By: John Foutz - 3/2000
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.
About the Gauges
The meter movement in all the gauges have a built in dampening mechanism which keeps the needle from bouncing around. The dampening mechanism is basically some thick grease on the movement's pivot points.
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 temperature 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.
About the Sending Units
The oil pressure sender is similar to the fuel gauge sender. The mechanical parts are different, but after that it is basically a coil of resistance wire wrapped around a card and the wiper moves across the winding to change the resistance. The higher the fuel level or engine oil pressure, the lower the resistance. With less resistance, more current flows and the gauge reads higher.
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.
Fuel and Temperature Gauges
|Back of speedometer, fuel, and temp gauge|
From the passenger side which is left to right in the picture:
Some manuals and gauges have the S and A terminals reversed on the temperature gauge. You can see the letters stamped in the insulation material around the posts. Either way, there is only one circuit loop inside the temperature gauge. The resistance test outlined below is still valid. A fine wire wraps around a bi-metallic strip and the heat caused by the resistance causes deflection of the strip and the connected meter. Like a light bulb, it will work no matter which way the current flows.
The jumper strap goes to a regulator that is inside the fuel gauge. A volt meter applied to the A terminal on the temperature gauge should fluctuate (plus and minus) near 5 volts. A reading of 12 volts on the temperature side indicates a bad regulator. 12 volts applied to the temperature gauge's A terminal will cook the gauge.
The fuel and temperature gauges can be bench tested by connecting a good ground to the gauge cluster and connecting positive 12v to the I terminal of the fuel gauge using a 3 amp fuse. The temperature gauge gets it power from the regulated output of the fuel gauge so the jumper strap connecting the A terminals of the fuel and temperature gauges must be left intact. The gauge needles shouldn't have moved from the off position until you take a resister from the S terminal to ground.
The fuel gauge should have the following resistance ...
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, just touch and release the wire. 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:
|23 ohms||1/2 tank|
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 resistance listed above. Run an appropriate resistor to the S terminal of the fuel gauge and to ground and check the readings.
|Inside of fuel gauge showing mechanical voltage regulator|
There is a mechanical voltage regulator inside the fuel gauge to reduce the voltage to 5 volts for both the fuel and temperature meters. When voltage is applied, the current flowing though the coil generates the heat necessary for the bi-metallic arm to react and open the contact which stops the current. The coil then cools down and the contact closes again. This process repeats itself over and over again.
This regulation process reduces the voltage directly to the fuel gauge meter and to the temperature gauge via the jumper strap. A volt meter applied to the A terminal of the fuel or temperature gauge should fluctuate (due to the breaking contact) near 5 volts. There have been reports of this regulated voltage being as high as 7-9 volts with no loss of gauge function. If 12 Volts is applied to the temperature gauge's A terminal, it will cook the temp gauge! (That notorious "puff of smoke").
A 12 volt reading at the A terminal indicates a non-functioning regulator due to the thin coil wire burning out or the contact has welded together giving continuous contact and sending 12 volts directly to both meters, often destroying them. A "0" volt reading at the A terminal can indicate a badly pitted contact which will prevent a voltage going through at all. Both gauges usually need to be replaced at the same time in either case.
The temp gauge has the following resistance ...
A volt meter can be used to measure the voltage between the A terminal of the temperature 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 resistance between the post and ground...
|Totally Cold||High Resistance|
|Slightly Warm||73 ohms|
|Beginning of Band||36 ohms|
|End of Band||13 ohms|
If an appropriate resistor is connected to the S terminal of the temperature gauge and to ground, the above resistance can be used to check the gauge. Use a resistor close to the specifications above to simulate the sending unit.
The temperature sender on 232 and 258 I6 engine is located at the top rear of the engine head. It is near the last head bolt and next to the valve cover on the manifold side of the engine right near the firewall. It is upright and cylindrical with one wire attached to its center post. The sensor's probe extends into the head's water jacket.
Oil Pressure Gauge
|Back of oil pressure gauge|
From the passenger side, left to right in the picture..
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. On some models, the oil pressure switch sends a signal to the engine's computer.
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.
It is easiest to test the sending unit by temporarily plumbing in a good mechanical gauge.
|Pressure (PSI)||Resistance (ohms)|
|Back of volt meter|
From the passenger side, left to right in the picture ...
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.
Many thanks to all those who contributed to this article. Use this information and your Jeep at your own risk.