Gauging Response

The early Falcon instruments are not exactly hi-tech in fact they appear to be a relic of the even earlier 6 volt days. Although the Falcons were among the first Fords to go 12 volt negative ground the guts of fuel and temperature gauges operate on about 6 volts.

Like all good inventions it's pure simplicity. The red pointer on the face of the gauge is fixed to a bimetallic strip inside, around which is wound a electric heating coil of wire. As the current flows around the coil the bimetallic strip heats up and bends, moving the pointer.

The bending is directly related to the heating and the heating is directly related to the current flow which is controlled by the resistance of the senders in the tank, or engine block. The lower the resistance the greater the movement.

So why do they pack up? In the Temp gauge is a multivibrator the device that allowed them to use 6 volt gauges on 12 volt cars. This little critter is also a bimetallic strip but its little coil is switched off as it bends away from a contact that feeds it. Only when the strip cools dose it bend back to complete the circuit again. This on off operation happens about twice a second.

As the voltage starts to climb it's cut off and drops, climes and drops, climes and drops to maintain an average voltage of around 5-6 volts. Great in principal but as we all know from ignition points these contacts don't like having their little heads banged together. Sooner or later they become pitted or worse weld themselves together. If the latter happens wires inside the gauges melt resulting in no readings or low readings from pitted points.

	Inside the Temp Gauge. The multivibrator section is to the left with the black wire feeding the contact, and from the top, the white ground wire to the case. To the right is the Temp bimetallic strip and the pointer with the white wire from the middle terminal around the strip and on to the sender terminal. The match is holding back the pointer during the removal of the multivibrator.

The cure is also simple, replace the burnt-out multivibrator with a 5 volt solid state voltage regulator No. 7805. These are available from stores like Radio Shack, Dick Smiths, etc. for about a dollar ten.

Remove the temp gauge from the instrument cluster. Then with a small screwdriver pry the face off the rim of the gauge body. This is best done while squeezing the upper and lower sides of the gauge together with the thumb and forefinger. When the face is free turn it a quarter of a turn anticlockwise and carefully slide it down the pointer. With the face off, swing the pointer to the right of the temp bimetallic strip and block its return with a match. With a pair of wire cutters snip out the burnt out multivibrator. See before and after in the diagram below.

The blue path is the current flow of the Multivibrator, from the Input terminal to the contact and along the bimetallic strip to ground. The red path is the Gauge, from the Multivibrator along the strip to the Sender terminal.

After soldering a 1µF Tantalum capacitor to each of the legs of the 7805 making sure the positive legs of the caps are outermost. Screw the 7805 to the body of the instrument cluster housing making sure its metal heatsink base is in good contact. Extend and solder the input and output legs of the 7805 with wire. Return the gauge to the instrument cluster housing with the terminal nuts holding the wires from the 7805. Refit and wire the cluster to the car, turn ignition on and test before refitting the face. If all is OK remove the gauge and refit the face, reverse order of remove.

	Voltage Regulator 7805 mounted on the rear of the instrument cluster housing. Make sure there is good electrical and heat transfer contact with the housing. The centre leg and the fixing plate of the 7805 are common points, electrically speaking. I = Input, S = Sender and A = Auxiliary, on the back of the temp gauge.

If the pointer deflection is not quite enough the 7805 output can be increased by half a volt by using a heatsink insulator and grounding the 7805 via a diode. Ask for diode and insulator with the 7805. If you need a bigger swing use a 7806 without insulation. This will give you six volts and an insulated 7806 plus a diode will produce six and a half volts.

Boosting the output of the 7805 or 7806, Note band at the ground end of the diode.

The gauge and metal instrument cluster housing must he grounded to the car body. Some times when restoration jobs are done owners have chosen to paint the inside of metal instrument cluster. It's most important that the area between the three temp-gauge terminal holes on the inside of the housing be clean bare metal as this is the point of grounding for the gauges. This point is valid for the original gauges as well as this modification.

The likelihood of an incorrect, or no, reading on a gauge from a faulty multivibrator is greater than a faulty sender though not always. To learn more about faulty fuel senders see the Restoration Hint Message Sender Generally speaking a faulty multivibrator will affect the readings on both the temperature and fuel gauges. Whereas a faulty Sender will only affect one gauge. To check either the fuel or temperature senders, remove the wire from the sender and with the ignition on very briefly ground the wire to the car's body while watching for movement on the relative gauge. No movement would suggest a faulty multivibrator.

© BigH 1996

DISCLAIMER While this method has been tried and works, NO responsibility will be excepted by the Author, the Club or any of its Members.