Jeep CJ Gauge Sender Diagnostics 72 86 by John Foutz 1300221252
Jeep CJ Gauge Sender Diagnostics 72 86 by John Foutz 1300221252
Jeep CJ Gauge Sender Diagnostics 72 86 by John Foutz 1300221252
by John Foutz
This article only covers gauges and sending units used in the AMC Jeep CJs built
from '72-'86, but some of this information will be useful for owners of other Jeeps.
There are some slight variations with the Jeeps built from '72-'75, but starting in '76
things remain pretty consistent for 10 years.
The reason all the meters are dampened is because the senders do not have a very
constant resistance. If you put a good testing meter on the sender while the motor is
running you will see the resistance bounce all over the place. The dampening
averages out the reading. This also is why it takes a few seconds for the needles to
come up to position instead of snapping to a reading.
When ordering replacement Fuel and Temp gauges for your Speedometer cluster,
Stewart Warner brand are considered the best. These have "C" and "H" as well as the
"E" and the "F" in the stock OEM orientation.
Imported after-market gauges have these ranges reversed, both are 12v, and have a
different wiring and post setup. Use their instructions and diagrams during
replacement.
The oil pressure sender and fuel sender are both electro-mechanical devices but the
temperature sender is not. The temperature sender is a temperature dependent
resistor (Thermistor). The type in the Jeep is a NTC (Negative Temperature
Coefficient) which means that as the temperature goes up, the resistance goes down.
Gauge Sizes
Gauge Diameter
Oil Pressure 2"
Volt meter 2"
Tachometer 2-7/16"
Clock 2-5/8"
Speedometer cluster 5-5/8"
Some manuals have the positions of these terminals incorrectly identified. You can
see the letters stamped in the insulation material around the posts.
Fuel Gauge
The fuel gauge should have the following resistances ...
S to Ground 68-72 ohms
S to I 19-21 ohms
S to A 19-21 ohms
I to A Zero
I to Ground 49-51 ohms
A to Ground 49-51 ohms
The fuel sending unit wires are located on top of the gas tank where they are hard to
get to without dropping the tank. The fuel sending unit should have a pink wire with
voltage on the isolated center post. The other black wire on the sending unit with a
tab style connector is a ground to the frame. Make sure it has good contact.
To be sure the problem is not the gauge, you can momentarily short the pink wire on
the output of the sender to ground, and this should show up as FULL on your gauge.
DO NOT hold it for very long in this position. If the gauge does not move from
EMPTY either the wiring has an open circuit (no voltage, or no connection to
ground) or he gauge is bad. If it does move, the sending unit is bad.
The sending unit can be checked with an ohmmeter to measure the resistance
between the round sender post (pink wire) and ground. It should be:
Resistance Reading
73 ohms Empty
23 ohms 1/2 tank
10 ohms Full
If the resistance falls in this ballpark (depending on how much gas you have in the
tank), then the sending unit is fine. If it shows infinitely HIGH resistance, then the
sending unit could be bad OR the wire from the tank to the gauge could be open.
The gauge can be tested with the resistances listed above. Run an appropriate
resistor to the S terminal of the fuel gauge and to ground and check the readings.
Temperature Gauge
The temp gauge has the following resistance ...
S to A 19-21 ohms
A volt meter can be used to measure the voltage between the A terminal of the Temp
gauge and ground. It should be pulsing and averaging about 5 volts. If it reads 12
volts the Jumper Strip/Regulator is bad. If it reads 0 volts, it has been burnt out.
The sending unit can be checked with the following resistances between the post and
ground...
The oil pressure sending unit is on the engine block and looks like a small 2 X 3 inch
filter with one terminal. There may be another sender plumbed in the same area that
has two connectors. It is an oil pressure switch that is supposed to close below 4 psi
to activate a dash warning light in some speedometer clusters.
To be sure the problem is not the gauge, you can momentarily short the wire from
the output of the Sender to ground. If there is no resistance, your gauge should read
80 psi. DO NOT hold it for long in this position. If the needle does not move from
zero psi then, either the wiring (open circuit) or the gauge is bad. If it does move, the
sender unit is bad.
It is very common for the sending units used with the 258 and 232 engines to be
inaccurate. Make sure you have a good connection to the sending unit.
Volt Meter
Testing the voltmeter is easy, you just need a good 12 volt connection to the (+) post
and have a good ground to the (-) post. If the gauge shows no activity, then the
gauge is bad.
Conclusion
Many thanks to all those who contributed to this article. Use this information and
your Jeep at your own risk.