A peek inside CJ gauges

By John Strenk

 

This article is a little more “in depth” review of the stock gauges used in the CJ between 1976 and 1987. With a better understanding of the gauges and the sensors used you will be better off trying to trouble shoot the gauges used in your jeep.

 

The hardest part of working on the instrument panel is trying to get behind the panel. After many contortions I have found it easiest to slide the dash out a few inches and work from the top. Remove the top screws from each end and insert a 3” bolt on the hole. This will support the dash after you remove the rest of the screws from the dash and allow you to slide the dash out a bit. You may find it necessary to remove the speedometer cable from the back of the speedometer. Just turn the ring on the connector to remove. Unless you are built like King Kong, you should now have enough room to reach behind the dash to work on your gauges.

 

One of the first things to do is to add an auxiliary ground to the dash. Over the years the metal under the bolts have probably gotten rusty and not making a good connection. If you have a metal tub you can ground the dash someplace on the firewall or better yet, to a common grounding block attached to the (-) post on the battery. Grounds are important for the proper operation of the voltmeter, fuel and temp gauges and the speedometer lights. I like to use a soldered ring terminal when attaching a ground wire to the dash. I just use a self-tapping screw into the bottom lip of the dash but if you want to keep your dash looking neat, you can attach the ring terminal to the stud used to hold the instrument cluster to the dash. You will need to have good grounds to the Tub/Frame/Engine for some of the gauges and lights to work properly.

 

Gauge Wiring Basics

 

First, lets talk a little about the basic wiring of the gauges before I bore you with details.

                                                                                 

Here is a basic diagram of the CJ Gauges:

 

Notice there one ground for the speedometer. This is for the temperature controlled current limiter in the fuel gauge Not for anything directly for the gauges.  The grounds for the fuel and temp gauges are made at the fuel level sender at the tank to the frame. The temperature and oil pressure gauges ground at the engine block. The case for the speedometer does have to be grounded for most of the lights to operate. If the lights start to flicker due to a bad ground, your gauges will probably start to fail also. There are two exceptions for the light grounds. The brake warning light and the 4-wheel drive or Emergency Drive lights. These lights have separate grounds. I ran out of room in the picture to show the ignition switch but it is between the fuse and the rest of the circuit. 

 

 The voltmeter has a ground connection on the back of the gauge. If you have installed new gauges and they still don’t work then I would check the grounds from the sensor to the negative (-) post on the battery. Remember the Tub/Frame/Engine grounding loop. I’ll talk more about this in the trouble shooting section near the end.

 

Speedometer Cluster

 

  

 

The speedometer cluster contains 2 gauges. The fuel gauge and the temperature gauge.

 

 There are 3 bulbs to illuminate the speedometer and have a blue filter over the bulb. This makes it necessary to use the correct bulb size so as not to melt the filter. These bulbs ground thru little clips on the socket so the speedometer housing has to be grounded in order for these to work.

 

There are 2 lights for the turn signal indicator. These indicators also ground through the sockets to the housing. These lights have a little plastic housing to prevent light from spilling in or out of the meter face. Other wise you might not know if your high beams are on or your turn signals have developed a bad ground.

 

 There is a light for the “Brake” system that will come ON under 3 conditions. 1. If the ignition switch is in the “START” position. 2. The Emergency or parking brake pedal is depressed and 3. You have an uneven pressure in your brake lines because of a leak. This light has it’s own ground going to 3 places as you would expect, on the ignition switch, e-brake pedal and switch on braking system combination valve. The RED wire goes to the ignition switch for power. On all models since 1976 there was a light for “Emergency Drive”. This light is connected to the transfer case used on the Quadratrac drive or if your lucky to have a Dana300 transfer case. This light will illuminate anytime you’re in 4WD. One wire goes to the switch in the transfer case and the other wire goes to the ignition switch for power. The last light is the High beam indicator. This light grounds thru the case and the wire goes to the dimmer switch on the floor. If you just started driving yesterday, this light will come on with the high beams. J

 

The speedometer gauges.

