Gas Gauge doesnít work
This is so typical. Many gas gauges donít work
for simple reasons. In would strongly suggest you read this whole
document before proceeding. You may want to attack this problem in a
different way. Please bear with us. This is a very difficult
thing to write.
Just a note. Our cars are usually molested pretty
well by the time we get them. Make sure your wiring under the dash is
in good shape. If itís not, I would suggest you start there. Having
seen some of these chimpanzees at work, youíre liable to find anything
under there. Maybe the problem.
Most people are apprehensive with auto wiring. It
is really quite simple and you canít get hurt with it. Just remember
your body is the ground, and the wires carry the voltage. BUT! There
are a few ground wires under the dash too. So be careful when you
Here are the components
Wiring from fuel tank to dashboard.
Voltage Limiter (Regulator)
There are FOUR of the most common problems.
1. Defective Dash Voltage regulator
(Voltage limiter) or gauge
2. Lost of a
ground at the sending unit
3. Break in the
wire going to the dash
4. On some
Chryslers, the Bulkhead connector on the firewall or connector behind
Actual bad Sending Unit
Fuel Gauge defective
Tools needed for the basics:
Alligator Clip jumper (Make sure it works)
Test light probe
Radio Shack or equal Multi-meter
If removing the gauge cluster, you will need a
socket set, a couple of #2 Phillips screw drives and a pair of needle
Gas Gauge and Temp gauge doesnít work. (Reason
Letís ask a simple question here. Does the temp
gauge work too? Both the Temp gauge and the fuel gauge use a voltage
limiter. Given the way these cares were built, it is hard to get too
get to that. But before we dig into this, check the fuse for the
instruments. That fuse feeds the voltage limiter. In time, voltage
limiters tend to go bad. If one burned out, it may have blown the
fuse. Which in a way is a good thing. If the fuse is good and the temp
and Fuel donít work, then chances are itís one of two things. Lost the
ground or the limiter is bad.
After you are sure the fuse is good, (Check it
with the Key ON), we can go for the checking for a ground to the
cluster. If your cluster is back lit, and the dash lights work, chances
are you have a good ground to the dash. If itís front lit like the
Mopars of the early 70ís, then we need to check the ground. If you have
any lights on the dash, oil light (Idiot Light) turn signal indicator
and they work, then a ground is established.
If all these things are good, then you have to go
for the voltage limiter. This is where the fun starts. Depending on
your brail skills, you should be able to at least find it on the back of
the cluster. If you canít, then youíll need to remove the cluster. A
good suggestion here is to make sure you have any other under Dash tasks on a list ready to be done. I strongly
advise you disconnect the battery while removing the cluster. Once out,
you can reconnect to finish your test.
It looks something like this.
Since you are going this far, itís a good idea to
just replace the regulator anyway. They are available from NAPA or any
decent auto parts store. Most of those chains donít know anything about
it unless itís in the computer. Remember, you are dealing with people
that only need a license to breath in the big chains, Brains werenít
Just the fuel gauge is out Supplement
IF the dash cluster has a printed circuit board,
double check to be sure itís in tact and there are no burned traces.
Take your test light and connect the one end to
ground. Turn on the ignition key and check the back of the fuel gauge.
On the red wire, (I think itís red. . they vary) you should get a dim
pulsing light. If that is present, then you need to remove the gauge
and inspect it. Look at the back of the gauge when itís removed. Make
sure the wires are connected to the posts. Make sure the coil isnít
charred black or cooked. If itís charred, the gauge is done and your
reason it doesnít work. Start looking around for a cluster. I found it
less expensive to buy a whole cluster than just the gauge.
If it gauge does work: (Reason #2)
The quickest way to determine if itís the wiring,
bulk head connector, or the voltage to the gauge is to ground the wire
at the sending unit.
Use an alligator clip jumper and ground the wire
to the sending unit to a chassis ground. Turn on the key and see if the
gauge goes to full.
Reconnect the wire to the sending unit. Using the
same alligator clip jumper. Ground the sending unit to the body. If the
gauge works, you know what you have to do. On non-molested cars, there
is usually a clip that jumps across the rubber hose from the sending
unit nipple to the fuel line. This clip establishes a ground between
the sending unit and the body.
If it is present, simply twist it around to
re-establish a ground. Over time, corrosion develops, loosing the
ground. A generous application of simple Vasoline around the ends of
the taps will prevent it from happening again.
Before you go any further, letís make sure the
sending unit is working. You can get a general idea if the sending unit
is working by checking it for resistance by using a simple multi-meter.
Set your Multi-meter to 0-100 Ohms. By now you know how your car, so
guess at how much fuel is in there and take a reading accordingly.
full tank will read 9 to 10 ohms. An empty tank will read 96 to 98
ohms. Put the red probe from the meter to the center post on your
sending unit, and the black to the body. Try using it to the fuel line
too. If you get something in between those two parameters, we can at
least determine itís putting out something. If the multi-meter is
reading 50 ohms, as an example, then you have approximately half a
tank. If there is no reading at all, then guess what? Run the tank
down and remove the unit. However, if you are getting a reading, then
itís enough to activate the gauge.
The gauge does not work: (Reason #3 and 4)
This is a little tougher because some cars come in
through the bulkhead connector or run through the body. You need to
look at the tank wire and see if it enters the body at the trunk or
under the rear seat. Usually it would head towards the drivers side.
Most cases, the wire goes into the cabin and joins the tail light
wiring. Note the color of the wire at the tank. Most likely it will be
the same color to the next connector. From there it may change.
Usually a blue or black wire.
Letís use a 71 Fury as an example
If the gauge does not work, we need to work our way
forward. Reground the wire to the sending unit with the alligator
jumper. Remove the driver sidekick panel and locate the multi-connector
going to the rear of the car. Check for corrosion and broken wires.
Determine which wire is the sending unit wire. In the 71 Fury itís a
bright blue wire. Connect your test light to a hot position on the fuse
block and test it to ground to make sure it lights up. Disconnect your
connector and check to see if the light goes on by probing the plug that
goes back. If it lights up, the wire is good to that point.
OK Put the tank back together and make sure youíve
maintained a good ground. Put the car down, weíre done back there.
Just for the heck of it, poke a ground wire into
the connector that goes up into the dash and see what happens. Turn on
the key and see if the fuel gauge comes up. If it does, go back and
check you wiring to the back again. Smear some Vasoline on the
connections and reconnect the plug.
Still not working? I assume you havenít put the
cluster back in by now. My suggestion would be to double-check your
ground to the cluster again at this point. If you replaced the voltage
limiter as mentioned in phase one, everything should be in order.
Youíll have to repeat step one over again. If the gauge is in good
condition, and the dash cluster is in order, you need to repeat all
Here is an idea on how to ground
the sending unit to the body.
Strip a piece of wire, lightly sand a small spot on the sending unit
fuel nipple. Using a small hose clamp, clamp the wire to the
nipple. Smear some Vasoline or a dot of grease on the connection
to prevent corrosion. Connect the other end to the body with a
screw, or slip it under a bolt somewhere nearby. Make sure the
body connection is clean. You can grease that too if you want.
This is not as complicated as it sounds. If you
are pretty good with tools, you should be able to diagnose and possibly
repair the gauge or gauges in half a day, and thatís allowing for time
to run to the parts store.
Trying to tell you how to do this is difficult
with so many different engineering schemes out there. But the basics
are all the same. All the clues and what to look for are all here.
Since I canít see what you are doing, I canít explain it any easier.
Itís all common sense.