Editor's Note: This article is being republished to provide you with updated information on the CJ guage and sending units.

August 2001 -- 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 these Gauges

From left to right (passenger side to driver): S - from fuel sender. A - fuel guage side of jumper strap. I - to ignition-on 12 volt battery source. S - from temp sender.

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 damped 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 damping makes the movement take the average reading. This also is why it akes 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 "factory replacements" (Mopar # 4740791) are considered the best. These have "C" and "H" as well as the "E" and the "F" in the stock OEM orientation. Note: Imported, after-market gauges have these ranges reversed. Both replacement brands are dual 12 volt units and have different wiring and post setups. Use their respective instructions and diagrams during replacement.

About their 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 oil pressure, the lower the resistance. With less resistance more current flows and the gauge's meter registers 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.

Gauge Sizes

• The Oil Pressure and the Volt meter gauges are 2 inches in diameter.
• The Tachometer is 2-7/16" diameter.
• The Clock is 2-5/8" diameter.
• The Speedometer cluster is 5-5/8" diameter.

Tip: There is a company in Minnesota that will rebuild your clock with a new Quartz movement for about $65: ATP Instruments, 9632 Humboldt Ave S., Bloomington, MN 55431, (612) 881-7095

Fuel and Temp Gauge post ID (From the passenger side:)

• Fuel gauge S terminal - Pink wire (from Fuel Sender)
• Fuel gauge A terminal - Jumper Strip to Temp gauge
• Fuel gauge I terminal - Red wire (Ignition-on hot 12V)
• Temp gauge A terminal - Jumper Strip from Fuel Gauge A terminal (regulated to approx. 5 volts)
• Temp gauge S terminal - Purple wire (from Temp Sender)
  Ending on the Drivers side.

Here's the view from inside of Fuel gauge illustrating the mechanical voltage regulator.

Note: Some manuals and gauge setups have the S and A terminal IDs reversed on the Temp gauge. In any case, there is only one circuit loop inside the Temp gauge. The resistance test (below) is still valid. A fine wire wraps around a bi-metallic strip and the heat caused by resistance causes deflection of the strip and the connected meter. Like a light bulb, it will work no matter which way the current flows.

This regulator reduces the incoming 12 volts on the I terminal to near 5 volts for both Fuel and Temp 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 Temp gauge via the jumper strap.

A volt meter applied to the A terminal of the Fuel or Temp gauge should fluctuate (due to the breaking contact) near 5 volts.

Note: there have been reports of this regulated voltage being as high as 7-9 volts with no loss of gauge function.

Warning: 12 Volts applied to the Temp gauge's A terminal 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.

CJ Fuel Gauge Resistance Tests

• 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

CJ Temp Gauge Resistance Tests

• S to A 19-21 ohms

Testing the Fuel Sender unit on a CJ

The sender should have one wire (pink) with voltage from the sensor's isolated center post. The tab style connector and wire is a ground to the frame. Make sure it has good contacts.

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. No resistance at all will peg the needle FULL (the whole 12 volts).

DO NOT hold it long in this position - just touch it and release. If the gauge does not move from EMPTY either the gauge's wiring has an open circuit (no voltage, or no connection to ground) or he gauge and/or its voltage regulator is bad. If it does move, the sender unit or its wiring is bad.

CJ Fuel Sender Resistance Tests

With an ohmmeter check the resistance between the round sender post (pink wire) and ground. It should be as follows:

• 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.

CJ Water Temperature Sender Resistance Tests

The Temp sender on a 232/258 I-6 cyl. engine is located at the top rear of the engine head. It is near the last head bolt and next to the valve cover and firewall on the manifold side of the engine. It is upright and cylindrical with one wire attached to its center post. The sensor's probe extends into the head's water jacket.

With an ohmmeter check the resistance between the post and ground.

• 73 ohms - Cold Test when engine is slightly warm. A stone cold engine will read a much higher resistance.
• 36 ohms - Beginning of Band
• 13 ohms - End of Band
• 9 ohms - Hot

Testing a Fuel and Temp gauge while it is out of the CJ

Looking from the back, left to right, the terminals should be marked by stamping on the insulator near by. For the fuel gauge. S, A, I. For the temp gauge. A, S.

Connect a good ground to the gauge cluster.

The Temp gauge gets it power from the regulated output of the fuel gauge so the Jumper Strap connecting the A terminals of the fuel and Temp gauges must be left intact.

Connect +12V to the I terminal of the Fuel gauge - use a 3 amp fuse.
The gauge needles shouldn't have moved from the off position.

Use a volt meter 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 regulator in the Fuel Gauge is bad.

Connect a resistor to the S terminal of the Fuel gauge and ground the other end of the resistor. This simulates the fuel sending unit.

• Use a 73 ohm resistor and the gauge should read "E".
• Use a 23 ohm resistor and the gauge should read "1/2".
• Use a 10 ohm resistor and the gauge should read "F".

Connect a resistor to the S terminal of the Temp gauge and ground the other end of the resistor. This simulates the temp sending unit.

• Use a 73 ohm resistor and the gauge should read "C".
  
• Use a 36 ohm resistor and the gauge should read "Beginning of Band".
  