   These gauges consist of a simple electro-mechanical gauge that uses a bimetallic strip to deflect the needle. The bimetallic strip has wire wrapped around the strip to heat the metal so it will bend. The more current that passes through the wire the hotter it will become causing greater deflection in the needle. The end of the strip has a little pin that goes into the needle that will move the needle. This allows it to be independent of the voltage polarity. It also makes them self-correcting for temperature changes. If you’re in the middle of winter and the inside of your Jeep is somewhere around –20* C It will take more current to heat up the bimetallic strip controlling the current so more current will flow thought the Fuel meter heater also and correct for the Temperature.

              

 No needle dampening is needed in these gauges because rapid changes in current will be dampened by how fast heat is transferred to the bi-metallic strip. It also makes them very rugged, as there are no jeweled bearings to break.

 

  

Temperature Gauge               Fuel Gauge

 

The bimetallic strip is used a lot in the CJ. It’s used as a “voltage regulator”, gauge movement, turn signal flasher, and circuit breaker in the headlight switch

 

Let’s talk about the fuel gauge first as this contains the so called “voltage regulator” for the gauges also.

The red arrow points to the bimetallic strip that moves the needle in the fuel gauge. As the strip is heated it bends moving the needle up scale. The temperature gauge works the same way. It’s identical to the fuel gauge except that it has no regulator installed. It gets it’s regulated current from the “A” terminal on the fuel gauge via a flat strip connecting both gauges.

 

The yellow arrow points the bimetallic strip that’s acts as the so called “voltage regulator”. As current passes thru the wire wrapped around the strip, it heats up the strip and opens the regulator. It’s really acting more as a temperature controlled current regulator than a voltage regulator. The reason the voltage is lower is really a result of the short amount of time the current is flowing, and the response of the meter testing it. A typical analog VOM meter has a response time of a couple seconds; the bimetallic strip is only closed for a few tenths of a second (0.1second). If you have a DVM, you could probably see the real voltage, but it’s gone so quickly from the display that you will hardly see any voltage. So many people assumed the regulator was regulating the voltage.

 

The heating element in the regulator has a resistance so does the meter heater and the sensor itself has a resistance. This creates the following circuit:

 

Figure 1

 

 

If we would put a scope on the “A” terminal you would probably see the following voltage trace:

In this picture each major vertical division is 5 volts and each horizontal division is 0.1 sec. So this fuel gauge regulator puts out a 12 volt pulse for 0.1seconds every 0.7 seconds. That’s a pretty small pulse huh.

 

In fact you will probably see a similar trace on each of the sender inputs on each gauge. This has lead many people into thinking they have a bad connection on the circuit and try to fix something that is not broken. The only thing different on the sensor is the height of the pulse. This is because of the resistance of the sensor in the circuit.

 

Why not just use a resistor to limit the current? The resistor will get extremely hot and waste power. Using the regulator conserves power and reduces the heat. Plus it will work for any voltage range your jeep can put out with out affecting the accuracy of the gauges. Plus it will work with any polarity. Most solid-state regulators have a problem if you reverse the polarity.

 

How does it work? Well if we look at Figure 1 above, the circuit on the left is an “equivalent circuit diagram”. The temperature controlled regulator and the fuel and temp gauges combine to create a parallel circuit.

 

The current through each part varies depending upon the resistance of the sending units. The regulator is always fixed so always will have the same current. If the voltage goes up then the current goes up and the strip heats faster and opens up sooner and longer. This prevents any more current to flow thru the gauges. So if the voltage varies it will still allow the correct amount of current to flow through each gauge. 

 

The short time the regulator is closed, current passes thru the gauges heating up the gauges. When the gauges heat up, the needle moves. So if the sending unit in the fuel tank is full. The resistance is small and there is more current through that gauge. The more current the more heating in the gauge and the more the needle moves.

