How Ford Gauge Unit Work

[JSeery]

Hard to say, did you mix the wiring up? Damage the sender? Because you are having two systems exhibit a problem I would guess a wiring problem. On the temperature gauge H indicates an open somewhere in the wiring or the sender.

[FireEngineMike]

I did two things at the temp sending unit. I removed and reinstalled the wire to the sending unit. Only one wire and one place to reinstall it. I replaced the screw with a socket head cap screw because the original straight head screw was buggered. Both gauges stopped working properly afterward.
I will double check continuity of the wire and try the 1.5 volt battery test procedure on both gauges and look for mid-range. Key off, 6 volt input wires and sending unit wires disconnected from gauges?
Operationally, with the temp sending unit wire disconnected, shouldn't the fuel gauge still work?
Since the fuel gauge deflected full range with momentary contact by the jumper from the sending unit end of the wire, I assume (uhoh, there's that word) that the gauge has 6 volt supply and the wire from the sending unit has continuity. The ground wire at the fuel sending unit is good, but I will double check it for continuity with the positive cable on the battery disconnect.

[GB SISSON]

Originally Posted by JSeery

Based on several resent post there seems to be a lot of confusion about how early Ford gauges work. They are NOT resistant units and are unique to Ford as far a I know. A lot of the following is directly lifted from a Ford Shop Manuel. There are also a lot of older post on the Barn on this same issue.

The 6v Ford gauges (Fuel Level, Oil Pressure & Temperature) all work on similar principles. They have two major components, a Sender Unit and a Gauge Unit. The senders use a bimetal element and a heating coil to control the average current flow through both units. The Gauge Unit pointer is controlled by another bimetal and heating coil unit.
When the ignition switch is on, current flows through the circuit and warms the Sending Unit bimetal by means of a heating coil, causing the bimetal strip to bend and open a set of contact points. When the points open the current is interrupted allowing the bimetal to cool and close the contact points again. This cycle then repeats and the points vibrate open and closed pulsing the current in the circuit.
Because the current through the heating coil in the Sending Unit also flows through the heating coil in the Gauge Unit, the amount of heat supplied to the gauge unit is about the same as the heat in the Sending Unit. The amount of heat in both units is controlled by the average current flowing through the circuit due to the repeated opening and closing of the contact points.
The Sending Unit varies the current required to open and close the contact points by varying the pressure on the bimetal strip making it harder or easier for the points to open and close. The more pressure on the points the more heat (current) that is required to open the points. Likewise, the less the pressure on the points the less heat (current) that is required. The different gauges use slightly different mechanical systems to accomplish this.
Fuel Level Sender Unit:When the tank is filled, the float rises with the fuel level in the tank and a cam moves the ground contact toward the bimetal arm, increasing the tension holding the contacts closed. A greater amount of current is required to heat the Sending Unit bimetal arm enough to cause it to open the contacts. A similar greater bending of the bimetal arm occurs in the Gauge Unit and results in a movement of the needle toward the full position on the scale.
Oil Pressure Sender Unit: When there is no oil pressure, the contact points are just touching and the gauge pointer register at the “0” position. Any increase in oil pressure bends a diaphragm, which in turn increases the tension on the bimetal arm. More heat must be supplied to cause the contacts to open and a resulting increase in the average current flow to supply this heat. This increase in average current flow in the circuit heats the coil in the Gauge Unit which bends the bimetal strip and moves the pointer.
Temperature Sender Unit(s): The Temperature Sender is coupled with a Thermal switch. When the engine is cool the bimetal arm in the Sending Unit has maximum tension holding the contacts closed. The Maximum average current is necessary to open the contacts. The heating effect of the current causes the Gauge Unit bimetal arm and pointer to defect toward the “C” position of the scale. As the engine temperature increases, less current is required to keep the contacts at the break point since the increase in engine temperature causes the Sending Unit bimetal to bend away from the grounded contact. The Gauge Unit pointer then registers toward the “H” position of the scale.
The Sending Unit has one electric terminal. The Thermal Switch can be identified by the two terminal connectors on it. The switch is set to open at 200-212° F. With a sending unit in one cylinder bank and the switch in the other cylinder bank, the Gauge Unit will indicate a boiling condition in either bank.
Testing a Gauge System
Fuel Level & Oil Pressure System Tests: Gauge Unit can be tested by disconnecting the wiring to the Gauge Unit and connecting it to a 1 ½ volt source (a “D” cell battery works well). The gauge should display mid-scale with a 1 ½ volt input and full-scale with 3 volt input (two “D” cell batteries). The suggested test of a Sending Unit is to test it by substituting a know good gauge to see if it will read correctly with the suspect Sending Unit. If it does not work correctly with a know good gauge the problem could be a bad Sender Unit or a bad connection between the Sender Unit and the Gauge Unit.
Oil Pressure System Test: The gauge test is the same as for the Fuel Level and Oil Pressure Gauge Unit test. The Sending Unit is checked the same as the Fuel Level and Oil Pressure Sending Unit. The Thermal Switch can be tested to see if it is closed at normal temperatures. It can be tested with a test light or an ohm meter. It should show continuity. The switch can then be tested to determine if it opens at boiling temperature by placing the bulb in boiling water and testing it. At temperature it show test as an open.

Edit: on another post it was pointed out that by shorting the electrical terminal on the sending unit you are causing full current to flow through the Gauge Unit which would move the pointer on the gauge to full scale. I would be very carful doing this test because it would be easy to damage the heater coil in the Gauge Unit. It should be a momentary test. If the Gauge Unit responds then the Sending Unit may be the problem. If not it could be the wire between the two or the Gauge Unit itself. I would lean toward the "D" cell battery test, much easier on the Gauge Unit.

