The temperature sender is grounded through the threads that contact the engine. A good sender ground is necessary for proper gauge operation. Therefore, Teflon tape should NOT be used on the threads of the temperature sender. Liquid sealant can be used sparingly if necessary. (The SN12mm temperature sender DOES REQUIRE sealant or Teflon tape to be used on the threads. A copper crush washer is included with this sender to provide a ground connection between the sender and the engine.)

Engine “core” temperature is typically 15-30 degrees F higher than the surface of the intake manifold where the sender is installed. It is normal (and expected) to get higher readings on the temperature gauge than readings obtained using an infrared temperature “gun”.

Volt Gauge

The volt gauge does not require a signal like the temperature, oil pressure, and fuel level gauges do. The volt gauge should be connected to a good ground and switched battery power. No connection is necessary to the alternator as with amp gauges.


Electronic speedometers operate on an electrical signal rather than a cable like the mechanical type. Classic Instruments’ 3-3/8- and 4-5/8-inch speedometers can operate on signals between 11,000 and 110,000 pulses per mile that have amplitude of at least 8 V.

The required signal can be produced a number of ways:

SN16 pulse signal generator: This is used on GM or Chrysler transmissions that have a mechanical speedometer cable drive. The pulse signal generator replaces the speedometer cable. It converts the mechanical movement of the cable to an electric pulse. The pulse is a “square wave” that alternates between 0 and plus 12 V. The SN16 is a powered signal generator. It requires plus 12 V to operate, unlike some two-wire pulse signal generators that create their own voltage.

SN17 Ford pulse signal generator adapter: This is used to convert the connection from Ford and similar transmissions that have a mechanical speedometer drive cable to work with the SN16. A speedometer cable driven gear needs to be installed on the SN17 adapter and is not provided from Classic Instruments.

SN74 speedometer signal interface: This is usually used to convert speed signals from electronic transmissions or vehicle computers to one that is compatible with the Classic Instruments speedometer. Benefits of the SN74 are: push-button speed calibration and supplemental speedometer signal output (for cruise controls, lockups, etc.). The SN74 will convert any signal between 3,000 and 200,000 pulses per mile to either 8,000 or 16,000 pulses per mile. Classic Instruments speedometers can be configured to work on one of these signal frequencies. Some Classic Instruments speedometers have the SN74 functionality built into them with no signal interface needed.

SN81 GPS speedometer signal: This can be used to provide a signal for any speedometer (not just Classic Instruments). Calibrating the SN81 can be done in the comfort of your garage. It is not necessary to drive the vehicle to get a proper calibration. The SN81 is especially useful for vehicles that lack a standard speed signal. Some common applications that would use an SN81: boat, ATV, golf cart, etc.

SN78SM magnetic proximity speed sensor: This sensor has a magnetic head, which senses ferrous targets. Targets can be bolts on a driveshaft or holes in a brake rotor. The SN78SM will provide a plus 12V pulse upon detection of each target. The number of pulses per mile should be calculated and should be at least 3,000 for good speedometer operation. The SN78SM sometimes requires a SN74 signal interface to convert a slower frequency signal to a signal compatible with the speedometer.


The tachometer can get a signal from a computer, ignition module, or coil. Some signals require an adapter to work correctly.

Classic Instruments’ 3-3/8- and 4-5/8-inch tachometers require a signal that has amplitude of at least 8 V. Some computer signals are 0-5V square waves (and need to be amplified. This can sometimes be done with a 1,000-ohm resistor installed between plus 12 V and the signal post of the tachometer.