Barney
I made some progress today. I removed the sending unit, measured the
resistance as you suggested. Good news! It works fine. I also installed
a new ground from the chassis wiring harness bracket six inched away
to the base of the sending unit. Unfortunately the gauge doesn't work.
With three gallons it says 7/8 full. I reversed both wires and no good.
When I turn off the car ignition the gas gauge goes back to E.Even
though I had the gauge checked several months ago but I am not
sure it is working correctly. How can I check the gauge? It is a
Smiths gauge but it does not have a number on the face. It may
have been new (reissue?) in 1985 when a lot of work was done.
Steve
'61 MGA 1600
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> From: Barney Gaylord <barneymg@ntsource.com>
> To: SSchultz@worldnet.att.net
> Cc: mgs@autox.team.net
> Subject: Re: MGA gas gauge?
> Date: Saturday, October 10, 1998 7:24 PM
>
> At 08:40 PM 10/10/98 -0400, Steven R Schultz wrote:
> >....
> >I know the tank currently has about three gallons in it. I haven't
> removed the sending unit yet. I checked the tank grounding by taking a
> test light and touching the sending unit wire stud with the ignition on
and
> touching the light probe to the tank. It is grounded.
>
> This can be a little deceiving, because the ground connection is not
> necessarily just connected (0 ohms) or not connected (infinite ohms). It
> is common for this ground connection between the tank and the chassis to
be
> a high resistance connection. When you have a low wattage bulb with a
high
> resistance, a little extra resistance in the circuit just makes the bulb
> glow a little dimmer, but it still lights.
>
> When I had trouble with my gauge the tank ground connection was reading
> about 30 ohms resistance, so the current would flow and the gauge would
> work, but the gauge wouldn't go all the way to the empty mark when the
tank
> was empty.
>
> >I touched the sending unit wire from the gauge to the chassis and the
> gauge went to full (not to empty).
>
> Uh, just a guess here, but you may have the wires on the back of the
gauge
> reversed. It should go to the empty mark when you ground the signal
wire.
>
> >.... multi-meter .... Grounded one end of the multi-meter to the chassis
> and touched the other end to the electric stud on the sending unit. With
> the ignition off and the gauge wire off I got a reading of 90.5
>
> Measuring the combined resistance of the sending unit and the tank ground
> connection here. Looks like a high resistance ground connection for the
> tank. The reading should go from about 70 ohms when full to 0 ohms when
> empty, never higher than 70 ohms. With 3 gallons of fuel in the tank the
> reading should be around 20-25 ohms. If you check the resistance between
> the tank and the frame, I think you will find at least 50 ohns resistance
> there, and it should be 0. If so, it needs a ground wire for the tank.
>
> >Grounded one end of the multi-meter to the chassis and touched the wire
to
> the gauge while it was disconnected . With the ignition off I got 63.8.
>
> Measuring here the combined resistance of the gauge and the ground
> connection of the gauge to the dash and chassis.
>
> >Connected the sending unit wire and touched the multi-meter to the
> electric stud and grounded to the chassis. With the ignition off I got
34.5
>
> Doesn't tell you much here, as you have two high resistance ground
circuits
> in parallel, but the numbers make sense.
>
> 1 1 1
> ---- + ---- = ---
> 90.5 63.8 X
>
> and solving fox X = 37.4, pretty close to you measurement.
>
> >Are any of these what you were suggesting?
>
> Yep, still looks like a high resistance ground connection for the tank.
>
> >Should I get the 0-70 with the ignition on, wire connected and ground
the
> other end of the multi-meter?
>
> NO !!! Don't try to measure the resistance with the ignition on. The
> supplied voltage could blow out your ohm meter. If it actually worked
you
> might be measuring negative resistance. No help that way.
>
> Add a ground wire from the tank to the chassis. If the gauge goes the
> wrong way, try reversing the wires on the back of the gauge. One
terminal
> on the gauge carries a single wire that goes to the tank sending unit.
The
> other terminal on the gauge should have three wires, one being power from
> the switched fuse, one going to the heater blower switch, and the other
> going to the turn signal switch.
>
> Point a browse to this address for diagram of the MGA fuel gauge circuit.
>
> http://www.ntsource.com/~barneymg/mgtech/electric/circ_f1.htm
>
> Barney Gaylord
> 1958 MGA with an attitude
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