Thanks for the guidance Barney. Your wiring diagram is much easier to
read than the shop manual. If I understand this correctly the 0-70 reading
registers when I connect one wire on the multi-meter to the chassis and
the other to the sending unit with the gas gauge wire connected and the
ignition off?
I reversed the wires on the back of the gauge but it only seems to work
one way.
I tried connecting a grounding wire from the chassis to the gas tank edge.
I will try it again with a heavier wire and a better connection to the
tank.
I think the next step is to drain the tank and pull the sending unit. To
check
the 0-70 resistance reading with the sending unit out do I ground the outer
edge of the unit to the chassis with an alligator clip. leave the gauge
wire
connected (probably not necessary) and connect the multi--meter between
the electric stud on the unit and the other wire to the chassis? Or do I
just pull the unit out of the car, connect one wire of the meter to the
electric
stud and touch the housing with the other end and move the float to vary
the
resistance?
Thanks once again.
Steve
'61 MGA 1600
----------
> 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
|