| The sending units for both oil and temperature gauges
operate in the same manner. They are nothing more than a heat or
pressure related device that screw into the engine block. Heat or
pressure causes the sender to vary the resistance in the sending unit
and this variance is detected at the gauge by reacting to different
readings that it receives. Low resistance results in more current flow
between the sender and the gauge, and high resistance results in
lowering the current flow between the sender and the gauge.
gauge is a simple device. The current from the sender passes through a
resistor that either wraps around or is located near a bimetallic
strip. The bimetallic strip is hooked up to the needle of the gauge
through a linkage. The bimetallic strip is a piece of metal made by
laminating two different types of metal together. The metals that make
up the strip expand and contract when they are heated or cooled. Each
type of metal has its own particular rate of expansion. The two metals
that make up the strip are chosen so that the rates of expansion and
contraction are different. When the strip is heated, one metal expands
less than the other, so the strip curves, with the metal that expands
more on the outside. This bending action is what moves the needle.
On the Temperature gauge is a white wire that is fed from the
Electroluminence Power Pack that creates the wonderful glow for
observations at night. Along with the white wire is a Violet blue wire
that comes from the temperature sending unit on the engine via the
bulkhead connector. There is a brown wire that provides +5 volts that
comes from the fuel gauge. The Oil gauge has a white wire that feeds
the Electroluminence part of the gauge and that wire is in the same
cluster as the rest of the EL gauges.
The Oil gauge has a gray wire that comes from the Oil sending unit on
the engine, via the bulkhead connector. There is brown wire that comes
from the Temperature gauge to provide the +5 volts to the Oil Pressure
Gauge. Both gauges must have +5 volts going to them for them to work,
and they must have a good circuit from the sender to the gauge for them
to work. Put in simple terms, the bulkhead connectors must be clean and
not corroded. Finally, the senders must be good for the gauges to work
properly. You can diagnose where the problem is using the following
- First you can turn on the key, like the car is running and
then take the wire coming from the oil or temperature sending unit and
see if the gauge reacts or the needle moves on the associated gauge,
when you place that wire on ground of the car.
- The gauge needle should reflect High oil pressure or High
Temperature condition because you have minimum resistance that the
gauge is seeing, and should have maximum current flow. If you don't get
a needle movement then time to move on. Remember that you must have +5
volts feeding the gauge, so check that point. If you have +5 volts on
the gauge, time to move on. Remember that you must have a good current
path from the gauge to the sending unit, so time to check the bulkhead
connector to see if it is corroded or dirty. You can do this with the
help of another person.
- Remove the battery cable so that you won't blow up a meter.
You do have an ohm meter don't you where you can check for continuity?
Get under the dash where the spiders are crawling around and get the
wire coming from the sending unit you want to check.
- Remove the wire going to the sending unit for the gauge you
- Place one lead of the ohm meter on the wire at the
gauge, and the other lead to ground of the car. You should be seeing an
infinity reading on the ohm meter.
- Now ground the wire that goes to the sending unit, and you
should see a dead short reading. You have just checked the ability of
the wire to provide a current path from the sender to the gauge in
question. You should now know where the trouble is
- Either the gauge is bad, sender is bad, or the current
path is bad or the voltage is not there to operate the gauge. If a
gauge is diagnosed to be bad, you can double check the gauge by
removing the gauge from the dash bezel.
- You can use the ohm meter to see if there are any faults
in the gauge. You should have infinity between any one post and the
case of the gauge. All the posts are there for, is to provide a current
path between the sender and +5 volts.
- All the gauge is looking for is current, so using a 9
volt battery connect the + side of the battery to one post, and using a
jumper go from the other post to the negative side of the battery.
- You should see the needle start moving. If it doesn't
move, then the gauge is bad. Don't leave the jumper on too long because
the gauge is set up to use +5 volts.
- While you are in there with the ohm meter, you can test
the EL portion of the gauge by checking between the white wire tab and
the case of the gauge. You should read infinity.
~ Don ~ Original Owner 1967 Red Dodge Charger 383 4BBL