• Use a 13 ohm resistor and the gauge should read "End of Band".
  
• Use a 9 ohm resistor and the gauge should read "H".

The resistor values above are what is specified in the AMC book. Some of these are not commonly available values. The closest standard values are 75 ohms, 22 ohms, 12 ohms, 10 ohms, and you may even have trouble finding the 75 ohm, so you could use either the 68 or 82.

Oil pressure guage.

Oil Pressure Gauge post ID (from left to right)

• Oil Pressure gauge S terminal (left) - Purple wire from sender
  
• Oil Pressure gauge middle terminal - Black wire (Ground)
  
• Oil Pressure gauge right terminal - Red wire (Ignition-on hot 12V)
  Ending on the Drivers side.

Oil Pressure Sender

This sender is on the engine block and looks like a small 2 X 3 inch filter with one terminal. The other sender with the two connectors is an oil pressure switch. It is supposed to close above 4 psi and goes to a dash warning light in some speedometer clusters and in other models it sends the 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. This should show up as 80 psi on your gauge. No resistance at all (full 12 volts). DO NOT hold it 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.

Oil Sender Resistance

These are hard to test but might help you to choose the correct replacement.

• 0 PSI - 234-246 ohms
  
• 20 PSI - 149-157 ohms
  
• 40 PSI - 100.5-105.5 ohms
  
• 60 PSI - 65-69 ohms
  
• 80 PSI - 32.5-34.5 ohms

Volt meter.

Volt Meter post ID (from left to right)

• Volt Meter gauge GND terminal (left) - Black wire (Ground)
  
• Volt Meter gauge terminal (right) - Red wire (Ignition-on hot 12V) (ending on the Drivers side.)

You 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.

Fuse block guage.

Blowing Fuses? Having a Short?

Larry Van Every has written a great trouble shooting guide for you ...|

1. Buy an inexpensive voltmeter... and learn to use the OHM meter section.
2. Get a schematic.
3. Look at the schematic, and find the fuse for the gauges.
4. Pull the fuse out of the panel to remove voltage to the gauges.
5. Notice that the wire going from the output of the fuse goes to three gauges and the 4WD drive lamp.
6. So, if this fuse blows one of two things has to be happening:
   a. One of the gauges, or the lamp is shorting to ground.
   b. The wire going from the fuse to the gauges and lamp must be shorting to ground.
7. To find out which, with the ignition off:
   Put one lead of the OHM meter to the Output of the fuse ( the red wire which powers the gauges) and the other lead to ground. You should see a VERY low Ohm reading... like zero ohm. That will blow a very BIG fuse, very quickly! Now pull off the power lead to each gauge, one at a time while observing the OHM meter.
   a. Off comes the power lead to the oil gauge.
   b. Off comes the power lead to the fuel gauge.
   c. Off comes the power lead to the temp gauge.

If the Ohm reading increased from zero in a; b; c, above then you know where the short is when you remove the wire and the Ohm reading jumps higher.

Still No Luck

If the reading remains the same, then the harness itself must be shorted to ground or the 4WD lamp/socket is shorting. Remove the 4WD lamp.

• Ohm meter still at zero?
  Remove the socket away from ground.
  
• Ohm meter still at zero?
  You have now isolated the problem to a bad red wire/harness feeding the gauges and the lamp. Physically trace the wire back from each termination point (the gauges) and look for:
  a. Chaffing.
  b. Pinching,
  c. Shorts to ground

If you find nothing, then cut the supply wire at the point where the wire separates and goes to the fuses. Check each half for shorts. This will tell you which half the short is in. Continue to bi-sect each suspected length of wire until you have isolated the short.

It's as easy as that! Once you have the process down, it's an hour job, tops! But it's not easy because you are always looking up into the dash. You may want to loosen the dash so you can see into it from between the dash and the cowl.

Good Jeepin'
Larry Van Every

Early 1980's CJ gauge wiring:

# 4A - Red -18 Ga. Fuse box to splice.
# 4B - Red -18 Ga. Splice to Fuel gauge
# 4C - Red -18 Ga. Splice to 4WD lamp
# 4D - Red -18Ga Splice to Oil gauge

Going to Ground

Are your headlights flickering? Do your gauges sometimes work and then not?
Dave Mellow has some great tips about grounding and your Jeep's electrical system.

Remember that you are actually driving three separate Jeeps around: the FRAME; the ENGINE/TRANNY GROUP; and the BODY. The three groups are separated by insulating bushings, rust (lots of that), grease, and just plain looseness.

When your Jeep was new and rust free, the factory thought enough about grounds to have little cables here and there going from the battery to these component groups, but when you and cousin Leroy jerked the engine in '89, you broke the cables and they never got put back.

Do your Jeep a favor, and run dedicated cables to EACH COMPONENT GROUP.

Go from the battery to the starter housing bolt for the ENGINE/TRANNY group; from the battery (or the engine) to the FRAME GROUP; and from the battery (or the engine) to the BODY GROUP.

Work hard to get good clean spots; ground the tub and front end body material separately; be diligent about this, and you will be AMAZED at what a difference it will make (gee, the gas gauge started working'!), and maybe now that measly twelve volts can actually find it's way to the headlights!