On the D cell test, how do I wire up the battery? Just the two battery terminals to the to instrument terminals? Does it matter which terminal is ground?

[JSeery]

The car use Positive ground and Negative for the power side. The gauge is wired with power from the ignition switch going to one terminal on the gauge and the other terminal connected to the sending unit. The negative side of the battery would connect to the ignition switch side of the gauge and the positive side would connect to the sender side of the gauge.

[glennsanders]

Great information thanks

[Paul Bennett]

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Has anyone used an Oscilloscope to determine the pulse speed of a Ford gauge at lo-mid-hi positions? I'm just curious.

Not a resistance direct current system, the Ford gauge isn't an alternating current system either. Information really depends upon the on-off pulse ratio not the pulse rate.

Sender open/pulses mostly off - gauge is at maximum.
Sender pulses mostly on - gauge is minimum.
Pulsing - gauge reads pulse off/on ratio.

I do find it interesting how the similar temperature coefficient of both sender and gauge work to cancel each other resulting in a system quite accurate across a wide ambient range.
.

[GB SISSON]

Originally Posted by JSeery

The car use Positive ground and Negative for the power side. The gauge is wired with power from the ignition switch going to one terminal on the gauge and the other terminal connected to the sending unit. The negative side of the battery would connect to the ignition switch side of the gauge and the positive side would connect to the sender side of the gauge.

Thanks so much for this info. I just tested a very nice looking instrument panel that I bought for 60 bucks and now I know all three gauges work and center out with the D battery. This one goes in the woodie. Excellent thread!

[50droptop]

So does converting to 12V affect the accuracy of the gauge? I was always under the impression you only needed to drop the voltage at the gauge, but it seems from the diagram that both the gauge and the sending unit would be impacted by the 12V.

[JSeery]

All of the gauges work the same way, the current is from the gauge to the sender and to ground. A voltage drop is needed at the power input side of the gauge, the sender is just a path to ground. The circuit is matching the current flow through both the gauge and the sender. There is no way or reason you would attempt to connect a voltage drop device to the sender.

[GB SISSON]

Originally Posted by JSeery

All of the gauges work the same way, the current is from the gauge to the sender and to ground. A voltage drop is needed at the power input side of the gauge, the sender is just a path to ground. The circuit is matching the current flow through both the gauge and the sender. There is no way or reason you would attempt to connect a voltage drop device to the sender.

I am a bit confused as to the resistor 'voltage drop' I need when converting to 12 volts. I see many ideas, but not sure what to do and what to use. One reducer for all 3 gauges, or 3 seperate? What and where to buy. Thanks GB

[rotorwrench]

Check out Randy Rundle's fifth avenue internet garage. He has about everything a person would need to switch to 12-volt. This includes dropping resistors & voltage regulators.

[JSeery]

Originally Posted by 50droptop

So does converting to 12V affect the accuracy of the gauge? I was always under the impression you only needed to drop the voltage at the gauge, but it seems from the diagram that both the gauge and the sending unit would be impacted by the 12V.

See if this helps any.

[rotorwrench]

The power in is at the gauge so it would need a runtz or a dropping resistor on there. The runtz units use one for each gauge I think but a dropping resistor or voltage regulator will drop all of them at once. Speedway carries this stuff too.

[50droptop]

Originally Posted by JSeery

See if this helps any.

Got it! I'm liking all the diagrams.

[GB SISSON]

So I looked at all Randy's stuff and he seems like a very knowledgeable straight shooter and I thank you. What has worked well over time, the speedway style that does all gauges or the runtz, needing three? Money seems about the same. I'm ready to pull the trigger on one way or the other.

[Curt in AZ]

I'd use the Runts units: one per gauge. Can't say for sure why, but I suspect the Runts may be more than a simple voltage dropping resitive load. In any event I like the idea of a single runtz unit responding to the individual load of each circuit rather than the average of the three parallel circuits.

[rotorwrench]

The runtz is a mini regulator. It will keep accuracy better with separate units for each gauge.

[GB SISSON]

Runtz it is! Thanks much.

[flatheadmurre]

Originally Posted by rotorwrench

The runtz is a mini regulator. It will keep accuracy better with separate units for each gauge.

Accuracy shouldn´t be an issue using 1 or 3 regulators since the heaters in sender/gauge is in series so exact same current goes through them.

Last conversion i did uses a switched stepdown converter and is now being under test for dependability.

The stepdown converter modules are dirtcheap today i can´t buy a regulator and breadbord for what they sell online.

If you use one for each gauge just about any dropping device including a simple resistor works fine.

The gauge doesn´t care about the voltage...you just need to limit the current rush a bit.

[rotorwrench]

The sender is not the problem so much as it is the generator output. It fluctuates even more on a generator than on an alternator but a change to 12-volts with an alternator changes things a lot. Fluctuation can be anything from 6-volts to 7.1 volts with the original set up but with 12-volts it's a different story (much more voltage spread at 12 to 14.7) and those little runtz regulators will keep it the same all the time and each gauge will be regulated separately so there are less chances of spikes creating any problems.

The gauges do care about voltage and especially amperage. Add twice the voltage to the mix also adds twice the amount of higher spike capability. Ford used constant voltage regulators on these systems for a long time after the change to 12-volt and for good reason.

The runtz is a step down switching converter or regulator if you choose. A person can build one cheaper but the labor involved along with soldering skills makes the runtz a relatively inexpensive way to do things. Definitely less hassle unless a person just likes to do that stuff themselves. A person could make 4 of them for the price of one prefabricated one if they were so inclined.