Baseline 4x4 Outfitters Newsletter

 Jeep CJ Speedometer Gauge Troubleshooting

Published 11/19/07
One of the common problems associated with the 1976 to 1986 series Jeep CJ series is the gauges in the speedometer cluster. No surprise with more than 30 years of abuse and neglect. The first thing that we tell any of our customers, that encounter a problem in this area, is to check all of their grounds. It is imperative that the vehicle is properly grounded for optimum performance of the electrical system. The negative post on the battery must be grounded to the engine block. The block should be grounded to the frame by the use of a jumper wire that is generally located at one of the engine mount bolt holes, on the block, and attaches directly to the frame. There is also another ground wire that attaches the engine block to the body. If any one of these cables is missing or is not sized properly, the electrical system on your vehicle will not operate properly. You also need to make sure that the connectors, on these cables, are properly installed and that they are properly terminated at the point of connection. It is easy for a little water to collect under the termination points and corrode causing a high resistance connection. This additional resistance will decrease the amount of voltage supplied, to the affected electrical components, within that system. In turn, that particular system or systems will not work to its designed potential. These high resistance connections can also produce heat due to the increase in resistance. This additional heat can have dramatic negative impact on the overall performance of the electrical system in your vehicle.

The best way to check your grounds is to use either a test light or a digital voltmeter (DVM). With the test light, attach the clip lead end of the light to the positive post on the battery. Touch the pointer end of the test light to the negative post of the battery. The test light should illuminate to its maximum level of light output. Keep in mind how bright this is and then touch the pointer to the engine block, the frame and the body. The test light should illuminate to the same level as it first did when you touched it to the negative terminal on the battery. If the same intensity is not shown at anyone of these areas, then check your cables and termination points for a good connection. One additional item to note is that when touching the test light to the engine block, frame or body, make sure the area that you are touching is a good clean metal surface. Any painted or rusted surfaces will not give you a good test point and could result in a false reading. If you are using a DVM, set your meter to the lowest range on the ohms scale as possible. Attach the negative lead to the negative terminal on the battery and touch the same points as described earlier with the test light. Your DVM should indicate from 0 to 4 ohms of resistance. If the indicated reading is any higher than this, then you need to check and clean the ground cables and contact areas as we discussed earlier.


The fuel gauge system consists of a sending unit mounted in the tank and an indicator that is mounted in the speedometer. The biggest problem encountered on this system is the ground wire that connects the sending unit to the frame. This wire is notorious for breaking or encountering a high resistance connection either at the frame or the sending unit tab that the wire is mounted to. Repair or replace this wire, as needed, making sure that the point that this wire is connected to at the frame is clean and free of any rust or corrosion.

The fuel indicator has 2 terminals. Both of these are located on the back side of the gauge. One is labeled “I” for indicator. This terminal is attached to a 12-volt supply through the ignition system. There is a voltage reduction circuit that is built internally into the gauge. The reduced voltage is required due to this being part of the fuel system. The second terminal is labeled ”S”. This terminal is wired directly to the fuel tank sending unit

The resistance chart below can be used to test both the indicator side of the circuit and the sending unit side too.

Fuel System Resistance Values
Empty	73 ohms
Half	23 ohms
Full	10 ohms

The temperature circuit is similar in design other than it operates off of full battery voltage. The primary components in this circuit are the temperature sensor and the temperature gauge. The temperature sensor is nothing more than a thermocouple that varies in resistance as its ambient temperature increases or decreases. The circuit operates by supplying battery voltage to the "I" terminal on the back of the gauge. The "S" terminal is connected directly to the temperature sensor. This sensor screws into the engine and senses the temperature, in the water jacket, of the coolant as it circulates through the engine during operating conditions. The sensor is grounded through the engine block back to the battery. Once again it is important that the ground cables are in place as we discussed earlier.  The gauge is calibrated to the sensor and indicates accordingly to the resistance that is supplied to it from the sensor.

Referring to the chart labeled “Engine Temperature Resistance Values” you can use a DVM to check the resistance of the temperature sensor. Place the positive lead of the DVM to the center post of the engine and the negative lead to a good system ground. Set the DVM to the R x 1 Scale and start the engine. If the engine was cold and the outside air temperature was approximately 70 degrees, the DVM should indicate some where between 200 to 400 ohms of resistance. As the vehicle runs and the temperature of the engine begins to increase, the resistance values will decrease also according to the supplied temperature scale. If your readings are different than the ones supplied, replace the temperature-sending unit.

The gauge can be checked for proper operation by building a simple circuit consisting of a power supply, a temperature gauge and a series of resistors of the indicated values listed below. Attach the positive side of the power supply to the "I" terminal on the temperature gauge. With the use of a couple of clip leads, attach the appropriate value of resistor from the "S" terminal to the negative side of the power supply. Ohm's law states that V = I x R where V = Voltage, I = Current and R = Resistance. Based on this theory, the minimum wattage of the resistors should be no less than 2 watts. Anything less than this value will overheat at the lower resistor values

Engine Temperature Resistance Values
70 °	200 - 400 ohms
130 °	73 ohms
171 °	36 ohms
242 °	13 ohms
270 °	9 ohms