Fuel Gauge Troubleshooting

[skk0819]

Okay, I admit it - i am clueless as to how some basic stuff works,
but I am learning....Just changed out my front bearings, installed
shims too, etc.....for me this is real progress..

Newest minor thing to fix - the fuel gauge. Original was toast, so
I bought a "redone" one from Gordon's. After hooking it up, still
doesn't work...much. The needle goes from below empty to right on
empty when the ignition switch is turned, even with a 3/4 full tank.

I tried another cluster gauge, my car's original, to see if the
problem was in the one that is newer, and currently installed. The
original one behaves the exact same way, so the problem isn't in the
gauge.

What else in the wiring can I verify? Should any of the connections
on either the sending unit or the Gauge have any Active power? I
can't seem to find any with my tester. I don't know the basics of
how the sender manipulates the gauge I guess. Maybe a hot circuit
isn't involved?

thx for any suggestions.....

Scott

 

[WB6WSN]

----- Original Message -----
From: skk0819
To: amphicar-lovers@yahoogroups.com
Sent: Thursday, October 28, 2004 2:03 PM
Subject: [amphicar-lovers] Fuel Gauge Troubleshooting



Okay, I admit it - i am clueless as to how some basic stuff works,
but I am learning....Just changed out my front bearings, installed
shims too, etc.....for me this is real progress..

Newest minor thing to fix - the fuel gauge. Original was toast, so
I bought a "redone" one from Gordon's. After hooking it up, still
doesn't work...much. The needle goes from below empty to right on
empty when the ignition switch is turned, even with a 3/4 full tank.

I tried another cluster gauge, my car's original, to see if the
problem was in the one that is newer, and currently installed. The
original one behaves the exact same way, so the problem isn't in the
gauge.

What else in the wiring can I verify? Should any of the connections
on either the sending unit or the Gauge have any Active power? I
can't seem to find any with my tester. I don't know the basics of
how the sender manipulates the gauge I guess. Maybe a hot circuit
isn't involved?

thx for any suggestions.....

Scott



The sending unit should have only two connections, one ground wire and one
wire to the instrument cluster.

Let's divide the problem in half by checking the operation of the instrument
cluster wiring and the gas gauge. (If you are using a light-bulb probe as a
tester, go buy an inexpensive digital multimeter at Radio Shack. A
light-bulb probe can't help you with these low-power circuits.)

You need a simple potentiometer (variable resistor) to simulate the sending
unit. Since I assume you don't have an electronic parts stash, a trip to
Radio Shack is needed. Buy a 1000 Ohm potentiometer (271-280) and a 270 Ohm
resistor (271-1112). (It would be nicer if Rat Shack had a 200 Ohm or so
variable resistor, but they don't....)

Now, connect the fixed resistor and the variable resistor in parallel, like
this (and if you can't read this ASCII graphic, ask me off-list and I'll
send you a jpg sketch):


270 Ohm
0-------------------xxxxxxxxxx--------------------0
l l
l l
l 1000 Ohm l
l----xxxxxxxxxx----l
l ^
l l
l l
-----------

You will be placing the 270 Ohm resistor across the far ends of the 1000 Ohm
variable resistor, making a net resistance of 212 Ohms. By connecting the
movable wiper arm of the variable resistor to one end, you will now have a
variable resistor that adjusts from zero to 212 Ohms.

Now disconnect the instrument wire from the sending unit (at the sending
unit) and connect the resistor package between the instrument wire and
ground. Turn on the instrument power. With the resistor set to zero Ohms,
you should read EMPTY on the gauge. With the resistor set to the other end
of its range, you should read FULL.

If this works, then the problem is with the sender unit.

You could also try this same circuit at the instrument cluster connection to
check the wiring between the sending unit and the gauge. But I think the
problem is more likely in the sending unit or the gauge.

You could also try checking the resistance of the sender unit, but I'm
worried that you may be using a cheapy analog multimeter, and they can put
too much current into a low-resistance unknown. Even a little spark around
the fuel tank is a very bad idea. If you have a better quality digital
multimeter, then maybe reading the resistance of the sender would be OK.
First, measure from the ground connection to the chassis. Is it zero Ohms?
It should be. Next, measure from the sender instrument wire connection to
the sender ground connection. The resistance should be somewhere between
zero and 180 Ohms (empty to full). Does the resistance value reasonably
indicate the existing fuel level?


Ed
El Cajon, CA USA
67 Rust Guppy