 

What happens when you accidentally hook the power up to the “A” terminal? You will get unregulated current moving thru your gauges heating up the coils on the meters and burning them up. It’s the “puff of smoke” failure you may hear people talking about. Here is a picture of a blown gauge windings. :

 

See all the insulation burned off. This will make the gauge inaccurate even it still manages to work.

 

Speaking of accuracy the blue and green arrows show you can adjust the gauges. The blue arrow adjusts the low end and the green arrow points to were you adjust the top end of the scale.

 

 

 

For some years the oil pressure gauge also used the regulator in the fuel gauge and had a

Bimetallic strip also. Now the oil pressure gauge is more of a voltmeter and uses a solid core meter movement. More on this when we cover the Voltmeter.

 

The fuel gauge has a ground contact on the rear of the meter. This is just a springy piece of metal clamped onto the back of the gauge that makes contact with the speedometer housing when it is installed. If there is any rust or corrosion this will make poor contact with the housing causing it to stop regulating the current and allowing full current to flow thru the gauges.

  

 

Fuel Sending unit:

The fuel-sending unit consists of float connected to an arm, a wiper arm and a resistance wire wrapped around an insulator. There is also a pickup tube with a filter on one end and a provision for a return line.

 

In this picture you can see the resistance wire and the wiper that makes contact with the resistance wire.:

 

 

If your eyes are good you could probably see were heat has distorted the end of this winding. This is because the current regulator stuck sometime or hooked up the +12 volts to the regulated connector. This will allow too much current to continue thru this part of the winding. This caused it to overheat distorting the insulator it’s wrapped around. If the contact arm does not have enough travel to contact the wiper it will cause an open circuit and the gauge will indicate empty. The one on the right is completely missing the wire wrap section and only makes contact when the tank is full. A new fuel level sender is nice and straight.

 

 

There are two connections on this unit. One is a screw type connector going to the fuel gauge and the other is a ground connection usually connected to the frame. If you loose the ground connection then it will be an open circuit and the fuel gauge will indicate open. The ground connection might be welded to the top of the fuel sensor. If it breaks off you need to find someone to weld the contact back into place or provide other means to ground the gauge. Some of the replacement sending units have a spade lug to connect the ground. Having to depend upon the gas tank to be your ground would not be wise.

 

Water Temperature Sensor: 

 

The water temperature sensor is located on the top of the head on the rear, drivers’ side of the engine.

 

To me, this would be the best location for the sensor as there is probably the least amount of circulation and cooling at this point. Unfortunately it’s also a great place to capture an air bubble throwing off the reading.  If your having trouble locating this sensor then look here:

This I6 engine is from another AMC product and the temperature sensor is located on the front of the head near the thermostat housing. This might happen if your jeeps previous owner had to replace the motor for some reason

 

 The sensor itself is a Thermistor were the resistance of the sensor decreases as the temperature increases. There is really not too much to say about this sensor unless you want to get into the physics of a Thermistor.

 

 The resistance varies as follows:

?? ohms at Room Temperature. Roughly 60* F.

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

 

 

Oil pressure

Pressure Sensor:

 

 The oil pressure sensor is located on the passenger side of the engine. On later model CJ’s it is also teed with an oil pressure switch that controls the electric choke and manifold heater. The oil pressure sensor has a pressure diaphragm connected to a moveable arm. This arm slides against a resistance wire wrapped around an insulator. As the arm moves it varies the resistance in the circuit.

 

 

As you can see in the picture, if the diaphragm becomes ruptured or develops a leak the insides can get filled with oil and cause problems. Normally the gauge is sealed pretty well against the environment with an o-ring between the cap and base. As this seal deteriorates because of heat, moisture can leak in and corrodes the contact. The screw connector on the top makes it connection just by pressing on the metal tab on the top of the sensor. If this seal fails, this contact can corrode and give erratic readings.

