frayed sending unit wire.

Gauge reads low.  Test the voltage at the gauge power wire.  Roughly 10v or a pulsed 12v signal?  Gauge needs replacing or recalibration.  Low voltage?  Replace the instrument voltage regulator. Clean connectors and green fuse cionnections.  Check the gauge ground at the same time.

Gauge calibration procedure to get a working gauge but not necessarily 100% accurate. Leave the sending unit out but hooked up.  Jimmy the float arm so it is all the way up.  Remove the two circular stickers to expose the adjustment slots.  Key off, tweak one adjustment or the other unit it just reads full when the key is turned on (do not make adjustments when the gauge is powered).  Lower the float and make sure it goes to empty.  Tweak the adjustments until you get both a proper full reading and a proper empty reading.  Should be OK now.

John

Offline Gary t

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Re: Fuel gauge
« Reply #3 on: Thursday,April 29, 2021, 06:23:39 PM »
Thanks bda I sent a email  see what they say.
And thanks John also good  list of trouble shooting tips.
 Sort of got past that stage the sending unit seems ok it is the gauge  itself.  I (with little to lose) opened the case. I now understand how they work. The heater coil on the bi-metal strip was open found the ends and rejoined them. The needle now moves. If I don't find a repair/ calibration shop I'll  have to come up with  some sort of sceam to calibrate it myself.
Gary Toffelmire
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Offline EuropaTC

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Re: Fuel gauge
« Reply #4 on: Thursday,April 29, 2021, 09:58:02 PM »
This page

http://lotus-europa.com/manuals/misc/electrical/gauges.html

gives you what the sender in the tank should be doing at each stage and the effect of voltage supplies.  If that's not the problem, then the usual thing is to do as John says, twiddle with the adjusting screws on the back.  Here's some pictures from the MGB crowd of how it's done....

http://www.mgb-stuff.org.uk/fuel.htm

Brian

Offline Gary t

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Re: Fuel gauge
« Reply #5 on: Friday,April 30, 2021, 06:26:23 AM »
Thanks Brian that mg document is great
Gary Toffelmire
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