 

The resistance of this sensor drops as the pressure increases:

 

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

 

The Oil Pressure Gauge:

 

The Oil pressure gauge has a solid core meter movement. There are a couple of wires wrapped around an iron core. The needle itself is mounted on a little magnet that sits atop a pin. The magnets normal attraction to iron performs a damping action for the needle. The amount of current flowing thru the coils will move the magnet a certain amount. The oil pressure sensors resistance wire controls the current. The only connection to the meter is +12 volts from the battery and connection to the sensor:

 

 

You probably notice I said “current”. A meter like this is really an ammeter. The more current passing thru the coils create a stronger magnetic field moving the magnet in the center of the windings. So when the pressure gauge moves down on the resistance wire, more current passes thru the windings because of less resistance and the needle moves up scale. If you were to ground the sensor lead to ground, the meter will go full scale and hopefully you wouldn’t burn the meter up or permanently damage the windings.

 

The Voltmeter:

 

If you notice the voltmeter looks a lot like the oil meter, you’re right. The voltmeter is also an ammeter with a fixed resistance (the coil windings). So as the voltage increases the current thru the windings increase and the needle moves. An interesting thing about the voltmeter is that there is a zener diode inside that prevents current from flowing until the voltage reaches a certain value. With out it the meter will read from 0 volts to 16 volts and since we are only interested in between 12 to 15 volts about 69% of the scale would be useless. Putting in a zener diode will make the lowest part of the scale to start at around 10 volts. There are other problems like linearity that are cured by using two different coils with different windings to make the movement linear. So instead of having the first ˝ if the scale reading from 10 volts to 10.5 volts and the last ˝ of the scale going from 11 volts to 16 volts, you end up with even spacing between all the volts on the scale. Pretty clever huh?

 

Since the voltmeter is an ammeter any resistance before the meter will affect the display. So for every switch, plug, wire, corrosion before the current can reach the meter will indicate a lower reading. Even if there is some device that might have a better connection some were in that circuit then it will draw more current from what is available and the voltmeter will get less lowering the current causing the meter to show a lower voltage on the scale. This is why the voltmeter bounces when you turn on your turn signal or turn on your heater fan.  So, one poor connection in a switch will limit the current for the rest of the circuit.

 

 If you really want to know if your alternator is working than put a voltmeter across the battery directly. You don’t want to do that with the factory voltmeter because the windings have such a low resistance that the meter can draw about Ľ amp and will slowly drain your battery.

 

Gauge Diagnostics:  

Here I will add some information and pictures of how to trouble shoot gauge problems.

 

The first thing to buy, borrow or steal is a DVM (Digital Volt Meter). You will need this to perform resistance and voltage measurements. There are many styles out there and even the cheapest ones will be good enough for our use.

 

The first problem that is likely to happen is the fuel and temp gauges fail. CJ’s are famous for water leaks and most of these leaks flow onto the speedometer housing. The grounds for the gauges are just a little dimple stamped into the housing making contact with a little strip of metal on the back of the fuel gauge. Any wonder why a little corrosion will stop the gauge from working? Don’t submerge your speedometer.

You can use you ohmmeter and check the ground between the case of the speedometer and the (-) side of the battery but that will only tell you if the housing is properly grounded. Take your ohm meter and pull of the wire on the “I” terminal on the back of the housing. Touch the ohmmeter to the “I” terminal and the (-) side of the battery. If the gauge is making good contact with the housing then this will show 51 ohms.  If not then you have a bad contact inside the speedometer housing. This also could indicate a badly pitted contact on the regulator not allowing any current to pass. You can check this by putting the ohmmeter between the “A” post and the “I” post. It should read 0 ohms with the DVM.

 

 

 

Note that if you have a bad ground to the housing then the thermally controlled current regulator is not operating and the fuel and temp gauges are receiving unregulated current that could fry the windings inside the gauges.

 

You can perform the tests:

 

  1. GRD to S = 71 ohms
  2. GRD to I = 51 ohms
  3. GRD to A = 51 ohms
  4. S to I = 21 ohms
  5. S to A = 21 ohms
  6. I to A =0 ohms

 

Some of these resistance may vary with newer style gauges.